# <img align="right" src="https://raw.github.com/magnars/dash.el/master/rainbow-dash.png"> dash.el [![Build Status](https://secure.travis-ci.org/magnars/dash.el.png)](http://travis-ci.org/magnars/dash.el)

A modern list api for Emacs. No 'cl required.

## Installation

It's available on [marmalade](http://marmalade-repo.org/) and [Melpa](http://melpa.milkbox.net/):

    M-x package-install dash

Or you can just dump `dash.el` in your load
path somewhere.

If you want the function combinators, then also:

    M-x package-install dash-functional

## Using in a package

Add this to the big comment block at the top:

    ;; Package-Requires: ((dash "2.12.0"))

To get function combinators:

    ;; Package-Requires: ((dash "2.12.0") (dash-functional "1.2.0") (emacs "24"))

## Syntax highlighting of dash functions

Font lock of dash functions in emacs lisp buffers is now optional.
Include this in your emacs settings to get syntax highlighting:

    (eval-after-load "dash" '(dash-enable-font-lock))

## Functions

All functions and constructs in the library are prefixed with a dash (-).

There are also anaphoric versions of functions where that makes sense,
prefixed with two dashes instead of one.

While `-map` takes a function to map over the list, you can also use
the anaphoric form with double dashes - which will then be executed
with `it` exposed as the list item. Here's an example:

```el
(-map (lambda (n) (* n n)) '(1 2 3 4)) ;; normal version

(--map (* it it) '(1 2 3 4)) ;; anaphoric version
```

of course the original can also be written like

```el
(defun square (n) (* n n))

(-map 'square '(1 2 3 4))
```

which demonstrates the usefulness of both versions.


### Maps


Functions in this category take a transforming function, which
is then applied sequentially to each or selected elements of the
input list.  The results are collected in order and returned as
new list.

* [-map](#-map-fn-list) `(fn list)`
* [-map-when](#-map-when-pred-rep-list) `(pred rep list)`
* [-map-first](#-map-first-pred-rep-list) `(pred rep list)`
* [-map-last](#-map-last-pred-rep-list) `(pred rep list)`
* [-map-indexed](#-map-indexed-fn-list) `(fn list)`
* [-annotate](#-annotate-fn-list) `(fn list)`
* [-splice](#-splice-pred-fun-list) `(pred fun list)`
* [-splice-list](#-splice-list-pred-new-list-list) `(pred new-list list)`
* [-mapcat](#-mapcat-fn-list) `(fn list)`
* [-copy](#-copy-arg) `(arg)`

### Sublist selection


Functions returning a sublist of the original list.

* [-filter](#-filter-pred-list) `(pred list)`
* [-remove](#-remove-pred-list) `(pred list)`
* [-remove-first](#-remove-first-pred-list) `(pred list)`
* [-remove-last](#-remove-last-pred-list) `(pred list)`
* [-remove-item](#-remove-item-item-list) `(item list)`
* [-non-nil](#-non-nil-list) `(list)`
* [-slice](#-slice-list-from-optional-to-step) `(list from &optional to step)`
* [-take](#-take-n-list) `(n list)`
* [-drop](#-drop-n-list) `(n list)`
* [-take-while](#-take-while-pred-list) `(pred list)`
* [-drop-while](#-drop-while-pred-list) `(pred list)`
* [-select-by-indices](#-select-by-indices-indices-list) `(indices list)`

### List to list


Bag of various functions which modify input list.

* [-keep](#-keep-fn-list) `(fn list)`
* [-concat](#-concat-rest-lists) `(&rest lists)`
* [-flatten](#-flatten-l) `(l)`
* [-flatten-n](#-flatten-n-num-list) `(num list)`
* [-replace](#-replace-old-new-list) `(old new list)`
* [-replace-first](#-replace-first-old-new-list) `(old new list)`
* [-replace-last](#-replace-last-old-new-list) `(old new list)`
* [-insert-at](#-insert-at-n-x-list) `(n x list)`
* [-replace-at](#-replace-at-n-x-list) `(n x list)`
* [-update-at](#-update-at-n-func-list) `(n func list)`
* [-remove-at](#-remove-at-n-list) `(n list)`
* [-remove-at-indices](#-remove-at-indices-indices-list) `(indices list)`

### Reductions


Functions reducing lists into single value.

* [-reduce-from](#-reduce-from-fn-initial-value-list) `(fn initial-value list)`
* [-reduce-r-from](#-reduce-r-from-fn-initial-value-list) `(fn initial-value list)`
* [-reduce](#-reduce-fn-list) `(fn list)`
* [-reduce-r](#-reduce-r-fn-list) `(fn list)`
* [-count](#-count-pred-list) `(pred list)`
* [-sum](#-sum-list) `(list)`
* [-product](#-product-list) `(list)`
* [-min](#-min-list) `(list)`
* [-min-by](#-min-by-comparator-list) `(comparator list)`
* [-max](#-max-list) `(list)`
* [-max-by](#-max-by-comparator-list) `(comparator list)`

### Unfolding


Operations dual to reductions, building lists from seed value rather than consuming a list to produce a single value.

* [-iterate](#-iterate-fun-init-n) `(fun init n)`
* [-unfold](#-unfold-fun-seed) `(fun seed)`

### Predicates

* [-any?](#-any-pred-list) `(pred list)`
* [-all?](#-all-pred-list) `(pred list)`
* [-none?](#-none-pred-list) `(pred list)`
* [-only-some?](#-only-some-pred-list) `(pred list)`
* [-contains?](#-contains-list-element) `(list element)`
* [-same-items?](#-same-items-list-list2) `(list list2)`
* [-is-prefix?](#-is-prefix-prefix-list) `(prefix list)`
* [-is-suffix?](#-is-suffix-suffix-list) `(suffix list)`
* [-is-infix?](#-is-infix-infix-list) `(infix list)`

### Partitioning


Functions partitioning the input list into a list of lists.

* [-split-at](#-split-at-n-list) `(n list)`
* [-split-with](#-split-with-pred-list) `(pred list)`
* [-split-on](#-split-on-item-list) `(item list)`
* [-split-when](#-split-when-fn-list) `(fn list)`
* [-separate](#-separate-pred-list) `(pred list)`
* [-partition](#-partition-n-list) `(n list)`
* [-partition-all](#-partition-all-n-list) `(n list)`
* [-partition-in-steps](#-partition-in-steps-n-step-list) `(n step list)`
* [-partition-all-in-steps](#-partition-all-in-steps-n-step-list) `(n step list)`
* [-partition-by](#-partition-by-fn-list) `(fn list)`
* [-partition-by-header](#-partition-by-header-fn-list) `(fn list)`
* [-group-by](#-group-by-fn-list) `(fn list)`

### Indexing


Return indices of elements based on predicates, sort elements by indices etc.

* [-elem-index](#-elem-index-elem-list) `(elem list)`
* [-elem-indices](#-elem-indices-elem-list) `(elem list)`
* [-find-index](#-find-index-pred-list) `(pred list)`
* [-find-last-index](#-find-last-index-pred-list) `(pred list)`
* [-find-indices](#-find-indices-pred-list) `(pred list)`
* [-grade-up](#-grade-up-comparator-list) `(comparator list)`
* [-grade-down](#-grade-down-comparator-list) `(comparator list)`

### Set operations


Operations pretending lists are sets.

* [-union](#-union-list-list2) `(list list2)`
* [-difference](#-difference-list-list2) `(list list2)`
* [-intersection](#-intersection-list-list2) `(list list2)`
* [-distinct](#-distinct-list) `(list)`

### Other list operations


Other list functions not fit to be classified elsewhere.

* [-rotate](#-rotate-n-list) `(n list)`
* [-repeat](#-repeat-n-x) `(n x)`
* [-cons*](#-cons-rest-args) `(&rest args)`
* [-snoc](#-snoc-list-elem-rest-elements) `(list elem &rest elements)`
* [-interpose](#-interpose-sep-list) `(sep list)`
* [-interleave](#-interleave-rest-lists) `(&rest lists)`
* [-zip-with](#-zip-with-fn-list1-list2) `(fn list1 list2)`
* [-zip](#-zip-rest-lists) `(&rest lists)`
* [-zip-fill](#-zip-fill-fill-value-rest-lists) `(fill-value &rest lists)`
* [-cycle](#-cycle-list) `(list)`
* [-pad](#-pad-fill-value-rest-lists) `(fill-value &rest lists)`
* [-table](#-table-fn-rest-lists) `(fn &rest lists)`
* [-table-flat](#-table-flat-fn-rest-lists) `(fn &rest lists)`
* [-first](#-first-pred-list) `(pred list)`
* [-some](#-some-pred-list) `(pred list)`
* [-last](#-last-pred-list) `(pred list)`
* [-first-item](#-first-item-list) `(list)`
* [-last-item](#-last-item-list) `(list)`
* [-butlast](#-butlast-list) `(list)`
* [-sort](#-sort-comparator-list) `(comparator list)`
* [-list](#-list-rest-args) `(&rest args)`
* [-fix](#-fix-fn-list) `(fn list)`

### Tree operations


Functions pretending lists are trees.

* [-tree-seq](#-tree-seq-branch-children-tree) `(branch children tree)`
* [-tree-map](#-tree-map-fn-tree) `(fn tree)`
* [-tree-map-nodes](#-tree-map-nodes-pred-fun-tree) `(pred fun tree)`
* [-tree-reduce](#-tree-reduce-fn-tree) `(fn tree)`
* [-tree-reduce-from](#-tree-reduce-from-fn-init-value-tree) `(fn init-value tree)`
* [-tree-mapreduce](#-tree-mapreduce-fn-folder-tree) `(fn folder tree)`
* [-tree-mapreduce-from](#-tree-mapreduce-from-fn-folder-init-value-tree) `(fn folder init-value tree)`
* [-clone](#-clone-list) `(list)`

### Threading macros

* [->](#--x-optional-form-rest-more) `(x &optional form &rest more)`
* [->>](#--x-optional-form-rest-more) `(x &optional form &rest more)`
* [-->](#---x-form-rest-more) `(x form &rest more)`
* [-some->](#-some--x-optional-form-rest-more) `(x &optional form &rest more)`
* [-some->>](#-some--x-optional-form-rest-more) `(x &optional form &rest more)`
* [-some-->](#-some---x-optional-form-rest-more) `(x &optional form &rest more)`

### Binding


Convenient versions of `let` and `let*` constructs combined with flow control.

* [-when-let](#-when-let-var-val-rest-body) `(var-val &rest body)`
* [-when-let*](#-when-let-vars-vals-rest-body) `(vars-vals &rest body)`
* [-if-let](#-if-let-var-val-then-rest-else) `(var-val then &rest else)`
* [-if-let*](#-if-let-vars-vals-then-rest-else) `(vars-vals then &rest else)`
* [-let](#-let-varlist-rest-body) `(varlist &rest body)`
* [-let*](#-let-varlist-rest-body) `(varlist &rest body)`
* [-lambda](#-lambda-match-form-rest-body) `(match-form &rest body)`

### Side-effects


Functions iterating over lists for side-effect only.

* [-each](#-each-list-fn) `(list fn)`
* [-each-while](#-each-while-list-pred-fn) `(list pred fn)`
* [-dotimes](#-dotimes-num-fn) `(num fn)`

### Destructive operations

* [!cons](#cons-car-cdr) `(car cdr)`
* [!cdr](#cdr-list) `(list)`

### Function combinators


These combinators require Emacs 24 for its lexical scope. So they are offered in a separate package: `dash-functional`.

* [-partial](#-partial-fn-rest-args) `(fn &rest args)`
* [-rpartial](#-rpartial-fn-rest-args) `(fn &rest args)`
* [-juxt](#-juxt-rest-fns) `(&rest fns)`
* [-compose](#-compose-rest-fns) `(&rest fns)`
* [-applify](#-applify-fn) `(fn)`
* [-on](#-on-operator-transformer) `(operator transformer)`
* [-flip](#-flip-func) `(func)`
* [-const](#-const-c) `(c)`
* [-cut](#-cut-rest-params) `(&rest params)`
* [-not](#-not-pred) `(pred)`
* [-orfn](#-orfn-rest-preds) `(&rest preds)`
* [-andfn](#-andfn-rest-preds) `(&rest preds)`
* [-iteratefn](#-iteratefn-fn-n) `(fn n)`
* [-fixfn](#-fixfn-fn-optional-equal-test-halt-test) `(fn &optional equal-test halt-test)`
* [-prodfn](#-prodfn-rest-fns) `(&rest fns)`


## Maps


Functions in this category take a transforming function, which
is then applied sequentially to each or selected elements of the
input list.  The results are collected in order and returned as
new list.

#### -map `(fn list)`

Return a new list consisting of the result of applying `fn` to the items in `list`.

```el
(-map (lambda (num) (* num num)) '(1 2 3 4)) ;; => '(1 4 9 16)
(-map 'square '(1 2 3 4)) ;; => '(1 4 9 16)
(--map (* it it) '(1 2 3 4)) ;; => '(1 4 9 16)
```

#### -map-when `(pred rep list)`

Return a new list where the elements in `list` that does not match the `pred` function
are unchanged, and where the elements in `list` that do match the `pred` function are mapped
through the `rep` function.

Alias: `-replace-where`

See also: [`-update-at`](#-update-at-n-func-list)

```el
(-map-when 'even? 'square '(1 2 3 4)) ;; => '(1 4 3 16)
(--map-when (> it 2) (* it it) '(1 2 3 4)) ;; => '(1 2 9 16)
(--map-when (= it 2) 17 '(1 2 3 4)) ;; => '(1 17 3 4)
```

#### -map-first `(pred rep list)`

Replace first item in `list` satisfying `pred` with result of `rep` called on this item.

See also: [`-map-when`](#-map-when-pred-rep-list), [`-replace-first`](#-replace-first-old-new-list)

```el
(-map-first 'even? 'square '(1 2 3 4)) ;; => '(1 4 3 4)
(--map-first (> it 2) (* it it) '(1 2 3 4)) ;; => '(1 2 9 4)
(--map-first (= it 2) 17 '(1 2 3 2)) ;; => '(1 17 3 2)
```

#### -map-last `(pred rep list)`

Replace first item in `list` satisfying `pred` with result of `rep` called on this item.

See also: [`-map-when`](#-map-when-pred-rep-list), [`-replace-last`](#-replace-last-old-new-list)

```el
(-map-last 'even? 'square '(1 2 3 4)) ;; => '(1 2 3 16)
(--map-last (> it 2) (* it it) '(1 2 3 4)) ;; => '(1 2 3 16)
(--map-last (= it 2) 17 '(1 2 3 2)) ;; => '(1 2 3 17)
```

#### -map-indexed `(fn list)`

Return a new list consisting of the result of (`fn` index item) for each item in `list`.

In the anaphoric form `--map-indexed`, the index is exposed as `it-index`.

```el
(-map-indexed (lambda (index item) (- item index)) '(1 2 3 4)) ;; => '(1 1 1 1)
(--map-indexed (- it it-index) '(1 2 3 4)) ;; => '(1 1 1 1)
```

#### -annotate `(fn list)`

Return a list of cons cells where each cell is `fn` applied to each
element of `list` paired with the unmodified element of `list`.

```el
(-annotate '1+ '(1 2 3)) ;; => '((2 . 1) (3 . 2) (4 . 3))
(-annotate 'length '(("h" "e" "l" "l" "o") ("hello" "world"))) ;; => '((5 "h" "e" "l" "l" "o") (2 "hello" "world"))
(--annotate (< 1 it) '(0 1 2 3)) ;; => '((nil . 0) (nil . 1) (t . 2) (t . 3))
```

#### -splice `(pred fun list)`

Splice lists generated by `fun` in place of elements matching `pred` in `list`.

`fun` takes the element matching `pred` as input.

This function can be used as replacement for `,@` in case you
need to splice several lists at marked positions (for example
with keywords).

See also: [`-splice-list`](#-splice-list-pred-new-list-list), [`-insert-at`](#-insert-at-n-x-list)

```el
(-splice 'even? (lambda (x) (list x x)) '(1 2 3 4)) ;; => '(1 2 2 3 4 4)
(--splice 't (list it it) '(1 2 3 4)) ;; => '(1 1 2 2 3 3 4 4)
(--splice (equal it :magic) '((list of) (magical) (code)) '((foo) (bar) :magic (baz))) ;; => '((foo) (bar) (list of) (magical) (code) (baz))
```

#### -splice-list `(pred new-list list)`

Splice `new-list` in place of elements matching `pred` in `list`.

See also: [`-splice`](#-splice-pred-fun-list), [`-insert-at`](#-insert-at-n-x-list)

```el
(-splice-list 'keywordp '(a b c) '(1 :foo 2)) ;; => '(1 a b c 2)
(-splice-list 'keywordp nil '(1 :foo 2)) ;; => '(1 2)
(--splice-list (keywordp it) '(a b c) '(1 :foo 2)) ;; => '(1 a b c 2)
```

#### -mapcat `(fn list)`

Return the concatenation of the result of mapping `fn` over `list`.
Thus function `fn` should return a list.

```el
(-mapcat 'list '(1 2 3)) ;; => '(1 2 3)
(-mapcat (lambda (item) (list 0 item)) '(1 2 3)) ;; => '(0 1 0 2 0 3)
(--mapcat (list 0 it) '(1 2 3)) ;; => '(0 1 0 2 0 3)
```

#### -copy `(arg)`

Create a shallow copy of `list`.

```el
(-copy '(1 2 3)) ;; => '(1 2 3)
(let ((a '(1 2 3))) (eq a (-copy a))) ;; => nil
```


## Sublist selection


Functions returning a sublist of the original list.

#### -filter `(pred list)`

Return a new list of the items in `list` for which `pred` returns a non-nil value.

Alias: `-select`

```el
(-filter (lambda (num) (= 0 (% num 2))) '(1 2 3 4)) ;; => '(2 4)
(-filter 'even? '(1 2 3 4)) ;; => '(2 4)
(--filter (= 0 (% it 2)) '(1 2 3 4)) ;; => '(2 4)
```

#### -remove `(pred list)`

Return a new list of the items in `list` for which `pred` returns nil.

Alias: `-reject`

```el
(-remove (lambda (num) (= 0 (% num 2))) '(1 2 3 4)) ;; => '(1 3)
(-remove 'even? '(1 2 3 4)) ;; => '(1 3)
(--remove (= 0 (% it 2)) '(1 2 3 4)) ;; => '(1 3)
```

#### -remove-first `(pred list)`

Return a new list with the first item matching `pred` removed.

Alias: `-reject-first`

See also: [`-remove`](#-remove-pred-list), [`-map-first`](#-map-first-pred-rep-list)

```el
(-remove-first 'even? '(1 3 5 4 7 8 10)) ;; => '(1 3 5 7 8 10)
(-remove-first 'stringp '(1 2 "first" "second" "third")) ;; => '(1 2 "second" "third")
(--remove-first (> it 3) '(1 2 3 4 5 6 7 8 9 10)) ;; => '(1 2 3 5 6 7 8 9 10)
```

#### -remove-last `(pred list)`

Return a new list with the last item matching `pred` removed.

Alias: `-reject-last`

See also: [`-remove`](#-remove-pred-list), [`-map-last`](#-map-last-pred-rep-list)

```el
(-remove-last 'even? '(1 3 5 4 7 8 10 11)) ;; => '(1 3 5 4 7 8 11)
(-remove-last 'stringp '(1 2 "last" "second" "third")) ;; => '(1 2 "last" "second")
(--remove-last (> it 3) '(1 2 3 4 5 6 7 8 9 10)) ;; => '(1 2 3 4 5 6 7 8 9)
```

#### -remove-item `(item list)`

Remove all occurences of `item` from `list`.

Comparison is done with `equal`.

```el
(-remove-item 3 '(1 2 3 2 3 4 5 3)) ;; => '(1 2 2 4 5)
(-remove-item 'foo '(foo bar baz foo)) ;; => '(bar baz)
(-remove-item "bob" '("alice" "bob" "eve" "bob" "dave")) ;; => '("alice" "eve" "dave")
```

#### -non-nil `(list)`

Return all non-nil elements of `list`.

```el
(-non-nil '(1 nil 2 nil nil 3 4 nil 5 nil)) ;; => '(1 2 3 4 5)
```

#### -slice `(list from &optional to step)`

Return copy of `list`, starting from index `from` to index `to`.

`from` or `to` may be negative.  These values are then interpreted
modulo the length of the list.

If `step` is a number, only each STEPth item in the resulting
section is returned.  Defaults to 1.

```el
(-slice '(1 2 3 4 5) 1) ;; => '(2 3 4 5)
(-slice '(1 2 3 4 5) 0 3) ;; => '(1 2 3)
(-slice '(1 2 3 4 5 6 7 8 9) 1 -1 2) ;; => '(2 4 6 8)
```

#### -take `(n list)`

Return a new list of the first `n` items in `list`, or all items if there are fewer than `n`.

```el
(-take 3 '(1 2 3 4 5)) ;; => '(1 2 3)
(-take 17 '(1 2 3 4 5)) ;; => '(1 2 3 4 5)
```

#### -drop `(n list)`

Return the tail of `list` without the first `n` items.

```el
(-drop 3 '(1 2 3 4 5)) ;; => '(4 5)
(-drop 17 '(1 2 3 4 5)) ;; => '()
```

#### -take-while `(pred list)`

Return a new list of successive items from `list` while (`pred` item) returns a non-nil value.

```el
(-take-while 'even? '(1 2 3 4)) ;; => '()
(-take-while 'even? '(2 4 5 6)) ;; => '(2 4)
(--take-while (< it 4) '(1 2 3 4 3 2 1)) ;; => '(1 2 3)
```

#### -drop-while `(pred list)`

Return the tail of `list` starting from the first item for which (`pred` item) returns nil.

```el
(-drop-while 'even? '(1 2 3 4)) ;; => '(1 2 3 4)
(-drop-while 'even? '(2 4 5 6)) ;; => '(5 6)
(--drop-while (< it 4) '(1 2 3 4 3 2 1)) ;; => '(4 3 2 1)
```

#### -select-by-indices `(indices list)`

Return a list whose elements are elements from `list` selected
as `(nth i list)` for all i from `indices`.

```el
(-select-by-indices '(4 10 2 3 6) '("v" "e" "l" "o" "c" "i" "r" "a" "p" "t" "o" "r")) ;; => '("c" "o" "l" "o" "r")
(-select-by-indices '(2 1 0) '("a" "b" "c")) ;; => '("c" "b" "a")
(-select-by-indices '(0 1 2 0 1 3 3 1) '("f" "a" "r" "l")) ;; => '("f" "a" "r" "f" "a" "l" "l" "a")
```


## List to list


Bag of various functions which modify input list.

#### -keep `(fn list)`

Return a new list of the non-nil results of applying `fn` to the items in `list`.

If you want to select the original items satisfying a predicate use [`-filter`](#-filter-pred-list).

```el
(-keep 'cdr '((1 2 3) (4 5) (6))) ;; => '((2 3) (5))
(-keep (lambda (num) (when (> num 3) (* 10 num))) '(1 2 3 4 5 6)) ;; => '(40 50 60)
(--keep (when (> it 3) (* 10 it)) '(1 2 3 4 5 6)) ;; => '(40 50 60)
```

#### -concat `(&rest lists)`

Return a new list with the concatenation of the elements in the supplied `lists`.

```el
(-concat '(1)) ;; => '(1)
(-concat '(1) '(2)) ;; => '(1 2)
(-concat '(1) '(2 3) '(4)) ;; => '(1 2 3 4)
```

#### -flatten `(l)`

Take a nested list `l` and return its contents as a single, flat list.

Note that because `nil` represents a list of zero elements (an
empty list), any mention of nil in `l` will disappear after
flattening.  If you need to preserve nils, consider [`-flatten-n`](#-flatten-n-num-list)
or map them to some unique symbol and then map them back.

Conses of two atoms are considered "terminals", that is, they
aren't flattened further.

See also: [`-flatten-n`](#-flatten-n-num-list)

```el
(-flatten '((1))) ;; => '(1)
(-flatten '((1 (2 3) (((4 (5))))))) ;; => '(1 2 3 4 5)
(-flatten '(1 2 (3 . 4))) ;; => '(1 2 (3 . 4))
```

#### -flatten-n `(num list)`

Flatten `num` levels of a nested `list`.

See also: [`-flatten`](#-flatten-l)

```el
(-flatten-n 1 '((1 2) ((3 4) ((5 6))))) ;; => '(1 2 (3 4) ((5 6)))
(-flatten-n 2 '((1 2) ((3 4) ((5 6))))) ;; => '(1 2 3 4 (5 6))
(-flatten-n 3 '((1 2) ((3 4) ((5 6))))) ;; => '(1 2 3 4 5 6)
```

#### -replace `(old new list)`

Replace all `old` items in `list` with `new`.

Elements are compared using `equal`.

See also: [`-replace-at`](#-replace-at-n-x-list)

```el
(-replace 1 "1" '(1 2 3 4 3 2 1)) ;; => '("1" 2 3 4 3 2 "1")
(-replace "foo" "bar" '("a" "nice" "foo" "sentence" "about" "foo")) ;; => '("a" "nice" "bar" "sentence" "about" "bar")
(-replace 1 2 nil) ;; => nil
```

#### -replace-first `(old new list)`

Replace the first occurence of `old` with `new` in `list`.

Elements are compared using `equal`.

See also: [`-map-first`](#-map-first-pred-rep-list)

```el
(-replace-first 1 "1" '(1 2 3 4 3 2 1)) ;; => '("1" 2 3 4 3 2 1)
(-replace-first "foo" "bar" '("a" "nice" "foo" "sentence" "about" "foo")) ;; => '("a" "nice" "bar" "sentence" "about" "foo")
(-replace-first 1 2 nil) ;; => nil
```

#### -replace-last `(old new list)`

Replace the last occurence of `old` with `new` in `list`.

Elements are compared using `equal`.

See also: [`-map-last`](#-map-last-pred-rep-list)

```el
(-replace-last 1 "1" '(1 2 3 4 3 2 1)) ;; => '(1 2 3 4 3 2 "1")
(-replace-last "foo" "bar" '("a" "nice" "foo" "sentence" "about" "foo")) ;; => '("a" "nice" "foo" "sentence" "about" "bar")
(-replace-last 1 2 nil) ;; => nil
```

#### -insert-at `(n x list)`

Return a list with `x` inserted into `list` at position `n`.

See also: [`-splice`](#-splice-pred-fun-list), [`-splice-list`](#-splice-list-pred-new-list-list)

```el
(-insert-at 1 'x '(a b c)) ;; => '(a x b c)
(-insert-at 12 'x '(a b c)) ;; => '(a b c x)
```

#### -replace-at `(n x list)`

Return a list with element at Nth position in `list` replaced with `x`.

See also: [`-replace`](#-replace-old-new-list)

```el
(-replace-at 0 9 '(0 1 2 3 4 5)) ;; => '(9 1 2 3 4 5)
(-replace-at 1 9 '(0 1 2 3 4 5)) ;; => '(0 9 2 3 4 5)
(-replace-at 4 9 '(0 1 2 3 4 5)) ;; => '(0 1 2 3 9 5)
```

#### -update-at `(n func list)`

Return a list with element at Nth position in `list` replaced with `(func (nth n list))`.

See also: [`-map-when`](#-map-when-pred-rep-list)

```el
(-update-at 0 (lambda (x) (+ x 9)) '(0 1 2 3 4 5)) ;; => '(9 1 2 3 4 5)
(-update-at 1 (lambda (x) (+ x 8)) '(0 1 2 3 4 5)) ;; => '(0 9 2 3 4 5)
(--update-at 2 (length it) '("foo" "bar" "baz" "quux")) ;; => '("foo" "bar" 3 "quux")
```

#### -remove-at `(n list)`

Return a list with element at Nth position in `list` removed.

See also: [`-remove-at-indices`](#-remove-at-indices-indices-list), [`-remove`](#-remove-pred-list)

```el
(-remove-at 0 '("0" "1" "2" "3" "4" "5")) ;; => '("1" "2" "3" "4" "5")
(-remove-at 1 '("0" "1" "2" "3" "4" "5")) ;; => '("0" "2" "3" "4" "5")
(-remove-at 2 '("0" "1" "2" "3" "4" "5")) ;; => '("0" "1" "3" "4" "5")
```

#### -remove-at-indices `(indices list)`

Return a list whose elements are elements from `list` without
elements selected as `(nth i list)` for all i
from `indices`.

See also: [`-remove-at`](#-remove-at-n-list), [`-remove`](#-remove-pred-list)

```el
(-remove-at-indices '(0) '("0" "1" "2" "3" "4" "5")) ;; => '("1" "2" "3" "4" "5")
(-remove-at-indices '(0 2 4) '("0" "1" "2" "3" "4" "5")) ;; => '("1" "3" "5")
(-remove-at-indices '(0 5) '("0" "1" "2" "3" "4" "5")) ;; => '("1" "2" "3" "4")
```


## Reductions


Functions reducing lists into single value.

#### -reduce-from `(fn initial-value list)`

Return the result of applying `fn` to `initial-value` and the
first item in `list`, then applying `fn` to that result and the 2nd
item, etc. If `list` contains no items, return `initial-value` and
`fn` is not called.

In the anaphoric form `--reduce-from`, the accumulated value is
exposed as `acc`.

See also: [`-reduce`](#-reduce-fn-list), [`-reduce-r`](#-reduce-r-fn-list)

```el
(-reduce-from '- 10 '(1 2 3)) ;; => 4
(-reduce-from (lambda (memo item) (concat "(" memo " - " (int-to-string item) ")")) "10" '(1 2 3)) ;; => "(((10 - 1) - 2) - 3)"
(--reduce-from (concat acc " " it) "START" '("a" "b" "c")) ;; => "START a b c"
```

#### -reduce-r-from `(fn initial-value list)`

Replace conses with `fn`, nil with `initial-value` and evaluate
the resulting expression. If `list` is empty, `initial-value` is
returned and `fn` is not called.

Note: this function works the same as [`-reduce-from`](#-reduce-from-fn-initial-value-list) but the
operation associates from right instead of from left.

See also: [`-reduce-r`](#-reduce-r-fn-list), [`-reduce`](#-reduce-fn-list)

```el
(-reduce-r-from '- 10 '(1 2 3)) ;; => -8
(-reduce-r-from (lambda (item memo) (concat "(" (int-to-string item) " - " memo ")")) "10" '(1 2 3)) ;; => "(1 - (2 - (3 - 10)))"
(--reduce-r-from (concat it " " acc) "END" '("a" "b" "c")) ;; => "a b c END"
```

#### -reduce `(fn list)`

Return the result of applying `fn` to the first 2 items in `list`,
then applying `fn` to that result and the 3rd item, etc. If `list`
contains no items, `fn` must accept no arguments as well, and
reduce return the result of calling `fn` with no arguments. If
`list` has only 1 item, it is returned and `fn` is not called.

In the anaphoric form `--reduce`, the accumulated value is
exposed as `acc`.

See also: [`-reduce-from`](#-reduce-from-fn-initial-value-list), [`-reduce-r`](#-reduce-r-fn-list)

```el
(-reduce '- '(1 2 3 4)) ;; => -8
(-reduce (lambda (memo item) (format "%s-%s" memo item)) '(1 2 3)) ;; => "1-2-3"
(--reduce (format "%s-%s" acc it) '(1 2 3)) ;; => "1-2-3"
```

#### -reduce-r `(fn list)`

Replace conses with `fn` and evaluate the resulting expression.
The final nil is ignored. If `list` contains no items, `fn` must
accept no arguments as well, and reduce return the result of
calling `fn` with no arguments. If `list` has only 1 item, it is
returned and `fn` is not called.

The first argument of `fn` is the new item, the second is the
accumulated value.

Note: this function works the same as [`-reduce`](#-reduce-fn-list) but the operation
associates from right instead of from left.

See also: [`-reduce-r-from`](#-reduce-r-from-fn-initial-value-list), [`-reduce`](#-reduce-fn-list)

```el
(-reduce-r '- '(1 2 3 4)) ;; => -2
(-reduce-r (lambda (item memo) (format "%s-%s" memo item)) '(1 2 3)) ;; => "3-2-1"
(--reduce-r (format "%s-%s" acc it) '(1 2 3)) ;; => "3-2-1"
```

#### -count `(pred list)`

Counts the number of items in `list` where (`pred` item) is non-nil.

```el
(-count 'even? '(1 2 3 4 5)) ;; => 2
(--count (< it 4) '(1 2 3 4)) ;; => 3
```

#### -sum `(list)`

Return the sum of `list`.

```el
(-sum '()) ;; => 0
(-sum '(1)) ;; => 1
(-sum '(1 2 3 4)) ;; => 10
```

#### -product `(list)`

Return the product of `list`.

```el
(-product '()) ;; => 1
(-product '(1)) ;; => 1
(-product '(1 2 3 4)) ;; => 24
```

#### -min `(list)`

Return the smallest value from `list` of numbers or markers.

```el
(-min '(0)) ;; => 0
(-min '(3 2 1)) ;; => 1
(-min '(1 2 3)) ;; => 1
```

#### -min-by `(comparator list)`

Take a comparison function `comparator` and a `list` and return
the least element of the list by the comparison function.

See also combinator [`-on`](#-on-operator-transformer) which can transform the values before
comparing them.

```el
(-min-by '> '(4 3 6 1)) ;; => 1
(--min-by (> (car it) (car other)) '((1 2 3) (2) (3 2))) ;; => '(1 2 3)
(--min-by (> (length it) (length other)) '((1 2 3) (2) (3 2))) ;; => '(2)
```

#### -max `(list)`

Return the largest value from `list` of numbers or markers.

```el
(-max '(0)) ;; => 0
(-max '(3 2 1)) ;; => 3
(-max '(1 2 3)) ;; => 3
```

#### -max-by `(comparator list)`

Take a comparison function `comparator` and a `list` and return
the greatest element of the list by the comparison function.

See also combinator [`-on`](#-on-operator-transformer) which can transform the values before
comparing them.

```el
(-max-by '> '(4 3 6 1)) ;; => 6
(--max-by (> (car it) (car other)) '((1 2 3) (2) (3 2))) ;; => '(3 2)
(--max-by (> (length it) (length other)) '((1 2 3) (2) (3 2))) ;; => '(1 2 3)
```


## Unfolding


Operations dual to reductions, building lists from seed value rather than consuming a list to produce a single value.

#### -iterate `(fun init n)`

Return a list of iterated applications of `fun` to `init`.

This means a list of form:

    (init (fun init) (fun (fun init)) ...)

`n` is the length of the returned list.

```el
(-iterate '1+ 1 10) ;; => '(1 2 3 4 5 6 7 8 9 10)
(-iterate (lambda (x) (+ x x)) 2 5) ;; => '(2 4 8 16 32)
(--iterate (* it it) 2 5) ;; => '(2 4 16 256 65536)
```

#### -unfold `(fun seed)`

Build a list from `seed` using `fun`.

This is "dual" operation to [`-reduce-r`](#-reduce-r-fn-list): while -reduce-r
consumes a list to produce a single value, [`-unfold`](#-unfold-fun-seed) takes a
seed value and builds a (potentially infinite!) list.

`fun` should return `nil` to stop the generating process, or a
cons (`a` . `b`), where `a` will be prepended to the result and `b` is
the new seed.

```el
(-unfold (lambda (x) (unless (= x 0) (cons x (1- x)))) 10) ;; => '(10 9 8 7 6 5 4 3 2 1)
(--unfold (when it (cons it (cdr it))) '(1 2 3 4)) ;; => '((1 2 3 4) (2 3 4) (3 4) (4))
(--unfold (when it (cons it (butlast it))) '(1 2 3 4)) ;; => '((1 2 3 4) (1 2 3) (1 2) (1))
```


## Predicates

#### -any? `(pred list)`

Return t if (`pred` x) is non-nil for any x in `list`, else nil.

Alias: `-any-p`, `-some?`, `-some-p`

```el
(-any? 'even? '(1 2 3)) ;; => t
(-any? 'even? '(1 3 5)) ;; => nil
(--any? (= 0 (% it 2)) '(1 2 3)) ;; => t
```

#### -all? `(pred list)`

Return t if (`pred` x) is non-nil for all x in `list`, else nil.

Alias: `-all-p`, `-every?`, `-every-p`

```el
(-all? 'even? '(1 2 3)) ;; => nil
(-all? 'even? '(2 4 6)) ;; => t
(--all? (= 0 (% it 2)) '(2 4 6)) ;; => t
```

#### -none? `(pred list)`

Return t if (`pred` x) is nil for all x in `list`, else nil.

Alias: `-none-p`

```el
(-none? 'even? '(1 2 3)) ;; => nil
(-none? 'even? '(1 3 5)) ;; => t
(--none? (= 0 (% it 2)) '(1 2 3)) ;; => nil
```

#### -only-some? `(pred list)`

Return `t` if at least one item of `list` matches `pred` and at least one item of `list` does not match `pred`.
Return `nil` both if all items match the predicate or if none of the items match the predicate.

Alias: `-only-some-p`

```el
(-only-some? 'even? '(1 2 3)) ;; => t
(-only-some? 'even? '(1 3 5)) ;; => nil
(-only-some? 'even? '(2 4 6)) ;; => nil
```

#### -contains? `(list element)`

Return non-nil if `list` contains `element`.

The test for equality is done with `equal`, or with `-compare-fn`
if that's non-nil.

Alias: `-contains-p`

```el
(-contains? '(1 2 3) 1) ;; => t
(-contains? '(1 2 3) 2) ;; => t
(-contains? '(1 2 3) 4) ;; => nil
```

#### -same-items? `(list list2)`

Return true if `list` and `list2` has the same items.

The order of the elements in the lists does not matter.

Alias: `-same-items-p`

```el
(-same-items? '(1 2 3) '(1 2 3)) ;; => t
(-same-items? '(1 2 3) '(3 2 1)) ;; => t
(-same-items? '(1 2 3) '(1 2 3 4)) ;; => nil
```

#### -is-prefix? `(prefix list)`

Return non-nil if `prefix` is prefix of `list`.

Alias: `-is-prefix-p`

```el
(-is-prefix? '(1 2 3) '(1 2 3 4 5)) ;; => t
(-is-prefix? '(1 2 3 4 5) '(1 2 3)) ;; => nil
(-is-prefix? '(1 3) '(1 2 3 4 5)) ;; => nil
```

#### -is-suffix? `(suffix list)`

Return non-nil if `suffix` is suffix of `list`.

Alias: `-is-suffix-p`

```el
(-is-suffix? '(3 4 5) '(1 2 3 4 5)) ;; => t
(-is-suffix? '(1 2 3 4 5) '(3 4 5)) ;; => nil
(-is-suffix? '(3 5) '(1 2 3 4 5)) ;; => nil
```

#### -is-infix? `(infix list)`

Return non-nil if `infix` is infix of `list`.

This operation runs in `o`(n^2) time

Alias: `-is-infix-p`

```el
(-is-infix? '(1 2 3) '(1 2 3 4 5)) ;; => t
(-is-infix? '(2 3 4) '(1 2 3 4 5)) ;; => t
(-is-infix? '(3 4 5) '(1 2 3 4 5)) ;; => t
```


## Partitioning


Functions partitioning the input list into a list of lists.

#### -split-at `(n list)`

Return a list of ((-take `n` `list`) (-drop `n` `list`)), in no more than one pass through the list.

```el
(-split-at 3 '(1 2 3 4 5)) ;; => '((1 2 3) (4 5))
(-split-at 17 '(1 2 3 4 5)) ;; => '((1 2 3 4 5) nil)
```

#### -split-with `(pred list)`

Return a list of ((-take-while `pred` `list`) (-drop-while `pred` `list`)), in no more than one pass through the list.

```el
(-split-with 'even? '(1 2 3 4)) ;; => '(nil (1 2 3 4))
(-split-with 'even? '(2 4 5 6)) ;; => '((2 4) (5 6))
(--split-with (< it 4) '(1 2 3 4 3 2 1)) ;; => '((1 2 3) (4 3 2 1))
```

#### -split-on `(item list)`

Split the `list` each time `item` is found.

Unlike [`-partition-by`](#-partition-by-fn-list), the `item` is discarded from the results.
Empty lists are also removed from the result.

Comparison is done by `equal`.

See also [`-split-when`](#-split-when-fn-list)

```el
(-split-on '| '(Nil | Leaf a | Node [Tree a])) ;; => '((Nil) (Leaf a) (Node [Tree a]))
(-split-on ':endgroup '("a" "b" :endgroup "c" :endgroup "d" "e")) ;; => '(("a" "b") ("c") ("d" "e"))
(-split-on ':endgroup '("a" "b" :endgroup :endgroup "d" "e")) ;; => '(("a" "b") ("d" "e"))
```

#### -split-when `(fn list)`

Split the `list` on each element where `fn` returns non-nil.

Unlike [`-partition-by`](#-partition-by-fn-list), the "matched" element is discarded from
the results.  Empty lists are also removed from the result.

This function can be thought of as a generalization of
`split-string`.

```el
(-split-when 'even? '(1 2 3 4 5 6)) ;; => '((1) (3) (5))
(-split-when 'even? '(1 2 3 4 6 8 9)) ;; => '((1) (3) (9))
(--split-when (memq it '(&optional &rest)) '(a b &optional c d &rest args)) ;; => '((a b) (c d) (args))
```

#### -separate `(pred list)`

Return a list of ((-filter `pred` `list`) (-remove `pred` `list`)), in one pass through the list.

```el
(-separate (lambda (num) (= 0 (% num 2))) '(1 2 3 4 5 6 7)) ;; => '((2 4 6) (1 3 5 7))
(--separate (< it 5) '(3 7 5 9 3 2 1 4 6)) ;; => '((3 3 2 1 4) (7 5 9 6))
(-separate 'cdr '((1 2) (1) (1 2 3) (4))) ;; => '(((1 2) (1 2 3)) ((1) (4)))
```

#### -partition `(n list)`

Return a new list with the items in `list` grouped into `n-`sized sublists.
If there are not enough items to make the last group `n-`sized,
those items are discarded.

```el
(-partition 2 '(1 2 3 4 5 6)) ;; => '((1 2) (3 4) (5 6))
(-partition 2 '(1 2 3 4 5 6 7)) ;; => '((1 2) (3 4) (5 6))
(-partition 3 '(1 2 3 4 5 6 7)) ;; => '((1 2 3) (4 5 6))
```

#### -partition-all `(n list)`

Return a new list with the items in `list` grouped into `n-`sized sublists.
The last group may contain less than `n` items.

```el
(-partition-all 2 '(1 2 3 4 5 6)) ;; => '((1 2) (3 4) (5 6))
(-partition-all 2 '(1 2 3 4 5 6 7)) ;; => '((1 2) (3 4) (5 6) (7))
(-partition-all 3 '(1 2 3 4 5 6 7)) ;; => '((1 2 3) (4 5 6) (7))
```

#### -partition-in-steps `(n step list)`

Return a new list with the items in `list` grouped into `n-`sized sublists at offsets `step` apart.
If there are not enough items to make the last group `n-`sized,
those items are discarded.

```el
(-partition-in-steps 2 1 '(1 2 3 4)) ;; => '((1 2) (2 3) (3 4))
(-partition-in-steps 3 2 '(1 2 3 4)) ;; => '((1 2 3))
(-partition-in-steps 3 2 '(1 2 3 4 5)) ;; => '((1 2 3) (3 4 5))
```

#### -partition-all-in-steps `(n step list)`

Return a new list with the items in `list` grouped into `n-`sized sublists at offsets `step` apart.
The last groups may contain less than `n` items.

```el
(-partition-all-in-steps 2 1 '(1 2 3 4)) ;; => '((1 2) (2 3) (3 4) (4))
(-partition-all-in-steps 3 2 '(1 2 3 4)) ;; => '((1 2 3) (3 4))
(-partition-all-in-steps 3 2 '(1 2 3 4 5)) ;; => '((1 2 3) (3 4 5) (5))
```

#### -partition-by `(fn list)`

Apply `fn` to each item in `list`, splitting it each time `fn` returns a new value.

```el
(-partition-by 'even? '()) ;; => '()
(-partition-by 'even? '(1 1 2 2 2 3 4 6 8)) ;; => '((1 1) (2 2 2) (3) (4 6 8))
(--partition-by (< it 3) '(1 2 3 4 3 2 1)) ;; => '((1 2) (3 4 3) (2 1))
```

#### -partition-by-header `(fn list)`

Apply `fn` to the first item in `list`. That is the header
value. Apply `fn` to each item in `list`, splitting it each time `fn`
returns the header value, but only after seeing at least one
other value (the body).

```el
(--partition-by-header (= it 1) '(1 2 3 1 2 1 2 3 4)) ;; => '((1 2 3) (1 2) (1 2 3 4))
(--partition-by-header (> it 0) '(1 2 0 1 0 1 2 3 0)) ;; => '((1 2 0) (1 0) (1 2 3 0))
(-partition-by-header 'even? '(2 1 1 1 4 1 3 5 6 6 1)) ;; => '((2 1 1 1) (4 1 3 5) (6 6 1))
```

#### -group-by `(fn list)`

Separate `list` into an alist whose keys are `fn` applied to the
elements of `list`.  Keys are compared by `equal`.

```el
(-group-by 'even? '()) ;; => '()
(-group-by 'even? '(1 1 2 2 2 3 4 6 8)) ;; => '((nil 1 1 3) (t 2 2 2 4 6 8))
(--group-by (car (split-string it "/")) '("a/b" "c/d" "a/e")) ;; => '(("a" "a/b" "a/e") ("c" "c/d"))
```


## Indexing


Return indices of elements based on predicates, sort elements by indices etc.

#### -elem-index `(elem list)`

Return the index of the first element in the given `list` which
is equal to the query element `elem`, or nil if there is no
such element.

```el
(-elem-index 2 '(6 7 8 2 3 4)) ;; => 3
(-elem-index "bar" '("foo" "bar" "baz")) ;; => 1
(-elem-index '(1 2) '((3) (5 6) (1 2) nil)) ;; => 2
```

#### -elem-indices `(elem list)`

Return the indices of all elements in `list` equal to the query
element `elem`, in ascending order.

```el
(-elem-indices 2 '(6 7 8 2 3 4 2 1)) ;; => '(3 6)
(-elem-indices "bar" '("foo" "bar" "baz")) ;; => '(1)
(-elem-indices '(1 2) '((3) (1 2) (5 6) (1 2) nil)) ;; => '(1 3)
```

#### -find-index `(pred list)`

Take a predicate `pred` and a `list` and return the index of the
first element in the list satisfying the predicate, or nil if
there is no such element.

```el
(-find-index 'even? '(2 4 1 6 3 3 5 8)) ;; => 0
(--find-index (< 5 it) '(2 4 1 6 3 3 5 8)) ;; => 3
(-find-index (-partial 'string-lessp "baz") '("bar" "foo" "baz")) ;; => 1
```

#### -find-last-index `(pred list)`

Take a predicate `pred` and a `list` and return the index of the
last element in the list satisfying the predicate, or nil if
there is no such element.

```el
(-find-last-index 'even? '(2 4 1 6 3 3 5 8)) ;; => 7
(--find-last-index (< 5 it) '(2 7 1 6 3 8 5 2)) ;; => 5
(-find-last-index (-partial 'string-lessp "baz") '("q" "foo" "baz")) ;; => 1
```

#### -find-indices `(pred list)`

Return the indices of all elements in `list` satisfying the
predicate `pred`, in ascending order.

```el
(-find-indices 'even? '(2 4 1 6 3 3 5 8)) ;; => '(0 1 3 7)
(--find-indices (< 5 it) '(2 4 1 6 3 3 5 8)) ;; => '(3 7)
(-find-indices (-partial 'string-lessp "baz") '("bar" "foo" "baz")) ;; => '(1)
```

#### -grade-up `(comparator list)`

Grade elements of `list` using `comparator` relation, yielding a
permutation vector such that applying this permutation to `list`
sorts it in ascending order.

```el
(-grade-up '< '(3 1 4 2 1 3 3)) ;; => '(1 4 3 0 5 6 2)
(let ((l '(3 1 4 2 1 3 3))) (-select-by-indices (-grade-up '< l) l)) ;; => '(1 1 2 3 3 3 4)
```

#### -grade-down `(comparator list)`

Grade elements of `list` using `comparator` relation, yielding a
permutation vector such that applying this permutation to `list`
sorts it in descending order.

```el
(-grade-down '< '(3 1 4 2 1 3 3)) ;; => '(2 0 5 6 3 1 4)
(let ((l '(3 1 4 2 1 3 3))) (-select-by-indices (-grade-down '< l) l)) ;; => '(4 3 3 3 2 1 1)
```


## Set operations


Operations pretending lists are sets.

#### -union `(list list2)`

Return a new list containing the elements of `list1` and elements of `list2` that are not in `list1`.
The test for equality is done with `equal`,
or with `-compare-fn` if that's non-nil.

```el
(-union '(1 2 3) '(3 4 5)) ;; => '(1 2 3 4 5)
(-union '(1 2 3 4) '()) ;; => '(1 2 3 4)
(-union '(1 1 2 2) '(3 2 1)) ;; => '(1 1 2 2 3)
```

#### -difference `(list list2)`

Return a new list with only the members of `list` that are not in `list2`.
The test for equality is done with `equal`,
or with `-compare-fn` if that's non-nil.

```el
(-difference '() '()) ;; => '()
(-difference '(1 2 3) '(4 5 6)) ;; => '(1 2 3)
(-difference '(1 2 3 4) '(3 4 5 6)) ;; => '(1 2)
```

#### -intersection `(list list2)`

Return a new list containing only the elements that are members of both `list` and `list2`.
The test for equality is done with `equal`,
or with `-compare-fn` if that's non-nil.

```el
(-intersection '() '()) ;; => '()
(-intersection '(1 2 3) '(4 5 6)) ;; => '()
(-intersection '(1 2 3 4) '(3 4 5 6)) ;; => '(3 4)
```

#### -distinct `(list)`

Return a new list with all duplicates removed.
The test for equality is done with `equal`,
or with `-compare-fn` if that's non-nil.

Alias: `-uniq`

```el
(-distinct '()) ;; => '()
(-distinct '(1 2 2 4)) ;; => '(1 2 4)
```


## Other list operations


Other list functions not fit to be classified elsewhere.

#### -rotate `(n list)`

Rotate `list` `n` places to the right.  With `n` negative, rotate to the left.
The time complexity is `o`(n).

```el
(-rotate 3 '(1 2 3 4 5 6 7)) ;; => '(5 6 7 1 2 3 4)
(-rotate -3 '(1 2 3 4 5 6 7)) ;; => '(4 5 6 7 1 2 3)
```

#### -repeat `(n x)`

Return a list with `x` repeated `n` times.
Return nil if `n` is less than 1.

```el
(-repeat 3 :a) ;; => '(:a :a :a)
(-repeat 1 :a) ;; => '(:a)
(-repeat 0 :a) ;; => nil
```

#### -cons* `(&rest args)`

Make a new list from the elements of `args`.

The last 2 members of `args` are used as the final cons of the
result so if the final member of `args` is not a list the result is
a dotted list.

```el
(-cons* 1 2) ;; => '(1 . 2)
(-cons* 1 2 3) ;; => '(1 2 . 3)
(-cons* 1) ;; => 1
```

#### -snoc `(list elem &rest elements)`

Append `elem` to the end of the list.

This is like `cons`, but operates on the end of list.

If `elements` is non nil, append these to the list as well.

```el
(-snoc '(1 2 3) 4) ;; => '(1 2 3 4)
(-snoc '(1 2 3) 4 5 6) ;; => '(1 2 3 4 5 6)
(-snoc '(1 2 3) '(4 5 6)) ;; => '(1 2 3 (4 5 6))
```

#### -interpose `(sep list)`

Return a new list of all elements in `list` separated by `sep`.

```el
(-interpose "-" '()) ;; => '()
(-interpose "-" '("a")) ;; => '("a")
(-interpose "-" '("a" "b" "c")) ;; => '("a" "-" "b" "-" "c")
```

#### -interleave `(&rest lists)`

Return a new list of the first item in each list, then the second etc.

```el
(-interleave '(1 2) '("a" "b")) ;; => '(1 "a" 2 "b")
(-interleave '(1 2) '("a" "b") '("A" "B")) ;; => '(1 "a" "A" 2 "b" "B")
(-interleave '(1 2 3) '("a" "b")) ;; => '(1 "a" 2 "b")
```

#### -zip-with `(fn list1 list2)`

Zip the two lists `list1` and `list2` using a function `fn`.  This
function is applied pairwise taking as first argument element of
`list1` and as second argument element of `list2` at corresponding
position.

The anaphoric form `--zip-with` binds the elements from `list1` as `it`,
and the elements from `list2` as `other`.

```el
(-zip-with '+ '(1 2 3) '(4 5 6)) ;; => '(5 7 9)
(-zip-with 'cons '(1 2 3) '(4 5 6)) ;; => '((1 . 4) (2 . 5) (3 . 6))
(--zip-with (concat it " and " other) '("Batman" "Jekyll") '("Robin" "Hyde")) ;; => '("Batman and Robin" "Jekyll and Hyde")
```

#### -zip `(&rest lists)`

Zip `lists` together.  Group the head of each list, followed by the
second elements of each list, and so on. The lengths of the returned
groupings are equal to the length of the shortest input list.

If two lists are provided as arguments, return the groupings as a list
of cons cells. Otherwise, return the groupings as a list of lists. 

```el
(-zip '(1 2 3) '(4 5 6)) ;; => '((1 . 4) (2 . 5) (3 . 6))
(-zip '(1 2 3) '(4 5 6 7)) ;; => '((1 . 4) (2 . 5) (3 . 6))
(-zip '(1 2 3 4) '(4 5 6)) ;; => '((1 . 4) (2 . 5) (3 . 6))
```

#### -zip-fill `(fill-value &rest lists)`

Zip `lists`, with `fill-value` padded onto the shorter lists. The
lengths of the returned groupings are equal to the length of the
longest input list.

```el
(-zip-fill 0 '(1 2 3 4 5) '(6 7 8 9)) ;; => '((1 . 6) (2 . 7) (3 . 8) (4 . 9) (5 . 0))
```

#### -cycle `(list)`

Return an infinite copy of `list` that will cycle through the
elements and repeat from the beginning.

```el
(-take 5 (-cycle '(1 2 3))) ;; => '(1 2 3 1 2)
(-take 7 (-cycle '(1 "and" 3))) ;; => '(1 "and" 3 1 "and" 3 1)
(-zip (-cycle '(1 2 3)) '(1 2)) ;; => '((1 . 1) (2 . 2))
```

#### -pad `(fill-value &rest lists)`

Appends `fill-value` to the end of each list in `lists` such that they
will all have the same length.

```el
(-pad 0 '()) ;; => '(nil)
(-pad 0 '(1)) ;; => '((1))
(-pad 0 '(1 2 3) '(4 5)) ;; => '((1 2 3) (4 5 0))
```

#### -table `(fn &rest lists)`

Compute outer product of `lists` using function `fn`.

The function `fn` should have the same arity as the number of
supplied lists.

The outer product is computed by applying fn to all possible
combinations created by taking one element from each list in
order.  The dimension of the result is (length lists).

See also: [`-table-flat`](#-table-flat-fn-rest-lists)

```el
(-table '* '(1 2 3) '(1 2 3)) ;; => '((1 2 3) (2 4 6) (3 6 9))
(-table (lambda (a b) (-sum (-zip-with '* a b))) '((1 2) (3 4)) '((1 3) (2 4))) ;; => '((7 15) (10 22))
(apply '-table 'list (-repeat 3 '(1 2))) ;; => '((((1 1 1) (2 1 1)) ((1 2 1) (2 2 1))) (((1 1 2) (2 1 2)) ((1 2 2) (2 2 2))))
```

#### -table-flat `(fn &rest lists)`

Compute flat outer product of `lists` using function `fn`.

The function `fn` should have the same arity as the number of
supplied lists.

The outer product is computed by applying fn to all possible
combinations created by taking one element from each list in
order.  The results are flattened, ignoring the tensor structure
of the result.  This is equivalent to calling:

    (-flatten-n (1- (length lists)) (-table fn lists))

but the implementation here is much more efficient.

See also: [`-flatten-n`](#-flatten-n-num-list), [`-table`](#-table-fn-rest-lists)

```el
(-table-flat 'list '(1 2 3) '(a b c)) ;; => '((1 a) (2 a) (3 a) (1 b) (2 b) (3 b) (1 c) (2 c) (3 c))
(-table-flat '* '(1 2 3) '(1 2 3)) ;; => '(1 2 3 2 4 6 3 6 9)
(apply '-table-flat 'list (-repeat 3 '(1 2))) ;; => '((1 1 1) (2 1 1) (1 2 1) (2 2 1) (1 1 2) (2 1 2) (1 2 2) (2 2 2))
```

#### -first `(pred list)`

Return the first x in `list` where (`pred` x) is non-nil, else nil.

To get the first item in the list no questions asked, use `car`.

Alias: `-find`

```el
(-first 'even? '(1 2 3)) ;; => 2
(-first 'even? '(1 3 5)) ;; => nil
(--first (> it 2) '(1 2 3)) ;; => 3
```

#### -some `(pred list)`

Return (`pred` x) for the first `list` item where (`pred` x) is non-nil, else nil.

Alias: `-any`

```el
(-some 'even? '(1 2 3)) ;; => t
(--some (member 'foo it) '((foo bar) (baz))) ;; => '(foo bar)
(--some (plist-get it :bar) '((:foo 1 :bar 2) (:baz 3))) ;; => 2
```

#### -last `(pred list)`

Return the last x in `list` where (`pred` x) is non-nil, else nil.

```el
(-last 'even? '(1 2 3 4 5 6 3 3 3)) ;; => 6
(-last 'even? '(1 3 7 5 9)) ;; => nil
(--last (> (length it) 3) '("a" "looong" "word" "and" "short" "one")) ;; => "short"
```

#### -first-item `(list)`

Return the first item of `list`, or nil on an empty list.

```el
(-first-item '(1 2 3)) ;; => 1
(-first-item nil) ;; => nil
```

#### -last-item `(list)`

Return the last item of `list`, or nil on an empty list.

```el
(-last-item '(1 2 3)) ;; => 3
(-last-item nil) ;; => nil
```

#### -butlast `(list)`

Return a list of all items in list except for the last.

```el
(-butlast '(1 2 3)) ;; => '(1 2)
(-butlast '(1 2)) ;; => '(1)
(-butlast '(1)) ;; => nil
```

#### -sort `(comparator list)`

Sort `list`, stably, comparing elements using `comparator`.
Return the sorted list.  `list` is `not` modified by side effects.
`comparator` is called with two elements of `list`, and should return non-nil
if the first element should sort before the second.

```el
(-sort '< '(3 1 2)) ;; => '(1 2 3)
(-sort '> '(3 1 2)) ;; => '(3 2 1)
(--sort (< it other) '(3 1 2)) ;; => '(1 2 3)
```

#### -list `(&rest args)`

Return a list with `args`.

If first item of `args` is already a list, simply return `args`.  If
not, return a list with `args` as elements.

```el
(-list 1) ;; => '(1)
(-list 1 2 3) ;; => '(1 2 3)
(-list '(1 2 3)) ;; => '(1 2 3)
```

#### -fix `(fn list)`

Compute the (least) fixpoint of `fn` with initial input `list`.

`fn` is called at least once, results are compared with `equal`.

```el
(-fix (lambda (l) (-non-nil (--mapcat (-split-at (/ (length it) 2) it) l))) '((1 2 3 4 5 6))) ;; => '((1) (2) (3) (4) (5) (6))
(let ((data '(("starwars" "scifi") ("jedi" "starwars" "warrior")))) (--fix (-uniq (--mapcat (cons it (cdr (assoc it data))) it)) '("jedi" "book"))) ;; => '("jedi" "starwars" "warrior" "scifi" "book")
```


## Tree operations


Functions pretending lists are trees.

#### -tree-seq `(branch children tree)`

Return a sequence of the nodes in `tree`, in depth-first search order.

`branch` is a predicate of one argument that returns non-nil if the
passed argument is a branch, that is, a node that can have children.

`children` is a function of one argument that returns the children
of the passed branch node.

Non-branch nodes are simply copied.

```el
(-tree-seq 'listp 'identity '(1 (2 3) 4 (5 (6 7)))) ;; => '((1 (2 3) 4 (5 (6 7))) 1 (2 3) 2 3 4 (5 (6 7)) 5 (6 7) 6 7)
(-tree-seq 'listp 'reverse '(1 (2 3) 4 (5 (6 7)))) ;; => '((1 (2 3) 4 (5 (6 7))) (5 (6 7)) (6 7) 7 6 5 4 (2 3) 3 2 1)
(--tree-seq (vectorp it) (append it nil) [1 [2 3] 4 [5 [6 7]]]) ;; => '([1 [2 3] 4 [5 [6 7]]] 1 [2 3] 2 3 4 [5 [6 7]] 5 [6 7] 6 7)
```

#### -tree-map `(fn tree)`

Apply `fn` to each element of `tree` while preserving the tree structure.

```el
(-tree-map '1+ '(1 (2 3) (4 (5 6) 7))) ;; => '(2 (3 4) (5 (6 7) 8))
(-tree-map '(lambda (x) (cons x (expt 2 x))) '(1 (2 3) 4)) ;; => '((1 . 2) ((2 . 4) (3 . 8)) (4 . 16))
(--tree-map (length it) '("<body>" ("<p>" "text" "</p>") "</body>")) ;; => '(6 (3 4 4) 7)
```

#### -tree-map-nodes `(pred fun tree)`

Call `fun` on each node of `tree` that satisfies `pred`.

If `pred` returns nil, continue descending down this node.  If `pred`
returns non-nil, apply `fun` to this node and do not descend
further.

```el
(-tree-map-nodes 'vectorp (lambda (x) (-sum (append x nil))) '(1 [2 3] 4 (5 [6 7] 8))) ;; => '(1 5 4 (5 13 8))
(-tree-map-nodes 'keywordp (lambda (x) (symbol-name x)) '(1 :foo 4 ((5 6 :bar) :baz 8))) ;; => '(1 ":foo" 4 ((5 6 ":bar") ":baz" 8))
(--tree-map-nodes (eq (car-safe it) 'add-mode) (-concat it (list :mode 'emacs-lisp-mode)) '(with-mode emacs-lisp-mode (foo bar) (add-mode a b) (baz (add-mode c d)))) ;; => '(with-mode emacs-lisp-mode (foo bar) (add-mode a b :mode emacs-lisp-mode) (baz (add-mode c d :mode emacs-lisp-mode)))
```

#### -tree-reduce `(fn tree)`

Use `fn` to reduce elements of list `tree`.
If elements of `tree` are lists themselves, apply the reduction recursively.

`fn` is first applied to first element of the list and second
element, then on this result and third element from the list etc.

See [`-reduce-r`](#-reduce-r-fn-list) for how exactly are lists of zero or one element handled.

```el
(-tree-reduce '+ '(1 (2 3) (4 5))) ;; => 15
(-tree-reduce 'concat '("strings" (" on" " various") ((" levels")))) ;; => "strings on various levels"
(--tree-reduce (cond ((stringp it) (concat it " " acc)) (t (let ((sn (symbol-name it))) (concat "<" sn ">" acc "</" sn ">")))) '(body (p "some words") (div "more" (b "bold") "words"))) ;; => "<body><p>some words</p> <div>more <b>bold</b> words</div></body>"
```

#### -tree-reduce-from `(fn init-value tree)`

Use `fn` to reduce elements of list `tree`.
If elements of `tree` are lists themselves, apply the reduction recursively.

`fn` is first applied to `init-value` and first element of the list,
then on this result and second element from the list etc.

The initial value is ignored on cons pairs as they always contain
two elements.

```el
(-tree-reduce-from '+ 1 '(1 (1 1) ((1)))) ;; => 8
(--tree-reduce-from (-concat acc (list it)) nil '(1 (2 3 (4 5)) (6 7))) ;; => '((7 6) ((5 4) 3 2) 1)
```

#### -tree-mapreduce `(fn folder tree)`

Apply `fn` to each element of `tree`, and make a list of the results.
If elements of `tree` are lists themselves, apply `fn` recursively to
elements of these nested lists.

Then reduce the resulting lists using `folder` and initial value
`init-value`. See [`-reduce-r-from`](#-reduce-r-from-fn-initial-value-list).

This is the same as calling [`-tree-reduce`](#-tree-reduce-fn-tree) after [`-tree-map`](#-tree-map-fn-tree)
but is twice as fast as it only traverse the structure once.

```el
(-tree-mapreduce 'list 'append '(1 (2 (3 4) (5 6)) (7 (8 9)))) ;; => '(1 2 3 4 5 6 7 8 9)
(--tree-mapreduce 1 (+ it acc) '(1 (2 (4 9) (2 1)) (7 (4 3)))) ;; => 9
(--tree-mapreduce 0 (max acc (1+ it)) '(1 (2 (4 9) (2 1)) (7 (4 3)))) ;; => 3
```

#### -tree-mapreduce-from `(fn folder init-value tree)`

Apply `fn` to each element of `tree`, and make a list of the results.
If elements of `tree` are lists themselves, apply `fn` recursively to
elements of these nested lists.

Then reduce the resulting lists using `folder` and initial value
`init-value`. See [`-reduce-r-from`](#-reduce-r-from-fn-initial-value-list).

This is the same as calling [`-tree-reduce-from`](#-tree-reduce-from-fn-init-value-tree) after [`-tree-map`](#-tree-map-fn-tree)
but is twice as fast as it only traverse the structure once.

```el
(-tree-mapreduce-from 'identity '* 1 '(1 (2 (3 4) (5 6)) (7 (8 9)))) ;; => 362880
(--tree-mapreduce-from (+ it it) (cons it acc) nil '(1 (2 (4 9) (2 1)) (7 (4 3)))) ;; => '(2 (4 (8 18) (4 2)) (14 (8 6)))
(concat "{" (--tree-mapreduce-from (cond ((-cons-pair? it) (concat (symbol-name (car it)) " -> " (symbol-name (cdr it)))) (t (concat (symbol-name it) " : {"))) (concat it (unless (or (equal acc "}") (equal (substring it (1- (length it))) "{")) ", ") acc) "}" '((elips-mode (foo (bar . booze)) (baz . qux)) (c-mode (foo . bla) (bum . bam))))) ;; => "{elips-mode : {foo : {bar -> booze}, baz -> qux}, c-mode : {foo -> bla, bum -> bam}}"
```

#### -clone `(list)`

Create a deep copy of `list`.
The new list has the same elements and structure but all cons are
replaced with new ones.  This is useful when you need to clone a
structure such as plist or alist.

```el
(let* ((a '(1 2 3)) (b (-clone a))) (nreverse a) b) ;; => '(1 2 3)
```


## Threading macros

#### -> `(x &optional form &rest more)`

Thread the expr through the forms. Insert `x` as the second item
in the first form, making a list of it if it is not a list
already. If there are more forms, insert the first form as the
second item in second form, etc.

```el
(-> '(2 3 5)) ;; => '(2 3 5)
(-> '(2 3 5) (append '(8 13))) ;; => '(2 3 5 8 13)
(-> '(2 3 5) (append '(8 13)) (-slice 1 -1)) ;; => '(3 5 8)
```

#### ->> `(x &optional form &rest more)`

Thread the expr through the forms. Insert `x` as the last item
in the first form, making a list of it if it is not a list
already. If there are more forms, insert the first form as the
last item in second form, etc.

```el
(->> '(1 2 3) (-map 'square)) ;; => '(1 4 9)
(->> '(1 2 3) (-map 'square) (-remove 'even?)) ;; => '(1 9)
(->> '(1 2 3) (-map 'square) (-reduce '+)) ;; => 14
```

#### --> `(x form &rest more)`

Thread the expr through the forms. Insert `x` at the position
signified by the token `it` in the first form. If there are more
forms, insert the first form at the position signified by `it` in
in second form, etc.

```el
(--> "def" (concat "abc" it "ghi")) ;; => "abcdefghi"
(--> "def" (concat "abc" it "ghi") (upcase it)) ;; => "ABCDEFGHI"
(--> "def" (concat "abc" it "ghi") upcase) ;; => "ABCDEFGHI"
```

#### -some-> `(x &optional form &rest more)`

When expr is non-nil, thread it through the first form (via [`->`](#--x-optional-form-rest-more)),
and when that result is non-nil, through the next form, etc.

```el
(-some-> '(2 3 5)) ;; => '(2 3 5)
(-some-> 5 square) ;; => 25
(-some-> 5 even? square) ;; => nil
```

#### -some->> `(x &optional form &rest more)`

When expr is non-nil, thread it through the first form (via [`->>`](#--x-optional-form-rest-more)),
and when that result is non-nil, through the next form, etc.

```el
(-some->> '(1 2 3) (-map 'square)) ;; => '(1 4 9)
(-some->> '(1 3 5) (-last 'even?) (+ 100)) ;; => nil
(-some->> '(2 4 6) (-last 'even?) (+ 100)) ;; => 106
```

#### -some--> `(x &optional form &rest more)`

When expr in non-nil, thread it through the first form (via [`-->`](#---x-form-rest-more)),
and when that result is non-nil, through the next form, etc.

```el
(-some--> "def" (concat "abc" it "ghi")) ;; => "abcdefghi"
(-some--> nil (concat "abc" it "ghi")) ;; => nil
(-some--> '(1 3 5) (-filter 'even? it) (append it it) (-map 'square it)) ;; => nil
```


## Binding


Convenient versions of `let` and `let*` constructs combined with flow control.

#### -when-let `(var-val &rest body)`

If `val` evaluates to non-nil, bind it to `var` and execute body.
`var-val` should be a (`var` `val`) pair.

Note: binding is done according to [`-let`](#-let-varlist-rest-body).

```el
(-when-let (match-index (string-match "d" "abcd")) (+ match-index 2)) ;; => 5
(-when-let ((&plist :foo foo) (list :foo "foo")) foo) ;; => "foo"
(-when-let ((&plist :foo foo) (list :bar "bar")) foo) ;; => nil
```

#### -when-let* `(vars-vals &rest body)`

If all `vals` evaluate to true, bind them to their corresponding
`vars` and execute body. `vars-vals` should be a list of (`var` `val`)
pairs.

Note: binding is done according to [`-let*`](#-let-varlist-rest-body).  `vals` are evaluated
sequentially, and evaluation stops after the first nil `val` is
encountered.

```el
(-when-let* ((x 5) (y 3) (z (+ y 4))) (+ x y z)) ;; => 15
(-when-let* ((x 5) (y nil) (z 7)) (+ x y z)) ;; => nil
```

#### -if-let `(var-val then &rest else)`

If `val` evaluates to non-nil, bind it to `var` and do `then`,
otherwise do `else`. `var-val` should be a (`var` `val`) pair.

Note: binding is done according to [`-let`](#-let-varlist-rest-body).

```el
(-if-let (match-index (string-match "d" "abc")) (+ match-index 3) 7) ;; => 7
(--if-let (even? 4) it nil) ;; => t
```

#### -if-let* `(vars-vals then &rest else)`

If all `vals` evaluate to true, bind them to their corresponding
`vars` and do `then`, otherwise do `else`. `vars-vals` should be a list
of (`var` `val`) pairs.

Note: binding is done according to [`-let*`](#-let-varlist-rest-body).  `vals` are evaluated
sequentially, and evaluation stops after the first nil `val` is
encountered.

```el
(-if-let* ((x 5) (y 3) (z 7)) (+ x y z) "foo") ;; => 15
(-if-let* ((x 5) (y nil) (z 7)) (+ x y z) "foo") ;; => "foo"
(-if-let* (((_ _ x) '(nil nil 7))) x) ;; => 7
```

#### -let `(varlist &rest body)`

Bind variables according to `varlist` then eval `body`.

`varlist` is a list of lists of the form (`pattern` `source`).  Each
`pattern` is matched against the `source` "structurally".  `source`
is only evaluated once for each `pattern`.  Each `pattern` is matched
recursively, and can therefore contain sub-patterns which are
matched against corresponding sub-expressions of `source`.

All the SOURCEs are evalled before any symbols are
bound (i.e. "in parallel").

If `varlist` only contains one (`pattern` `source`) element, you can
optionally specify it using a vector and discarding the
outer-most parens.  Thus

    (-let ((`pattern` `source`)) ..)

becomes

    (-let [`pattern` `source`] ..).

[`-let`](#-let-varlist-rest-body) uses a convention of not binding places (symbols) starting
with _ whenever it's possible.  You can use this to skip over
entries you don't care about.  However, this is not *always*
possible (as a result of implementation) and these symbols might
get bound to undefined values.

Following is the overview of supported patterns.  Remember that
patterns can be matched recursively, so every a, b, aK in the
following can be a matching construct and not necessarily a
symbol/variable.

Symbol:

    a - bind the `source` to `a`.  This is just like regular `let`.

Conses and lists:

    (a) - bind `car` of cons/list to `a`

    (a . b) - bind car of cons to `a` and `cdr` to `b`

    (a b) - bind car of list to `a` and `cadr` to `b`

    (a1 a2 a3  ...) - bind 0th car of list to `a1`, 1st to `a2`, 2nd to `a3` ...

    (a1 a2 a3 ... aN . rest) - as above, but bind the Nth cdr to `rest`.

Vectors:

    [a] - bind 0th element of a non-list sequence to `a` (works with
          vectors, strings, bit arrays...)

    [a1 a2 a3 ...] - bind 0th element of non-list sequence to `a0`, 1st to
                     `a1`, 2nd to `a2`, ...
                     If the `pattern` is shorter than `source`, the values at
                     places not in `pattern` are ignored.
                     If the `pattern` is longer than `source`, an `error` is
                     thrown.

    [a1 a2 a3 ... &rest rest] - as above, but bind the rest of
                                the sequence to `rest`.  This is
                                conceptually the same as improper list
                                matching (a1 a2 ... aN . rest)

Key/value stores:

    (&plist key0 a0 ... keyN aN) - bind value mapped by keyK in the
                                   `source` plist to aK.  If the
                                   value is not found, aK is nil.

    (&alist key0 a0 ... keyN aN) - bind value mapped by keyK in the
                                   `source` alist to aK.  If the
                                   value is not found, aK is nil.

    (&hash key0 a0 ... keyN aN) - bind value mapped by keyK in the
                                  `source` hash table to aK.  If the
                                  value is not found, aK is nil.

Further, special keyword &keys supports "inline" matching of
plist-like key-value pairs, similarly to &keys keyword of
`cl-defun`.

    (a1 a2 ... aN &keys key1 b1 ... keyN bK)

This binds `n` values from the list to a1 ... aN, then interprets
the cdr as a plist (see key/value matching above).

You can name the source using the syntax `symbol` &as `pattern`.
This syntax works with lists (proper or improper), vectors and
all types of maps.

    (list &as a b c) (list 1 2 3)

binds `a` to 1, `b` to 2, `c` to 3 and `list` to (1 2 3).

Similarly:

    (bounds &as beg . end) (cons 1 2)

binds `beg` to 1, `end` to 2 and `bounds` to (1 . 2).

    (items &as first . rest) (list 1 2 3)

binds `first` to 1, `rest` to (2 3) and `items` to (1 2 3)

    [vect &as _ b c] [1 2 3]

binds `b` to 2, `c` to 3 and `vect` to [1 2 3] (_ avoids binding as usual).

    (plist &as &plist :b b) (list :a 1 :b 2 :c 3)

binds `b` to 2 and `plist` to (:a 1 :b 2 :c 3).  Same for &alist and &hash.

This is especially useful when we want to capture the result of a
computation and destructure at the same time.  Consider the
form (function-returning-complex-structure) returning a list of
two vectors with two items each.  We want to capture this entire
result and pass it to another computation, but at the same time
we want to get the second item from each vector.  We can achieve
it with pattern

    (result &as [_ a] [_ b]) (function-returning-complex-structure)

Note: Clojure programmers may know this feature as the ":as
binding".  The difference is that we put the &as at the front
because we need to support improper list binding.

```el
(-let (([a (b c) d] [1 (2 3) 4])) (list a b c d)) ;; => '(1 2 3 4)
(-let [(a b c . d) (list 1 2 3 4 5 6)] (list a b c d)) ;; => '(1 2 3 (4 5 6))
(-let [(&plist :foo foo :bar bar) (list :baz 3 :foo 1 :qux 4 :bar 2)] (list foo bar)) ;; => '(1 2)
```

#### -let* `(varlist &rest body)`

Bind variables according to `varlist` then eval `body`.

`varlist` is a list of lists of the form (`pattern` `source`).  Each
`pattern` is matched against the `source` structurally.  `source` is
only evaluated once for each `pattern`.

Each `source` can refer to the symbols already bound by this
`varlist`.  This is useful if you want to destructure `source`
recursively but also want to name the intermediate structures.

See [`-let`](#-let-varlist-rest-body) for the list of all possible patterns.

```el
(-let* (((a . b) (cons 1 2)) ((c . d) (cons 3 4))) (list a b c d)) ;; => '(1 2 3 4)
(-let* (((a . b) (cons 1 (cons 2 3))) ((c . d) b)) (list a b c d)) ;; => '(1 (2 . 3) 2 3)
(-let* (((&alist "foo" foo "bar" bar) (list (cons "foo" 1) (cons "bar" (list 'a 'b 'c)))) ((a b c) bar)) (list foo a b c bar)) ;; => '(1 a b c (a b c))
```

#### -lambda `(match-form &rest body)`

Return a lambda which destructures its input as `match-form` and executes `body`.

Note that you have to enclose the `match-form` in a pair of parens,
such that:

    (-lambda (x) body)
    (-lambda (x y ...) body)

has the usual semantics of `lambda`.  Furthermore, these get
translated into normal lambda, so there is no performance
penalty.

See [`-let`](#-let-varlist-rest-body) for the description of destructuring mechanism.

```el
(-map (-lambda ((x y)) (+ x y)) '((1 2) (3 4) (5 6))) ;; => '(3 7 11)
(-map (-lambda ([x y]) (+ x y)) '([1 2] [3 4] [5 6])) ;; => '(3 7 11)
(funcall (-lambda ((_ . a) (_ . b)) (-concat a b)) '(1 2 3) '(4 5 6)) ;; => '(2 3 5 6)
```


## Side-effects


Functions iterating over lists for side-effect only.

#### -each `(list fn)`

Call `fn` with every item in `list`. Return nil, used for side-effects only.

```el
(let (s) (-each '(1 2 3) (lambda (item) (setq s (cons item s))))) ;; => nil
(let (s) (-each '(1 2 3) (lambda (item) (setq s (cons item s)))) s) ;; => '(3 2 1)
(let (s) (--each '(1 2 3) (setq s (cons it s))) s) ;; => '(3 2 1)
```

#### -each-while `(list pred fn)`

Call `fn` with every item in `list` while (`pred` item) is non-nil.
Return nil, used for side-effects only.

```el
(let (s) (-each-while '(2 4 5 6) 'even? (lambda (item) (!cons item s))) s) ;; => '(4 2)
(let (s) (--each-while '(1 2 3 4) (< it 3) (!cons it s)) s) ;; => '(2 1)
```

#### -dotimes `(num fn)`

Repeatedly calls `fn` (presumably for side-effects) passing in integers from 0 through `num-1`.

```el
(let (s) (-dotimes 3 (lambda (n) (!cons n s))) s) ;; => '(2 1 0)
(let (s) (--dotimes 5 (!cons it s)) s) ;; => '(4 3 2 1 0)
```


## Destructive operations

#### !cons `(car cdr)`

Destructive: Set `cdr` to the cons of `car` and `cdr`.

```el
(let (l) (!cons 5 l) l) ;; => '(5)
(let ((l '(3))) (!cons 5 l) l) ;; => '(5 3)
```

#### !cdr `(list)`

Destructive: Set `list` to the cdr of `list`.

```el
(let ((l '(3))) (!cdr l) l) ;; => '()
(let ((l '(3 5))) (!cdr l) l) ;; => '(5)
```


## Function combinators


These combinators require Emacs 24 for its lexical scope. So they are offered in a separate package: `dash-functional`.

#### -partial `(fn &rest args)`

Takes a function `fn` and fewer than the normal arguments to `fn`,
and returns a fn that takes a variable number of additional `args`.
When called, the returned function calls `fn` with `args` first and
then additional args.

```el
(funcall (-partial '- 5) 3) ;; => 2
(funcall (-partial '+ 5 2) 3) ;; => 10
```

#### -rpartial `(fn &rest args)`

Takes a function `fn` and fewer than the normal arguments to `fn`,
and returns a fn that takes a variable number of additional `args`.
When called, the returned function calls `fn` with the additional
args first and then `args`.

```el
(funcall (-rpartial '- 5) 8) ;; => 3
(funcall (-rpartial '- 5 2) 10) ;; => 3
```

#### -juxt `(&rest fns)`

Takes a list of functions and returns a fn that is the
juxtaposition of those fns. The returned fn takes a variable
number of args, and returns a list containing the result of
applying each fn to the args (left-to-right).

```el
(funcall (-juxt '+ '-) 3 5) ;; => '(8 -2)
(-map (-juxt 'identity 'square) '(1 2 3)) ;; => '((1 1) (2 4) (3 9))
```

#### -compose `(&rest fns)`

Takes a list of functions and returns a fn that is the
composition of those fns. The returned fn takes a variable
number of arguments, and returns the result of applying
each fn to the result of applying the previous fn to
the arguments (right-to-left).

```el
(funcall (-compose 'square '+) 2 3) ;; => (square (+ 2 3))
(funcall (-compose 'identity 'square) 3) ;; => (square 3)
(funcall (-compose 'square 'identity) 3) ;; => (square 3)
```

#### -applify `(fn)`

Changes an n-arity function `fn` to a 1-arity function that
expects a list with n items as arguments

```el
(-map (-applify '+) '((1 1 1) (1 2 3) (5 5 5))) ;; => '(3 6 15)
(-map (-applify (lambda (a b c) (\` ((\, a) ((\, b) ((\, c))))))) '((1 1 1) (1 2 3) (5 5 5))) ;; => '((1 (1 (1))) (1 (2 (3))) (5 (5 (5))))
(funcall (-applify '<) '(3 6)) ;; => t
```

#### -on `(operator transformer)`

Return a function of two arguments that first applies
`transformer` to each of them and then applies `operator` on the
results (in the same order).

In types: (b -> b -> c) -> (a -> b) -> a -> a -> c

```el
(-sort (-on '< 'length) '((1 2 3) (1) (1 2))) ;; => '((1) (1 2) (1 2 3))
(-min-by (-on '> 'length) '((1 2 3) (4) (1 2))) ;; => '(4)
(-min-by (-on 'string-lessp 'int-to-string) '(2 100 22)) ;; => 22
```

#### -flip `(func)`

Swap the order of arguments for binary function `func`.

In types: (a -> b -> c) -> b -> a -> c

```el
(funcall (-flip '<) 2 1) ;; => t
(funcall (-flip '-) 3 8) ;; => 5
(-sort (-flip '<) '(4 3 6 1)) ;; => '(6 4 3 1)
```

#### -const `(c)`

Return a function that returns `c` ignoring any additional arguments.

In types: a -> b -> a

```el
(funcall (-const 2) 1 3 "foo") ;; => 2
(-map (-const 1) '("a" "b" "c" "d")) ;; => '(1 1 1 1)
(-sum (-map (-const 1) '("a" "b" "c" "d"))) ;; => 4
```

#### -cut `(&rest params)`

Take n-ary function and n arguments and specialize some of them.
Arguments denoted by <> will be left unspecialized.

See `srfi-26` for detailed description.

```el
(funcall (-cut list 1 <> 3 <> 5) 2 4) ;; => '(1 2 3 4 5)
(-map (-cut funcall <> 5) '(1+ 1- (lambda (x) (/ 1.0 x)))) ;; => '(6 4 0.2)
(-filter (-cut < <> 5) '(1 3 5 7 9)) ;; => '(1 3)
```

#### -not `(pred)`

Take an unary predicates `pred` and return an unary predicate
that returns t if `pred` returns nil and nil if `pred` returns
non-nil.

```el
(funcall (-not 'even?) 5) ;; => t
(-filter (-not (-partial '< 4)) '(1 2 3 4 5 6 7 8)) ;; => '(1 2 3 4)
```

#### -orfn `(&rest preds)`

Take list of unary predicates `preds` and return an unary
predicate with argument x that returns non-nil if at least one of
the `preds` returns non-nil on x.

In types: [a -> Bool] -> a -> Bool

```el
(-filter (-orfn 'even? (-partial (-flip '<) 5)) '(1 2 3 4 5 6 7 8 9 10)) ;; => '(1 2 3 4 6 8 10)
(funcall (-orfn 'stringp 'even?) "foo") ;; => t
```

#### -andfn `(&rest preds)`

Take list of unary predicates `preds` and return an unary
predicate with argument x that returns non-nil if all of the
`preds` returns non-nil on x.

In types: [a -> Bool] -> a -> Bool

```el
(funcall (-andfn (-cut < <> 10) 'even?) 6) ;; => t
(funcall (-andfn (-cut < <> 10) 'even?) 12) ;; => nil
(-filter (-andfn (-not 'even?) (-cut >= 5 <>)) '(1 2 3 4 5 6 7 8 9 10)) ;; => '(1 3 5)
```

#### -iteratefn `(fn n)`

Return a function `fn` composed `n` times with itself.

`fn` is a unary function.  If you need to use a function of higher
arity, use [`-applify`](#-applify-fn) first to turn it into an unary function.

With n = 0, this acts as identity function.

In types: (a -> a) -> Int -> a -> a.

This function satisfies the following law:

    (funcall (-iteratefn fn n) init) = (-last-item (-iterate fn init (1+ n))).

```el
(funcall (-iteratefn (lambda (x) (* x x)) 3) 2) ;; => 256
(funcall (-iteratefn '1+ 3) 1) ;; => 4
(funcall (-iteratefn 'cdr 3) '(1 2 3 4 5)) ;; => '(4 5)
```

#### -fixfn `(fn &optional equal-test halt-test)`

Return a function that computes the (least) fixpoint of `fn`.

`fn` must be a unary function. The returned lambda takes a single
argument, `x`, the initial value for the fixpoint iteration. The
iteration halts when either of the following conditions is satisified:

 1. Iteration converges to the fixpoint, with equality being
      tested using `equal-test`. If `equal-test` is not specified,
      `equal` is used. For functions over the floating point
      numbers, it may be necessary to provide an appropriate
      appoximate comparsion test.

 2. `halt-test` returns a non-nil value. `halt-test` defaults to a
      simple counter that returns t after `-fixfn-max-iterations`,
      to guard against infinite iteration. Otherwise, `halt-test`
      must be a function that accepts a single argument, the
      current value of `x`, and returns non-nil as long as iteration
      should continue. In this way, a more sophisticated
      convergence test may be supplied by the caller.

The return value of the lambda is either the fixpoint or, if
iteration halted before converging, a cons with car `halted` and
cdr the final output from `halt-test`.

In types: (a -> a) -> a -> a.

```el
(funcall (-fixfn 'cos 'approx-equal) 0.7) ;; ~> 0.7390851332151607
(funcall (-fixfn (lambda (x) (expt (+ x 10) 0.25))) 2.0) ;; => 1.8555845286409378
(funcall (-fixfn 'sin 'approx-equal) 0.1) ;; => '(halted . t)
```

#### -prodfn `(&rest fns)`

Take a list of n functions and return a function that takes a
list of length n, applying i-th function to i-th element of the
input list.  Returns a list of length n.

In types (for n=2): ((a -> b), (c -> d)) -> (a, c) -> (b, d)

This function satisfies the following laws:

    (-compose (-prodfn f g ...) (-prodfn f' g' ...)) = (-prodfn (-compose f f') (-compose g g') ...)
    (-prodfn f g ...) = (-juxt (-compose f (-partial 'nth 0)) (-compose g (-partial 'nth 1)) ...)
    (-compose (-prodfn f g ...) (-juxt f' g' ...)) = (-juxt (-compose f f') (-compose g g') ...)
    (-compose (-partial 'nth n) (-prod f1 f2 ...)) = (-compose fn (-partial 'nth n))

```el
(funcall (-prodfn '1+ '1- 'int-to-string) '(1 2 3)) ;; => '(2 1 "3")
(-map (-prodfn '1+ '1-) '((1 2) (3 4) (5 6) (7 8))) ;; => '((2 1) (4 3) (6 5) (8 7))
(apply '+ (funcall (-prodfn 'length 'string-to-int) '((1 2 3) "15"))) ;; => 18
```


## Contribute

Yes, please do. Pure functions in the list manipulation realm only,
please. There's a suite of tests in `dev/examples.el`, so remember to add
tests for your function, or I might break it later.

You'll find the repo at:

    https://github.com/magnars/dash.el

Run the tests with

    ./run-tests.sh

Create the docs with

    ./create-docs.sh

I highly recommend that you install these as a pre-commit hook, so that
the tests are always running and the docs are always in sync:

    cp pre-commit.sh .git/hooks/pre-commit

Oh, and don't edit `README.md` directly, it is auto-generated.
Change `readme-template.md` or `examples-to-docs.el` instead.

## Changelist

### From 2.11 to 2.12

- Add GNU ELPA support. (Phillip Lord)
- Add `-some->`, `-some->>`, and `-some-->` macros. (Cam Saul)
- `-is-suffix?` no longer destroys input list.
- Faster hashtable implementation for `-union`.
- Improvements to docstrings and examples

### From 2.10 to 2.11

- Lots of clean up wrt byte compilation, debug macros and tests

### From 2.9 to 2.10

- Add `-let` destructuring to `-if-let` and `-when-let` (Fredrik Bergroth)

### From 2.8 to 2.9

- Add `-let`, `-let*` and `-lambda` with destructuring
- Add `-tree-seq` and `-tree-map-nodes`
- Add `-non-nil`
- Add `-fix`
- Add `-fixfn` (dash-functional 1.2)
- Add `-copy` (Wilfred Hughes)

### From 2.7 to 2.8

- Add `-butlast`

### From 2.6 to 2.7

- `-zip` now supports more than two lists (Steve Lamb)
- Add  `-cycle` ,  `-pad` ,  `-annotate` ,  `-zip-fill` (Steve Lamb)
- Add `-table`, `-table-flat` (finite cartesian product)
- Add `-flatten-n`
- `-slice` now supports "step" argument
- Add functional combinators `-iteratefn`, `-prodfn`
- Add `-replace`, `-splice`, `-splice-list` which generalize `-replace-at` and `-insert-at`
- Add `-compose`, `-iteratefn` and `-prodfn` (dash-functional 1.1)

### From 2.5 to 2.6

- Add `-is-prefix-p`, `-is-suffix-p`, `-is-infix-p` (Matus Goljer)
- Add `-iterate`, `-unfold` (Matus Goljer)
- Add `-split-on`, `-split-when` (Matus Goljer)
- Add `-find-last-index` (Matus Goljer)
- Add `-list` (Johan Andersson)

### From 2.4 to 2.5

- Add `-same-items?` (Johan Andersson)
- A few bugfixes

### From 2.3 to 2.4

- Add `-snoc` (Matus Goljer)
- Add `-replace-at`, `-update-at`, `-remove-at`, and `-remove-at-indices` (Matus Goljer)

### From 2.2 to 2.3

- Add tree operations (Matus Goljer)
- Make font-lock optional

### From 2.1 to 2.2

- Add `-compose` (Christina Whyte)

### From 2.0 to 2.1

- Add indexing operations (Matus Goljer)

### From 1.8 to 2.0

- Split out `dash-functional.el` (Matus Goljer)
- Add `-andfn`, `-orfn`, `-not`, `-cut`, `-const`, `-flip` and `-on`. (Matus Goljer)
- Fix `-min`, `-max`, `-min-by` and `-max-by` (Matus Goljer)

### From 1.7 to 1.8

- Add `-first-item` and `-last-item` (Wilfred Hughes)

### From 1.6 to 1.7

- Add `-rotate` (Matus Goljer)

### From 1.5 to 1.6

- Add `-min`, `-max`, `-min-by` and `-max-by` (Johan Andersson)

### From 1.4 to 1.5

- Add `-sum` and `-product` (Johan Andersson)

### From 1.3 to 1.4

- Add `-sort`
- Add `-reduce-r` (Matus Goljer)
- Add `-reduce-r-from` (Matus Goljer)

### From 1.2 to 1.3

- Add `-partition-in-steps`
- Add `-partition-all-in-steps`

### From 1.1 to 1.2

- Add `-last` (Matus Goljer)
- Add `-insert-at` (Emanuel Evans)
- Add `-when-let` and `-if-let` (Emanuel Evans)
- Add `-when-let*` and `-if-let*` (Emanuel Evans)
- Some bugfixes

## Contributors

 - [Matus Goljer](https://github.com/Fuco1) contributed lots of features and functions.
 - [Takafumi Arakaki](https://github.com/tkf) contributed `-group-by`.
 - [tali713](https://github.com/tali713) is the author of `-applify`.
 - [Víctor M. Valenzuela](https://github.com/vemv) contributed `-repeat`.
 - [Nic Ferrier](https://github.com/nicferrier) contributed `-cons*`.
 - [Wilfred Hughes](https://github.com/Wilfred) contributed `-slice`, `-first-item` and `-last-item`.
 - [Emanuel Evans](https://github.com/shosti) contributed `-if-let`, `-when-let` and `-insert-at`.
 - [Johan Andersson](https://github.com/rejeep) contributed `-sum`, `-product` and `-same-items?`
 - [Christina Whyte](https://github.com/kurisuwhyte) contributed `-compose`
 - [Steve Lamb](https://github.com/steventlamb) contributed `-cycle`, `-pad`, `-annotate`, `-zip-fill` and an n-ary version of `-zip`.
 - [Fredrik Bergroth](https://github.com/fbergroth) made the `-if-let` family use `-let` destructuring and improved script for generating documentation.
 - [Mark Oteiza](https://github.com/holomorph) contributed the script to create an info manual.
 - [Vasilij Schneidermann](https://github.com/wasamasa) contributed `-some`.
 - [William West](https://github.com/occidens) made `-fixfn` more robust at handling floats.
 - [Cam Saül](https://github.com/cammsaul) contributed `-some->`, `-some->>`, and `-some-->`.

Thanks!

## License

Copyright (C) 2012-2014 Magnar Sveen

Authors: Magnar Sveen <magnars@gmail.com>
Keywords: lists

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
