# Module BatPrint

    module BatPrint: sig .. end

Formatted output functions (also known as unparsing).

This module define a printf facility similar to the one defined by `BatPrintf` but in a safer and extensible way.

Classical printf implementation (in ocaml and other languages) deals with format strings as regular strings which are parsed at runtime. This approach is problematic considering typing and make it almost impossible to define new directives.

This module attempts to handle the problem in a more functional way. For convenience it can be used with the syntax extension `batteries.pa_string.syntax`.

Look at the example `"examples/snippets/test_printf.ml"` for an example of the use of this module.
Author(s): Jeremie Dimino, David Rajchenbach-Teller (documentation)

###### General overview

The functions of this module produce output, according to `BatPrint.directive`s, as described below. Output may be used to write to the screen, to error channels, to strings, to buffers, to the network, etc.

This module is very powerful and allows concise and extensible definition of output formats. The downside is that this module may be quite confusing at first. Don't worry, it's worth it.

The foremost concept is that of `format`. A `format` is a description of how informations should be printed. For instance, a format such as `p"%d\n"` may be used to display an integer (this is what

    %d

means -- `d` stands for decimal) and will end the line at the end of this integer (this is what

    \n

means everywhere in OCaml -- it stands for newline). Similarly, a format such as `p"(%d, %d)\n"` may be used to display two integers as a pair, and then end the line:

    %d

and

    \n

have the same meaning as previously, while the other characters are interpreted as regular text. Therefore, when applied to numbers 5 and 10, this format will output `"(5, 10)\n"`.

Formats may be applied using functions such as `BatPrint.printf`. For instance, `Print.printf p"(%d, %d)\n" 5 10` will print on the screen `"(5, 10)\n"`. Similarly, if `foo` is an `BatIO.output` which may be used to write on a file, `Printf.fprintf foo p"(%d, %d)\n" 5 10` will write `"(5, 10)\n"` to this file.

Let us consider formats more in detail.

Formats

The simplest format is the empty format, or `p""` -- notice the `p` (for "print"), all formats start with this letter. This format is unable to display any information. Actually, this format is only able to output the empty string, which is not very useful. Its type is `('a, 'a, 'b) format`. This `'b` states that it can be used with any kind of output.

Slightly more complex is format `p"foo"`. This format is also unable to display any information but, when applied, it will output text `"foo"`. Its type is also `('a, 'a, 'b) format`. More generally, any text without special characters may be used as a format, which can be used to display exactly that text, without any additional information.

Formats also support directives, which serve to insert additional pieces of information inside the text. Directives are always introduced with special character `%`. For instance, format `p"%d"` may be used to display integers. Its type is `(int -> 'a, 'a, 'b) format`. Here, type parameter `int -> 'a` states that this directive expects an integer. A more complex format such as `p"some text before the number%d"` or, equivalently, `p"some text before the number%(d)"` would have the same type `(int -> 'a, 'a, 'b) format`. Again,`p"some text before the number%(d)some text after the number"` would have the same type -- in this case, parenthesis are compulsory, for reasons we will detail later. When applied with argument 5, this last format will output text `p"some text before the number5some text after the number"`.

Directives

General-purpose directives

General-purpose directives start with `{%{}` and end with `{}}`. By default, functions of this module recognize the following general-purpose directives:

- `{%{int}}` or `{%{Int.t}}` format an integer with the usual notation (i.e. signed decimal).
- `{%{char}}` or `{%{Char.t}}` insert a character
- `{%{string}}` or `{%{String.t}}` insert a string
- `{%{float}}` or `{%{Float.t}}` format a floating-point number as a decimal, in the style `dddd.ddd`
- `{%{bool}}` or `{%{Bool.t}}` format a boolean argument to the string `"true"` or `"false"`
- `{%{int32}}` or `{%{Int32.t}}` format an integer with the usual notation (i.e. signed decimal).
- `{%{int64}}` or `{%{Int64.t}}` format an integer with the usual notation (i.e. signed decimal).
- `{%{nativeint}}` or `{%{Native_int.t}}` format an integer with the usual notation (i.e. signed decimal).
- `{%{Rope.t}}` insert a `Rope`
- `{%{foo list}}` or `{%{foo List.t}}`, where `foo` is the name of another directive, such as `int`, `char`, `string`, `int list`. etc., format a list of items (for more control on list formatting, see `List.print`
- `{%{foo array}}` or `{%{foo Array.t}}`, where `foo` is the name of another directive, such as `int`, `char`, `string`, etc., format a array of items (for more control on array formatting, see `Array.print`
- `{%{foo maybe}}` , where `foo` is the name of another directive, such as `int`, `char`, `string`, etc., format an optional `x` item to nothing if `x` is `None` or to the result of directive `foo` applied to `y`, if `x` is `Some y`
- `{%{foo option}}` or `{%{foo Option.t}}`, where `foo` is the name of another directive, such as `int`, `char`, `string`, etc., format an `'a option` to OCaml syntax, i.e. `"None"` or `"Some bar"`

Additional directives may be plugged-in.

Short-hand directives

A number of short-hand directives are also provided for most common situations. By default, functions of this module recognize the following short-hand directives:

- `%d`, `%i`, `%n`, `%l`, `%L`, or `%N`: format an integer with the usual notation (i.e. signed decimal).
- `%u`: format an integer as an unsigned decimal.
- `%x`: format an integer as an unsigned hexadecimal, using lowercase letters.
- `%X`: format an integer as an unsigned hexadecimal, using upper letters.
- `%o`: format an integer as an unsigned octal.
- `%s`: insert a string.
- `%S`: insert a quoted string in Caml syntax (double quotes, escapes).
- `%sc`: insert a read-only/read-write string (see `String.Cap`).
- `%Sc`: insert a quoted read-only/read-write string in Caml syntax (double quotes, escapes).
- `%rope`: insert a rope (see `Rope`).
- `%utf8`: insert a UTF-8 encoded string.
- `%c`: insert a character argument.
- `%C`: insert a character argument in Caml syntax (single quotes, escapes).
- `%f`: format a floating-point number as a decimal, in the style `dddd.ddd`.
- `%F`: format a floating-point number in Caml syntax (`dddd.` or `dddd.ddd` or `d.ddd e+-dd`).
- `%e` or `%E`: format a floating-point number as a decimal, in the style `d.ddd e+-dd` (mantissa and exponent).
- `%g` or `%G`: format a floating-point number as a decimal in style `%f` or `%e`, `E` (whichever is more compact).
- `%B`: format a boolean argument to the string `"true"` or `"false"`
- `%b`: format a boolean argument (for backward compatibility; do not use in new programs).
- `%ld`, `%li`, `%lu`, `%lx`, `%lX`, `%lo`: format an `int32` respectively as a signed decimal/a signed decimal/an unsigned decimal/ an unsigned hexadecimal in lowercase, an unsigned decimal in uppercase/ an unsigned octal.
- `%nd`, `%ni`, `%nu`, `%nx`, `%nX`, `%no`: format a `nativeint` respectively as a signed decimal/a signed decimal/an unsigned decimal/ an unsigned hexadecimal in lowercase, an unsigned decimal in uppercase/ an unsigned octal.
- `%Ld`, `%Li`, `%Lu`, `%Lx`, `%LX`, `%Lo`: format an `int64` respectively as a signed decimal/a signed decimal/an unsigned decimal/ an unsigned hexadecimal in lowercase, an unsigned decimal in uppercase/ an unsigned octal.
- `%obj`: format an object using its method `print: 'a output -> unit`
- `%!`: take no argument and flush the output.
- `%%`: take no argument and output one `%` character.

Additional short-hand directives may also be plugged-in.

Extending formats

In addition to the above default set of directives, you may define your own directives or override existing ones. This is actually quite simple.

Adding general-purpose directives (with a module)

When defining a new data structure `Foo.t`, it is usually a good idea to implement a function `Foo.t_printer`, with type `Foo.t BatValue_printer`. That's it. Once this function is created, you may use directive `{%{Foo.t}}` in any printing function of this module.

If the data structure has type arguments, as is the case of lists, arrays, etc., you will probably want to be able to print the contents of your data structure. For this purpose, you may define printers which accept as arguments other printers. For instance, the type of `List.t_printer` is actually `'a BatValue_printer.t -> 'a list BatValue_printer.t`, etc. If you have given such a type to `Foo.t_printer`, you may use `{%{Foo.t bar}}` in any printing function of this module, where `bar` is the name of another general-purpose directive.

Adding general-purpose directives (without a module)

An alternative technique for adding general-purpose directives is to define functions outside of their module. For instance, to plug-in a general-purpose directive for elements of type `foo`, it is sufficient to define a function `bar_printer`, with type `foo BatValue_printer`. That's it. Once this function is created, you may use directive `{%{bar}}` in any printing function of this module.

Again, you may also choose to define functions which take other printer as arguments, as before.

Adding short-hand directives

Short-hands are convenient and easier to write but are also less powerful than general-purpose directives, insofar as they may not accept printers as arguments and may not be passed as arguments to printers.

To define a short-hand directive `%foo` for formatting elements of type `bar`, it is sufficient to create a function `printer_foo` with type `(bar -> 'a, 'a) directive`. Assuming that you already have a function `string_of_bar: bar -> string`, `printer_foo` may be implemented simply as

       let printer_foo k x = k (fun oc -> BatIO.nwrite oc (string_of_bar x))

That's it. Once this function is created you may use directive `%foo` in any printing function of this module.

To improve the flexibility of your newly created directive, you may wish to add an optional argument `flags` to function `printer_foo`, to handle flags, as detailed in the following section.

Flags

The general format of directives is

`% ( [options] [width] [.precision] name )`

`name` is one of `d`, `i`, `n`, `l`, `L`, `N`, `u`, `x` ..., and behaves as explained above.

The optional `options` are:

- `-`: left-justify the output (default is right justification).
- `0`: for numerical conversions, pad with zeroes instead of spaces.
- `+`: for numerical conversions, prefix number with a `+` sign if positive.
- space: for numerical conversions, prefix number with a space if positive.
- `#`: request an alternate formatting style for numbers.

The optional `width` is an integer indicating the minimal width of the result. For instance, `%6d` prints an integer, prefixing it with spaces to fill at least 6 characters.

The optional `precision` is a dot `.` followed by an integer indicating how many digits follow the decimal point in the `%f`, `%e`, and `%E` conversions. For instance, `%.4f` prints a `float` with 4 fractional digits.

The integer in a `width` or `precision` can also be specified as `*`, in which case an extra integer argument is taken to specify the corresponding `width` or `precision`. This integer argument precedes immediately the argument to print. For instance, `%.*f` prints a `float` with as many fractional digits as the value of the argument given before the float.

For more informations on supporting flags in your directives, see the documentation of type `BatPrint.directive`.

###### Directives and formats

You can skip this section if you are only interested in using this module and not in extending its behavior or understanding its internal mechanisms.

    type ('a, 'b) directive = ((unit BatInnerIO.output -> unit) -> 'b) -> 'a

The underlying type of a directive. Directives are the basic elements of formats.

A directive takes as arguments:

- a continuation
- any arguments they need

its goal is to create a printer which prints the arguments of the directives and to pass it to the continuation.

For example, directive `%d` has the following type

            val printer_d : (int -> 'a, 'a) directive

And here is a possible implementation:

            let printer_d k x = k (fun oc -> BatIO.nwrite oc (string_of_int x))

Additionally, directives can takes ``flags'' (like in `"%2d"` where `2` is here a width). Flags are passed to the directive as optional argument before the continuation, for example:

            val printer_d : ?width : int -> (int -> 'a, 'a) directive

with:

            let printer_d ?width k x =
              let str = string_of_int x in
              let str = match width with
                | None ->
                    str
                | Some n ->
                    let len = String.length str in
                    if len < n then
                      String.make (n - len) ' ' ^ str
                    else
                      str
              in
              k (fun oc -> BatIO.nwrite oc str)

Standard flags, i.e. the ones supported by the syntax extension, are:

- the boolean flags (which must default to `false`):

- `-` with label `left_justify`
- `0` with label `pad_with_zeros`
- `+` with label `prefix_with_plus`
- `` with lavel `prefix_with_space`
- `#` with label `alternate`

- the integer flags `width` and `precision`

    val literal : string -> ('a, 'a) directive

`literal str` create a directive which do not take any argument from a literal string

###### Formatting functions

    val printf : ('a, unit) format -> 'a

`printf fmt args` formats the arguments in `args` as specified by `fmt` and prints the result on the standard output `BatIO.stdout`, i.e. normally to the screen. If you are lost, this is probably the function you're looking for.

    val eprintf : ('a, unit) format -> 'a

`eprintf fmt args` behaves as `printf fmt args` but prints the result on the error out put `BatIO.stderr` rather on the screen. This function is typically used to display warnings and errors, which may later be separated from "regular" printouts.

    val sprintf : ('a, string) format -> 'a

A function which doesn't print its result but returns it as a string. Useful for building messages, for translation purposes or for display in a window, for instance.

While this function is quite convenient, don't abuse it to create very large strings such as files, that's not its role. For this kind of usage, prefer the more modular and usually faster `BatPrint.fprintf`.

###### Generic functions

    val fprintf : 'a BatInnerIO.output -> ('b, unit) format -> 'b

General formatting function.

This function behaves mostly as `BatPrint.printf` or `BatPrint.eprintf` but, instead of printing to a predefined output, it takes as argument the output to which it should print.

Typically, if you are attempting to build a large output such as a file, this is probably the function you are looking for. If you are writing a pretty-printer, this is probably the function you are looking for. If you are you are looking for a function to combine with directive `%a`, this is also probably the function you are looking for.

    val ifprintf : 'a -> ('b, unit) format -> 'b

As `BatPrint.fprintf` but doesn't actually print anything. Sometimes useful for debugging.

    val bprintf : Buffer.t -> ('a, unit) format -> 'a

This function behaves as `BatPrint.fprintf` but prints into a buffer rather than into an `output`.

###### Functions with continuations

    val kfprintf : ('a BatInnerIO.output -> 'b) ->
           'a BatInnerIO.output -> ('c, 'b) format -> 'c

`kfprintf k oc fmt` prints on `oc` then call `k` with `oc` as argument

    val rprintf : ('a, BatRope.t) format -> 'a

`rprintf fmt` returns the result as a rope

    val krprintf : (BatRope.t -> 'a) -> ('b, 'a) format -> 'b

`krprintf k fmt` creates a rope from the format and other arguments and pass it to `k`

    val ksprintf : (string -> 'a) -> ('b, 'a) format -> 'b

`ksprintf k fmt` creates a string from the format and other arguments and pass it to `k`

    val kbprintf : (Buffer.t -> 'a) -> Buffer.t -> ('b, 'a) format -> 'b

`bprintf k buf fmt` prints into the buffer `buf`, then call `k` with `buf` as argument.