
=========================================
Gnumeric/Pure: A Pure Plugin for Gnumeric
=========================================

Version 0.16, |today|

Albert Gräf <aggraef@gmail.com>

Gnumeric/Pure is a Gnumeric_ extension which lets you use Pure_ functions in
Gnumeric, the Gnome spreadsheet. It offers better execution speed than the
existing Perl and Python plugins, and provides some powerful features not
found in other Gnumeric scripting plugins, such as asynchronous data sources
created from Pure streams.

.. _Gnumeric: http://www.gnumeric.org/
.. _Pure: http://purelang.bitbucket.org/
.. _MATLAB: http://www.mathworks.com/
.. _Octave: http://www.octave.org/

.. only:: html

   .. contents:: :local:


Introduction
============

This package provides a Gnumeric_ extension which gives you access to the
Pure_ programming language in Gnumeric. It works pretty much like the Perl and
Python plugin loaders which are distributed with Gnumeric, but Gnumeric/Pure
offers some powerful features which aren't found in other Gnumeric scripting
plugins:

* Pure is a functional programming language which fits the computational model
  of spreadsheet programs very well.

* Pure is based on term rewriting and thus enables you to do symbolic
  computations in addition to the usual numeric calculations.

* Pure has a built-in MATLAB_/Octave_-like matrix data structure which makes
  it easy to deal with cell ranges in a spreadsheet in an efficient manner.

* Pure also provides a bridge to Octave_ so that you can call arbitrary Octave
  functions using this extension.

* Pure also has built-in support for lazy data structures and thus allows you
  to handle potentially infinite amounts of data such as the list of all prime
  numbers. Gnumeric/Pure lets you turn such lazy values into asynchronous data
  sources computed in the background, which update the spreadsheet
  automatically as results become available.

* Last but not least, Pure is compiled to native code on the fly. This means
  that, while startup times are a bit longer due to Pure's JIT compiler
  kicking in (you'll notice this if you open a spreadsheet with Pure
  functions), the resulting compiled code then typically executes *much*
  faster than equivalent interpreted Perl and Python code.

Once the plugin loader is installed and enabled, you can try the Pure
functions in the provided examples and start adding your own plugin scripts.
As of version 0.12, there's a new helper script ``pure-gnm`` which generates
the required plugin.xml files to make this easy. Various examples can be found
in the examples folder in the distribution, which should help you to get
started with Gnumeric/Pure fairly quickly.

For more advanced uses, Gnumeric/Pure also provides a programming interface
which lets you do various special tasks such as modifying entire ranges of
cells with one Pure call, calling Gnumeric functions from Pure, and setting up
asynchronous data sources. The manual explains all this in detail.

.. note:: This manual assumes that you're already familiar with Gnumeric as
   well as the Pure language and its programming environment. If not then you
   should consult the corresponding documentation to learn more about these.

Copying
=======

Copyright (c) 2009-2017 by Albert Graef.

Gnumeric/Pure 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 2 of the License, or (at your option) any later
version.

Gnumeric/Pure 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/>.

Installation
============

Obviously, you need to have both Pure and Gnumeric installed. Pure 0.36 and
Gnumeric 1.9.13 or later are known to work. The current release of this module
will work with the latest, GTK3-based versions of Gnumeric (Gnumeric 1.11 and
later, various versions starting from 1.12.23 have been tested and are known
to work).

Please note that the Gnumeric plugin interface is a moving target, which means
that from time to time the gnumeric-pure module sources are updated to
accommodate the latest changes in Gnumeric. Thus, depending on which Gnumeric
version you are running, you may have to go with an older version of
gnumeric-pure. In particular, if you are still running Gnumeric 1.10 or
earlier then you should use version 0.12 of the module instead:

  https://bitbucket.org/purelang/pure-lang/downloads/gnumeric-pure-0.12.tar.gz

Run ``make`` to compile the software. You might have to adjust the settings at
the beginning of the Makefile to make this work. Once the compile goes
through, you should now have a ``pure_loader.so`` file in the ``pure-loader``
subdirectory. You can install the plugin and related stuff with ``sudo make
install`` in the global Gnumeric plugin directory, or if you prefer to install
it into your personal plugin directory then run ``make install-local``
instead. (The latter is recommended if you plan to customize any of the sample
plugin scripts included in the distribution for your purposes.)

Typically, ``make install`` and ``make install-local`` will install the
plugins into the following directories by default (here and in the following
``<version>`` denotes the version of Gnumeric you have installed):

* System-wide installations go into
  ``/usr/local/lib/gnumeric/<version>/plugins`` or similar, depending on
  Gnumeric's installation prefix (usually either ``/usr/local`` or ``/usr``).

* User-specific installations go into ``~/.gnumeric/<version>/plugins``.

The Makefile tries to guess the installation path and version number of
Gnumeric on its own. If it guesses wrong, you can change these using the
Makefile variables ``prefix`` and ``gnmversion``, respectively. For instance:

.. code-block:: sh

  $ make prefix=/usr gnmversion=1.12.4

In either case, ``make install`` also installs the ``pure-gnm`` helper script
under the Pure installation prefix. (This is a little convenience script to
generate the ``plugin.xml`` files used by Gnumeric to load a plugin; see
`Defining Your Own Functions`_ for details.)

If ``make install`` doesn't work for some reason, you can also just copy the
``pure-func`` and ``pure-loader`` directories manually to your Gnumeric plugin
directory. You can still run ``make install`` in the ``pure-gnm`` subdirectory
to get the ``pure-gnm`` script installed in this case.

Setup
=====

Once Gnumeric/Pure has been properly installed, you should see it in
Gnumeric's Tools/Plug-ins dialog. There are actually two main entries, one
labelled "Pure functions" and the other one labelled "Pure plugin loader". You
need to enable both before you can start using Pure functions in your Gnumeric
spreadsheets.

Gnumeric doesn't provide much in the way of GUI customization options right
now, but at least it's possible for plugins to install and configure
additional menu and toolbar options. Gnumeric/Pure adds three additional
options to the Tools menu which allow you to stop asynchronous data sources,
reload Pure scripts and edit them. After installation, the definitions of
these items can be found in the ``pure-loader/pure-ui.xml`` file in your
Gnumeric plugin directory. Have a look at this file and edit is as
desired. E.g., if you want to put the Pure-related options into a submenu and
enable toolbar buttons for these options, then your ``pure-ui.xml`` file
should look as follows:

.. code-block:: xml

  <ui>
    <menubar>
      <menu name="Tools" action="MenuTools">
        <separator/>
        <menu name="Pure" action="PureMenu">
          <menuitem action="PureStop"/>
          <menuitem action="PureReload"/>
          <menuitem action="PureEdit"/>
        </menu>
      </menu>
    </menubar>
    <toolbar name="StandardToolbar">
      <separator/>
      <toolitem action="PureStop"/>
      <toolitem action="PureReload"/>
      <toolitem action="PureEdit"/>
    </toolbar>
  </ui>

Basic Usage
===========

With Pure/Gnumeric installed and enabled, you should be ready to join the fun
now. Start up Gnumeric, click on a cell and invoke the "f(x)" dialog. The Pure
functions available for use are shown in the "Pure" category. E.g., click on
``pure_hello``. Now the Pure interpreter will be loaded and the function
description displayed. Click "Ok" to select the ``pure_hello`` function and
then "Ok" again to actually insert the function call (without arguments) into
the current cell. You should now be able to read the friendly greeting
returned by the function.

Of course, you can also enter the function call directly as a formula into a
cell as usual. Click on a cell, then enter the following::

  =pure_hello(getenv("USER"))

The greeting should now be displayed with your login name in it.

Play around a bit with the other Pure functions. These functions are nothing
special; they are just ordinary Pure functions which are defined by the
``pure_func.pure`` script in the ``pure-func`` subdirectory of your Gnumeric
plugin directory. You can have a look at them by invoking the "Edit Pure
Scripts" option which gets added to the Tools/Pure menu once the Pure plugin
loader is enabled. (This will invoke the emacs editor by default, or the
editor named by the ``EDITOR`` environment variable. You can set this
environment variable in your shell's startup files.) The Tools/Pure menu
contains a second Pure-related option, "Reload Pure Scripts" which can be used
to quickly reload all loaded Pure scripts after edits; more about that later.

Please note that most of the functions in ``pure_func.pure`` are rather
useless, they are only provided for illustrative purposes. However, there are
some useful examples in there, too, in particular:

* ``pure_eval`` lets you evaluate any Pure expression, given as a string in
  its first argument. E.g., try something like ``=pure_eval("foldl (+) 0
  (1..100)")``. Additional parameters are accessible as ``x!0``, ``x!1``,
  etc. For instance: ``=pure_eval("x!0+x!1",A1,B1)``.

* ``pure_echo`` just displays its arguments as a string in Pure syntax, as the
  interpreter sees them. This is useful for debugging purposes. E.g.,
  ``=pure_echo(A1:B10)`` shows the given range as a Pure matrix.

* ``pure_shell`` is a variation of ``pure_eval`` which executes arbitrary Pure
  code and returns the last evaluated expression (if any) as a string. This is
  mainly provided as a convenience to create an "interactive Pure shell" which
  lets you evaluate Pure code inside Gnumeric. To these ends, simply prepare a
  text cell for entering the code to be evaluated, and then apply
  ``pure_shell`` on this text cell in another cell to display the result.

A spreadsheet showing most of the predefined functions in action can be found
in ``pure-examples.gnumeric`` example distributed with Gnumeric/Pure.

Interactive Pure Shell
======================

The ``pure-examples.gnumeric`` spreadsheet also includes an instance of
``pure_shell`` which lets you evaluate arbitrary Pure code in the same
interpreter instance that executes Gnumeric/Pure functions. This is very
helpful if you're developing new Pure functions to be used in Gnumeric. It
also lets you use Gnumeric as a kind of GUI frontend to the Pure
interpreter. You can try this now. Open the ``pure-examples`` spreadsheet in
Gnumeric and enter the following into the input cell of the Pure shell::

  > scanl (+) 0 (1..20)
    [0,1,3,6,10,15,21,28,36,45,55,66,78,91,105,120,136,153,171,190,210]

Note that here and in the following the prompt ``>`` indicates a Pure
expression to be evaluated in *Gnumeric* (rather than the standalone Pure
interpreter), which is followed by another line indicating the result (printed
in the output cell below the input cell of the Pure shell). You can find the
Pure shell at the bottom of the first sheet in ``pure-examples``, see the
screenshot below. For your convenience, there's also a second, bigger one on
the second sheet. You might want to copy this over to a separate spreadsheet
which you can use as a scratchpad for experimentation purposes.

.. figure:: shell.png

   The Pure shell.

Also note that this is in fact *Pure code* (not a Gnumeric formula) being
evaluated there. You can execute any Pure code, including Pure declarations,
so you can type::

  > using system; puts "Hello, world!";
    14

This prints the string ``"Hello, world!"`` on standard output, visible in the
terminal window where you launched Gnumeric. Here is another example, showing
how you can invoke any function from the C library, by declaring it as a Pure
``extern`` function::

  > extern int rand(); [rand | i = 1..5];
    [1810821799,2106746672,1436605662,1363610028,695042099]

All functions in the Pure prelude are readily available in the Gnumeric Pure
shell, as well as the functions defined in ``pure_func.pure`` and its imports,
including the programming interface described in `Advanced Features`_. For
instance, here's how you can retrieve a cell value from the current sheet::

  > get_cell "A1"
    "Gnumeric/Pure Examples"

Using ``call`` (see `Calling Gnumeric from Pure`_), you can also invoke any
Gnumeric function::

  > call "product" (1..10)
    3628800.0

Defining Your Own Functions
===========================

After playing around with ``pure_func.pure`` and the interactive Pure shell
for a while, of course you will want to write your own functions, that's what
this extension is about after all! This section shows you how to do this.

Creating a Simple Plugin
------------------------

Let's consider a simple example: the factorial function. In Pure this function
can be implemented as follows::

  factorial [x] = foldl (*) 1 (1..x);

Note the list bracket around the argument ``x``. You wouldn't normally pass a
single numeric argument that way in Pure, but this is needed here since by
default Gnumeric passes arguments as a list to a Pure function. There are
other ways to configure the call interface to Pure functions, but these
require that we tell Gnumeric about the number and types of arguments, see
`Gnumeric/Pure Interface`_ below. For the moment let's stick to the default
scheme, however, in order to keep things simple.

Put the above definition into a script file, say, ``myplugin.pure``. Next we
need to create a ``plugin.xml`` file to tell Gnumeric about our plugin and
which functions it provides. While these files can be written by hand, this is
tedious and error-prone. Fortunately, recent Gnumeric/Pure versions provide
the ``pure-gnm`` helper script which makes this quite easy. To use
``pure-gnm`` with our plugin script, we have to add a special "hashbang"
comment block to our script which supplies the needed information. In our
case, this might look as follows::

  #! N: My Pure functions
  #! C: Pure
  #! D: My Pure functions.
  #! F: factorial

You can add this comment block anywhere in your plugin script, but usually it
is placed near the beginning. The different fields have the following meaning:

* ``N``: the name of the plugin

* ``C``: the function category

* ``D``: a more detailed description of the plugin

* ``F``: a whitespace-delimited list of Pure function names

The contents of the ``N`` and ``D`` fields (name and description) are visible
in Gnumeric's "Plugin Manager" dialog. You should specify at least the name
field (otherwise the plugin will be displayed as "Unnamed" in the dialog),
while the description is optional (if you don't specify one, the description
of the plugin will be empty). The ``C`` field denotes the category under which
the functions listed in the ``F`` field will be shown in Gnumeric's "f(x)"
dialog; if you don't specify this, the functions will be in the "Unknown"
category. The ``F`` field is the most crucial part. It must contain all Pure
functions defined in the plugin script or its imports that you want to be
visible in Gnumeric, so you have to keep this in sync with the actual function
definitions in the script; if you don't specify this, the plugin will provide
no functions at all.

The ``D`` and ``F`` fields can also be split into multiple lines (each
prefixed with the "hashbang" comment marker and the corresponding field
identifier) if necessary.

So our ``myplugin.pure`` script now looks like this::

  #! N: My Pure functions
  #! C: Pure
  #! D: My Pure functions.
  #! F: factorial

  factorial [x] = foldl (*) 1 (1..x);

Once you've added the comment block, you can generate the ``plugin.xml`` file
for the plugin simply as follows:

.. code-block:: sh

  $ pure-gnm myplugin.pure > plugin.xml

Note that by default ``pure-gnm`` writes the ``plugin.xml`` file to standard
output which is useful if you want to check the generated file first. To
actually create the file, we simply redirect the output to ``plugin.xml``.

You'll have to redo this every time your plugin changes (i.e., you've added
new functions or deleted or renamed old ones, or changed the name or
description of the plugin). It's easy to automate this step using
``make``. E.g., the following Makefile will do the trick:

.. code-block:: make

  myplugin = myplugin.pure

  all: plugin.xml

  plugin.xml: $(myplugin)
          pure-gnm $< > $@

  clean:
          rm -f plugin.xml

Now you can just run ``make`` in the plugin directory and it will rebuild the
``plugin.xml`` file as needed.

The plugin.xml File
-------------------

The ``plugin.xml`` file resulting from the previous step looks like this:

.. code-block:: xml

  <?xml version="1.0" encoding="UTF-8"?>
  <plugin id="Gnumeric_myplugin">
    <information>
      <name>My Pure functions</name>
      <description>My Pure functions.</description>
      <require_explicit_enabling/>
    </information>
    <loader type="Gnumeric_PureLoader:pure">
      <attribute name="module_name" value="myplugin.pure"/>
    </loader>
    <services>
      <service type="function_group" id="myplugin">
        <category>Pure</category>
        <functions>
          <function name="factorial"/>
        </functions>
      </service>
    </services>
  </plugin>

You can also edit this file by hand if you know what you're doing; in that
case, please check the Gnumeric documentation for details about the format of
these files. The template used by ``pure-gnm`` to generate these files can be
found in the source distribution (see ``plugin.xml`` in the ``pure-gnm``
folder) or under ``/usr/local/lib/pure-gnm`` after installation. You can edit
this file (carefully!) in order to implement global changes that you want to
be in every ``plugin.xml`` file generated by ``pure-gnm``.

Two specific items that you might want to edit by hand are the
``<require_explicit_enabling/>`` tag and the ``id`` properties of the
``<plugin>`` and ``<service>`` tags:

* The ``<require_explicit_enabling/>`` tag indicates that Gnumeric shouldn't
  enable the new plugin until you explicitly tell it to. You can remove that
  line if you want Gnumeric to automatically enable new plugins as they are
  added to the system.

* The ``pure-gnm`` script automatically derives the ``id`` properties of the
  ``<plugin>`` and ``<service>`` tags from the name of the plugin script,
  which is a sensible default in most cases. However, you might have to change
  these identifiers if they happen to collide with other Gnumeric plugins and
  services. This can be done by either editing the generated ``plugin.xml``
  file or by renaming the plugin script accordingly.

Note that the only really Pure-specific part in the xml file is the loader
description which also names the Pure script implementing the plugin in the
value of the ``module_name`` attribute. In this case this is just
``"myplugin.pure"``. This path is taken relative to the directory containing
the ``plugin.xml`` file, but you can also specify an absolute path there if
you want to keep the plugin script elsewhere. To achieve this with
``pure-gnm``, you can just invoke it with an absolute path name, e.g.:

.. code-block:: sh

  $ pure-gnm $PWD/myplugin.pure > plugin.xml

Now you can move the ``plugin.xml`` file whereever you like and still have
Gnumeric find the script file in its prescribed location. Again, this can be
automatized using ``make`` fairly easily; we'll return to that in the
following section.

Loading the Plugin
------------------

We now have the plugin script ``myplugin.pure`` and the ``plugin.xml`` file in
the same directory, say, ``/some/path/myplugin``. We still need to tell
Gnumeric about the new plugin, though, so that it can find it. Unfortunately,
the mechanics of making a plugin known to Gnumeric are somewhat involved, so
we discuss the necessary steps in detail below. There are basically three ways
you can go about this:

* If you want to keep plugin script and the ``plugin.xml`` file where they
  are, you'll have to change Gnumeric's plugin path so that it includes the
  *parent* directory ``/some/path`` (not ``/some/path/myplugin``). This is
  done by adding the directory under the Directories tab in Gnumeric's
  Tools/Plug-ins dialog, after which you'll have to restart Gnumeric so that
  it picks up the changes in the plugin search path.

* Second, you can also move or copy the entire ``/some/path/myplugin``
  directory to your personal Gnumeric plugin folder (usually
  ``~/.gnumeric/<version>/plugins``). Gnumeric will always search this
  directory for new plugins by default, so modifying the plugin search path is
  not necessary. However, keeping the plugin script in a hidden location in
  your home directory may not be very convenient if you want to modify the
  script later.

* Third, you can get the best of both previous methods by keeping the plugin
  script where it is and copying just the ``plugin.xml`` file to your personal
  Gnumeric plugin folder.

The third method tends to be the easiest, but note that it requires that the
plugin script needs to be specified as an absolute path (as sketched out
previously). Fortunately, it's fairly easy to automate this with ``make``.
The following requires GNU make to work, and you'll also need to have the
Gnumeric development files installed, so that the Gnumeric version can be
determined easily with a shell command. These rules are to be added to the end
of the Makefile described previously under `Creating a Simple Plugin`_.

.. code-block:: make

  gnmversion=$(shell pkg-config --modversion libspreadsheet-1.12)
  plugindir=$(HOME)/.gnumeric/$(gnmversion)/plugins/myplugin

  install: $(myplugin)
          test -d $(plugindir) || mkdir -p $(plugindir)
          pure-gnm $(CURDIR)/$< > $(plugindir)/plugin.xml

  uninstall:
          rm -rf $(plugindir)

Now you can just run ``make install`` to make the plugin known to Gnumeric.
Note that we also added a rule that allows you to uninstall the plugin if it
isn't needed any more.

In any case, once you fire up Gnumeric again, the new plugin should be listed
as "My Pure functions" on the Plugin List tab in the Tools/Plug-ins
dialog. Check it to enable it. The ``factorial`` function defined in the
plugin should now be available and ready to be called just like any other
Gnumeric function. For instance, type this into a cell to have the factorial
of 10 computed::

  =factorial(10)

Also try saving the spreadsheet and loading it again after restarting
Gnumeric. The plugin will now be loaded automatically and the spreadsheet
should display the proper value of the factorial.

.. note:: Once you start playing around with your own Pure plugins, you may
   run into one common mishap: You open an existing spreadsheet without having
   enabled the plugins it uses. An easily visible symptom of that is that
   you'll see cells showing the ``#NAME?`` error. You will then have to enable
   those plugins again *and* reload the spreadsheet afterwards, so that
   everything is recalculated properly.

   In contrast, just changing the body of a function in a plugin usually needs
   neither a restart of Gnumeric nor a reloading of the spreadsheet. In this
   case it's often sufficient to reload all scripts with the "Reload Pure
   Scripts" option in the Tools/Pure menu, after which you can use
   “Recalculate” (F9) to recompute the spreadsheet.

It is also worth mentioning here that the Pure loader can load multiple Pure
plugins (and of course each plugin can provide as many functions as you
want). You only need to tell Gnumeric about them after creating the scripts
and ``plugin.xml`` files and placing them into corresponding plugin
directories. Just enable the ones that you want in Tools/Plug-ins. All scripts
are loaded in the same Pure interpreter (and thus are treated like one big
script) so that functions in one script can use the function and variable
definitions in another. If you need to access the definitions in the
``pure_func.pure`` "mother script", you can also just import it into your
scripts with a ``using`` clause, i.e.: ``using pure_func;``

Another important point is that a Pure plugin script is always loaded in the
directory where it is located, as indicated by the corresponding
``plugin.xml`` file, even if it is different from the plugin directory. That
is, the current working directory (which is normally the directory that
Gnumeric was started in) is temporarily set to the directory holding the
plugin script while the script is being loaded. This enables the script to
find imported scripts and other files (such as media files or scripts written
in other languages) that it may need at load time. This wasn't needed in this
simple example, but you can find other examples in the Gnumeric/Pure
distribution which make good use of this feature.

Spicing It Up
-------------

Our plugin example is now essentially complete, but in order to make it really
convenient to use, we may want to add some information about how the
``factorial`` function is to be called in Gnumeric. Gnumeric doesn't keep this
kind of information in the ``plugin.xml`` file, but expects it to be provided
by the plugin itself. In the Gnumeric/Pure interface this can be done by
adding a rule for the ``gnm_info`` function. In our example we tell Gnumeric
that ``factorial`` expects a single numeric argument. While we're at it, we
might as well add some helpful documentation to be displayed in Gnumeric's
"f(x)" dialog. The details of this are described in the following section, but
to give you a sneak preview, here's a beefed-up version of our script which
implements all this (you can also find this version of the example along with
a GNU Makefile in the Gnumeric/Pure distribution)::

  #! N: My Pure functions
  #! C: Pure
  #! D: My Pure functions.
  #! F: factorial

  factorial x = foldl (*) 1 (1..x);

  using pure_func; // for the gnm_help function
  gnm_info "factorial" = "f", gnm_help "factorial:factorial of a number"
   ["x:number"] "Computes the factorial of @{x}." [] ["=factorial(10)"] [];

Fire up Gnumeric again, press the "f(x)" button and select ``factorial`` under
the ``Pure`` category. The "f(x)" dialog should now display the additional
information we added above. Also note that Gnumeric now knows that this
function is supposed to be called with exactly one ``f`` (numeric) argument.
Therefore the list brackets around the argument of ``factorial`` aren't needed
any more, so don't forget to remove them, as shown in the above code sample.

This completes our little example. As an exercise, you're invited to add more
functions on your own. (Don't forget to change the ``#! F`` line accordingly
and rerun ``pure-gnm`` when you do this, so that Gnumeric knows about the new
functions.) It also pays off to take a look at some of the other included
examples, you can find these in the ``examples`` folder of the distribution
tarball.

Gnumeric/Pure Interface
=======================

We already explained in the previous section that, when a Pure function is
called from Gnumeric, it receives its arguments in a list by default. However,
it is possible to tell Gnumeric about the expected arguments of the function
and also specify a help text to be displayed in the "f(x)" dialog, by giving a
definition of the ``gnm_info`` function as explained below.

Note that ``gnm_info`` is really an ordinary Pure function. Thus, rather than
hardcoding this information as static text (such as the "docstrings" used in
Gnumeric's Python extension), the function descriptions can also be
constructed dynamically in corresponding Pure code. This offers an opportunity
for programmatic customizations. But note that the ``gnm_info`` function will
only be invoked when the plugin script is loaded, so once that is done the
function description remains the same for the entire Gnumeric session.

Function Descriptions
---------------------

To describe a given function to Gnumeric, define ``gnm_info "<name>"`` (where
``<name>`` is the name of the function) as a pair with the following elements:

* The first element, a string, gives the signature of the function. E.g.,
  ``""`` denotes a function without arguments, ``"f"`` a function taking a
  single float parameter, ``"fs"`` a function taking a float and a string
  argument (in that order), etc. Optional parameters can be indicated using
  ``|``, as in ``"ff|s"`` (two non-optional floats, followed by an optional
  string). See below for a complete list of the supported parameter types.

* The second element is a list of hash pairs ``key=>text`` which together make
  up the help text shown in Gnumeric's "f(x)" dialog. You should at least
  specify the function name along with a short synopsis here, e.g.
  ``GNM_FUNC_HELP_NAME => "frob:the frob function"``. Parameter descriptions
  take the form ``GNM_FUNC_HELP_ARG => "x:integer"``. There are a number of
  other useful elements, see below for details.

Both the signature and the function description are optional. That is,
``gnm_info`` may return either just a signature string, or a list of hash
pairs with the function description, or both. The signature defaults to a
variadic function which takes any number of parameters of any type (see
below), and the description defaults to some boilerplate text which says that
the function hasn't been documented yet.

Note that if no signature is given, then the function accepts any number of
parameters of any type. In that case, or if there are optional parameters, the
function becomes variadic and the (optional) parameters are passed as a Pure
list (in addition to the non-optional parameters).

Here's the list of valid parameter types, as they are documented in the
Gnumeric sources:

.. code-block:: none

    f : float 		no errors, string conversion attempted
    b : boolean		identical to f
    s : string		no errors
    S : scalar		any non-error scalar
    E : scalar		any scalar, including errors

    r : cell range	content may not be evaluated yet
    A : area		array, range (as above), or scalar
    ? : anything        any value (scalars, non-scalars, errors, whatever)

The keys used in the function description may be any of the following, along
with sample text for each type of field::

    GNM_FUNC_HELP_NAME         => "name:synopsis"
    GNM_FUNC_HELP_ARG	       => "name:parameter description"
    GNM_FUNC_HELP_DESCRIPTION  => "Long description."
    GNM_FUNC_HELP_NOTE	       => "Note."
    GNM_FUNC_HELP_EXAMPLES     => "=sample_formula()"
    GNM_FUNC_HELP_SEEALSO      => "foo,bar,..."

The following keys are only supported in the latest Gnumeric versions::

    GNM_FUNC_HELP_EXTREF       => "wiki:en:Trigonometric_functions"
    GNM_FUNC_HELP_EXCEL	       => "Excel compatibility information."
    GNM_FUNC_HELP_ODF	       => "OpenOffice compatibility information."

Note that inside the descriptions, the notation ``@{arg}`` (``@arg`` in older
Gnumeric versions) can be used to refer to a parameter value. For instance,
here's a sample description for a binary function which also includes a help
text::

  gnm_info "pure_max" = "ff",
  [GNM_FUNC_HELP_NAME     => "pure_max:maximum of two numbers",
   GNM_FUNC_HELP_ARG	  => "x:number",
   GNM_FUNC_HELP_ARG	  => "y:number",
   GNM_FUNC_HELP_DESCRIPTION =>
   "Computes the maximum of two numbers @{x} and @{y}.",
   GNM_FUNC_HELP_EXAMPLES => "=pure_max(17,22)"];

As you can see, the function descriptions are a bit unwieldy, so it's
convenient to construct them using this little helper function defined in
``pure_func.pure``::

  gnm_help name::string args descr::string notes examples see_also =
    [GNM_FUNC_HELP_NAME         => name] +
    [GNM_FUNC_HELP_ARG          => x | x::string = args ] +
    [GNM_FUNC_HELP_DESCRIPTION  => descr ] +
    [GNM_FUNC_HELP_NOTE         => x | x::string = notes ] +
    [GNM_FUNC_HELP_EXAMPLES     => x | x::string = examples ] +
    (if null see_also then [] else
     [GNM_FUNC_HELP_SEEALSO     => join "," see_also]);

Now the description can be written simply as follows::

  gnm_info "pure_max" = "ff", gnm_help "pure_max:maximum of two numbers"
    ["x:number", "y:number"]
    "Computes the maximum of two numbers @{x} and @{y}."
    [] ["=pure_max(17,22)"] [];

Since this function only has fixed arguments, it will be called in curried
form, i.e., as ``pure_max x y``. For instance, the actual definition of
``pure_max`` may look as follows::

  pure_max x y = max x y;

Conversely, if no signature is given, then the function accepts any number of
parameters of any type, which are passed as a list. For instance::

  gnm_info "pure_sum" = gnm_help "pure_sum:sum of a collection of numbers"
    [] "Computes the sum of a collection of numbers."
    [] ["=pure_sum(1,2,3,4,5,6)"] ["pure_sums"];

Here the function will be called as ``pure_sum [x1,x2,...]``, where ``x1``,
``x2``, etc. are the arguments the function is invoked with. Note that in this
case there may be any number of arguments (including zero) of any type, so
your function definition must be prepared to handle this. If a function does
not have a ``gnm_info`` description at all then it is treated in the same
fashion. The ``pure_func.pure`` script contains some examples showing how to
write functions which can deal with any numbers of scalars, arrays or ranges,
see the ``pure_sum`` and ``pure_sums`` examples. These employ the following
``ranges`` function to "flatten" a parameter list to a list holding all
denoted values::

  ranges xs = cat [ case x of _::matrix = list x; _ = [x] end | x = xs ];

E.g., the ``pure_sum`` function can now be defined as follows::

  pure_sum xs = foldl (+) 0 (ranges xs);

A function may also have both fixed and optional arguments (note that in what
follows we're going to omit the detailed function descriptions for brevity)::

  gnm_info "foo" = "ff|ff";

In this case the fixed arguments are passed in curried form as usual, while
the optional parameters are passed as a list. That is, ``foo`` may be called
as ``foo x y []``, ``foo x y [z]`` or ``foo x y [z,t]``, depending on whether
it is invoked with two, three or four arguments.

Conversions Between Pure and Gnumeric Values
--------------------------------------------

The marshalling of types between Gnumeric and Pure is pretty straightforward;
basically, Pure numbers, strings and matrices map to Gnumeric numbers, strings
and arrays, respectively. The following table summarizes the available
conversions:

===============================    ===============================
Pure                               Gnumeric
===============================    ===============================
``gnm_error "#N/A"``               error
``4711``, ``4711L``, ``4711.0``    scalar (number)
``"Hello world"``                  string
``()``                             empty
``(1,2,3)``                        array
``[1,2,3]``                        array
``{1,2,3;4,5,6}``                  array (or cell range)
``"A1:B10"``                       cell range (``"r"`` conversion)
===============================    ===============================

These conversions mostly work both ways. Note that on input, cell ranges are
usually passed as matrices to Pure functions (i.e., they are passed "by
value"), unless the function signature specifies a ``"r"`` conversion in which
case the cell ranges themselves are passed to the function in string form.
(Such values can also be passed on to Gnumeric functions which expect a cell
range (``"r"`` ) parameter, see `Calling Gnumeric from Pure`_ below.)

Conversely, matrices, lists and tuples all become Gnumeric arrays on output,
so usually you'll want to enter these as array functions (``Ctrl-Shift-Enter``
in Gnumeric). As a special case, the empty tuple can be used to denote empty
cell values (but note that empty Gnumeric values may become zeros when passed
as float or array arguments to Pure functions).

Another special case is a term of the form ``gnm_error msg``, where ``msg`` is
a string value indicating a Gnumeric error value such as ``"#N/A"``,
``"#NAME?"``, ``"#NULL!"``, etc. When returned by a plugin function, the error
text will be displayed in the corresponding Gnumeric cell.

If a Pure function returns a value that doesn't match any of the above then it
is converted to a string in Pure expression syntax and that string is returned
as the result of the function invocation in Gnumeric. This makes it possible
to return any kind of symbolic Pure value, but note that if such a value is
then fed into another Pure function, that function will have to convert the
string value back to the internal representation if needed; this can be done
very conveniently using Pure's ``eval`` function, see the Pure documentation
for details.

Advanced Features
=================

This section explains various additional features provided by the
Gnumeric/Pure interface that should be useful for writing your own functions.
Note that for your convenience all functions discussed in this section are
declared in ``pure_func.pure``.

Calling Gnumeric from Pure
--------------------------

It is possible to call Gnumeric functions from Pure using the ``call``
function which takes the name of the function (a string) as its first, and the
parameters as the second (list) argument. For instance::

  gnm_info "gnm_bitand" = "ff";
  gnm_bitand x y = call "bitand" [x,y];

Note that ``call`` is an external C function provided by Gnumeric/Pure. If
you want to use it, it must be declared in your Pure script as follows::

  extern expr* pure_gnmcall(char* name, expr* args) = call;

However, ``pure_func.pure`` already contains the above declaration, so you
don't have to do this yourself if you import ``pure_func.pure`` in your
scripts.

Also note that ``call`` doesn't do any of Gnumeric's automatic conversions on
the parameters, so you have to pass the proper types of arguments as required
by the function.

Accessing Spreadsheet Cells
---------------------------

Gnumeric/Pure provides the following functions to retrieve and modify the
contents of spreadsheet cells and ranges of such cells::

  extern expr* pure_get_cell(char* s) = get_cell;
  extern expr* pure_get_cell_text(char* s) = get_cell_text;
  extern expr* pure_get_cell_format(char* s) = get_cell_format;
  extern expr* pure_set_cell(char* s, expr *x) = set_cell;
  extern expr* pure_set_cell_text(char* s, expr *x) = set_cell_text;
  extern expr* pure_set_cell_format(char* s, expr *x) = set_cell_format;
  extern expr* pure_get_range(char* s) = get_range;
  extern expr* pure_get_range_text(char* s) = get_range_text;
  extern expr* pure_get_range_format(char* s) = get_range_format;
  extern expr* pure_set_range(char* s, expr *x) = set_range;
  extern expr* pure_set_range_text(char* s, expr *x) = set_range_text;
  extern expr* pure_set_range_format(char* s, expr *x) = set_range_format;

For instance, here's how you use these functions to write and then read some
cell values (try this in the `interactive Pure shell`_)::

  > set_cell "A14" 42
    ()
  > get_cell "A14"
    42.0
  > set_range "A14:G14" $ scanl (*) 1 (1..6)
    ()
  > get_range "A14:G14"
    {1.0,1.0,2.0,6.0,24.0,120.0,720.0}
  > set_cell_text "A14" "=sum(B14:G14)"
    ()
  > get_cell "A14"
    873.0
  > get_cell_text "A14"
    "=sum(B14:G14)"
  > get_range_text "A14:G14"
    {"=sum(B14:G14)","1","2","6","24","120","720"}

Note that while the ``set_cell`` function sets the given cell to a constant
value, ``set_cell_text`` also allows you to store a formula in a cell which
will then be evaluated as usual. Similarly, ``get_cell`` retrieves the cell
value, while ``get_cell_text`` yields the text in the cell, as entered by the
user (which will either be a formula or the textual representation of a
constant value). The ``set_range``, ``set_range_text``, ``get_range`` and
``get_range_text`` functions work analogously, but are used to manipulate
entire ranges of cells, which can be set from Pure tuples, lists or matrices,
and retrieved as Pure matrices.

Functions to retrieve and change the cell format are also provided (watch the
contents of the cell ``A14`` change its color to blue on entering the first
expression)::

  > set_cell_format "A14" "[Blue]0.00"
    ()
  > get_range_format "A14:C14"
    {"[Blue]0.00","General","General"}

There are also functions to get the position of the "current" cell (i.e., the
cell from which a Pure function was called), and to translate between cell
ranges in Gnumeric syntax and the corresponding internal representation
consisting of a pointer to a Gnumeric sheet and the cell or range
coordinates::

  extern expr* pure_this_cell() = this_cell;
  extern expr* pure_parse_range(char* s) = parse_range;
  extern expr* pure_make_range(expr* x) = make_range;

Examples::

  > this_cell
    "B4"
  > parse_range this_cell
    #<pointer 0x875220>,1,3
  > make_range (NULL,0,0,10,10)
    "Sheet2!A1:K11"

Asynchronous Data Sources
-------------------------

Gnumeric/Pure makes it easy to set up asynchronous data sources which draw
values from a Pure computation executed in a background process. This facility
is useful to carry out lengthy computations in the background while you can
continue to work with your spreadsheet. It also allows you to process incoming
data and asynchronous events from special devices (MIDI, sensors, stock
tickers, etc.) in (soft) realtime.

To do this, you simply pass an expression to the ``datasource`` function. This
is another external C function provided by Gnumeric/Pure, which is declared in
``pure_func.pure`` as follows::

  extern expr* pure_datasource(expr* x) = datasource;

The argument to ``datasource`` is typically a thunk or stream (lazy list)
which is to be evaluated in the background. The call to ``datasource``
initially returns a ``#N/A`` value (``gnm_error "#N/A"``) while the
computation is still in progress. The cell containing the data source then
gets updated automatically as soon as the value becomes available, at which
point the ``datasource`` call now returns the computed value. E.g., here's how
you would wrap up a lengthy calculation as a thunk and submit it to
``datasource`` which carries out the computation as a background task::

  gnm_info "pure_frob" = "f";
  pure_frob x = datasource (lengthy_calculation x&);
  lengthy_calculation x = sleep 3 $$ foldl (*) 1 (1..x);

Note that a cell value may draw values from as many independent data sources
as you want, so the definition of a cell may also involve multiple invocations
of ``datasource``::

  gnm_info "pure_frob2" = "ff";
  pure_frob2 x y = datasource (lengthy_calculation x&),
    datasource (lengthy_calculation y&);

Special treatment is given to (lazy) lists, in this case ``datasource``
returns a new value each time a list element becomes available. For instance,
the following function uses an infinite stream to count off the seconds
starting from a given initial value::

  gnm_info "pure_counter" = "f";
  pure_counter x = datasource [sleep (i>x) $$ i | i = x..inf];

You can also try this interactively in the Pure shell::

  > datasource [sleep (i>0) $$ i | i = 0..inf]
    0
    1
    ...

Here's another example for the Pure shell which prints the prime numbers::

  > datasource primes with primes = sieve (2..inf);
      sieve (p:qs) = p : (sleep 1 $$ sieve [q | q = qs; q mod p])& end
    2
    3
    5
    ...

Note that when processing a lazy list, the cell containing the call will keep
changing as long as new values are produced (i.e., forever in this
example). The "Stop Data Sources" option in the Tools/Pure menu can be used to
stop all active data sources. "Reload Pure Scripts" also does this. You can
then restart the data sources at any time by using "Recalculate" (``F9``) to
recompute the spreadsheet.

Also note that because of the special way that ``datasource`` handles list
values, you cannot return a list directly as the result of ``datasource``, if
it is to be treated as a single result. Instead, you'll have to wrap the
result in a singleton list (e.g., ``datasource [[lengthy_calculation
x,lengthy_calculation y]&]``), or return another aggregate (i.e., a matrix or
a tuple).

Finally, note that when the arguments of a call involving ``datasource``
change (because they depend on other cells which may have been updated), the
computation is automatically restarted with the new parameters. The default
behaviour in this case is that the entire computation will be redone from
scratch, but it's also possible to wrap up calls to ``datasource`` in a manner
which enables more elaborate communication between Gnumeric and background
tasks initiated with ``datasource``. This is beyond the scope of this manual,
however, so we leave this as an exercise to the interested reader.

Triggers
--------

In addition to asynchronous data sources, the ``trigger`` function is provided
to compute values or take actions depending on some external condition, such
as the availability of data on a special device or the creation of some widget
(see the next section)::

  extern expr* pure_trigger(int timeout, expr* cond, expr *val, expr *data)
    = trigger;

Thus a typical invocation of the function looks as follows::

  trigger timeout condition value data

The ``condition`` and ``value`` arguments are callback functions which get
invoked by ``trigger``, passing them the given ``data`` argument. The trigger
reevaluates the given condition in regular intervals (1 second in the current
implementation) and, as soon as it becomes ``true``, computes the given value
and returns that value as the result of the ``trigger`` call. As long as the
condition doesn't hold, ``trigger`` returns a ``#N/A`` value (``gnm_error
"#N/A"``). Note that, in difference to ``datasource``, both the condition and
the value are computed in Gnumeric (rather than a child process), so that it
is possible to access the current information in the loaded spreadsheet.

The ``timeout`` value determines how often the condition is checked. If it is
positive, the condition will be reevaluated ``timeout+1`` times (once
initially, and then once per second for a total duration of ``timeout``
seconds). If it is negative, the trigger never times out and the condition
will be checked repeatedly until the trigger expression is removed (or
Gnumeric is exited). In either case ``value data`` will be recomputed each
time ``condition data`` yields ``true``. (This is most useful if the computed
value, as a side effect, arranges for the condition to become ``false`` again
afterwards.) Finally, if ``timeout`` is zero then the trigger fires at most
once, as soon as the condition becomes ``true``, at which point ``value data``
is computed just once.

Here's a (rather useless) example of a trigger which fires exactly once, as
soon as a certain cell goes to a certain value, and then modifies another cell
value accordingly::

  > trigger 0 (\_->get_cell "A14"==="Hello") (\_->set_cell "A15" "World") ()

Now, as soon as you type ``Hello`` in the cell A14, the trigger will print
``World`` in cell A15. Note that the ``data`` argument isn't used here. A
more useful example will be discussed in the following section.

Sheet Objects
-------------

Gnumeric offers some kinds of special objects which can be placed on a
sheet. This comprises the chart and image objects which can be found in the
"Insert" menu, as well as a number of useful graphical elements and GUI
widgets on the "Object" toolbar, accessible via "View/Toolbars". The latter
are also useful for providing control input to Pure functions.

Gnumeric/Pure provides the following function to retrieve information about
the special objects in a spreadsheet::

  extern expr* pure_sheet_objects() = sheet_objects;

For instance, with one button object in your spreadsheet, the output of
``sheet_objects`` might look like this::

  > sheet_objects
    [("Sheet1","button","Push Me!","A11",[#<pointer 0x2a1dcd0>])]

Each object is described by a tuple which lists the name of the sheet on which
the object is located, the type of object, the object's content or label (if
applicable), the cell which the object is linked to (if applicable), and a
list of pointers to the corresponding ``GtkWidgets`` (if any). Note that in
general a GUI object may be associated with several widgets, as Gnumeric
allows you to have multiple views on the same spreadsheet, so there will be
one widget for each view an object is visible in. Also note that the
content/label information depends on the particular type of object:

* List and combo widgets return the content link (referring to the cells in
  the spreadsheet holding the items shown in the list).

* Frame and button widgets return the label shown on the widget.

* Graphic objects like rectangles and ellipses return the text content of the
  object.

* Image objects (type ``"image/xyz"``, where ``xyz`` is the type of image,
  such as ``svg`` or ``png``) return a pointer to the image data in this
  field.

The ``sheet_objects`` function is a bit tricky to use, since some of the
objects or their associated widgets might not have been created yet when the
spreadsheet is loaded. Therefore it is necessary to use a trigger to make sure
that the information is updated once all objects are fully displayed. The
``pure_func.pure`` script contains the following little wrapper around
``sheet_objects`` which does this::

  pure_objects = trigger 0 (\_->all realized sheet_objects)
    (\_->matrix$map list sheet_objects) ()
  with realized (_,_,_,_,w) = ~listp w || ~null w && ~any null w end;

See the ``widgets.gnumeric`` spreadsheet in the distribution for an example.

Possible uses of this facility are left to your imagination. Using Gnumeric's
internal APIs and Pure's Gtk interface, you might manipulate the GUI widgets
in various ways (add icons to buttons or custom child widgets to frames,
etc.).
