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# pure-octave

Version 0.2, June 03, 2012

Albert Graef <Dr.Graef@t-online.de>

A Pure interface to GNU Octave.

- Introduction
- Copying
- Installation
- Basic Usage
- Direct Function Calls
- Data Conversions
- Calling Back Into Pure
- Caveats and Notes

## Introduction

This is an Octave module for the Pure programming language, based on Paul Kienzle’s octave_embed which allows Octave to be embedded in other languages. It allows you to execute arbitrary Octave commands and Octave functions from Pure.

## Copying

pure-octave 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.

pure-octave 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.

Please see the accompanying COPYING file for the precise license terms. The GPL can also be read online at http://www.gnu.org/licenses/.

## Installation

Get the latest source from http://pure-lang.googlecode.com/files/pure-octave-0.2.tar.gz.

Run `make` to compile the module and `make install` (as root) to install it in the Pure library directory. This requires GNU make, and of course you need to have both Pure and Octave installed (including Octave’s `mkoctfile` utility and the corresponding header files and libraries).

`make` tries to guess your Pure installation directory and platform-specific setup. If it gets this wrong, you can set some variables manually. In particular, `make install prefix=/usr` sets the installation prefix. Please see the Makefile for details.

NOTE: This release of pure-gen has been tested with Octave 3.4.0. Older versions might require some fiddling with the sources to get the embedded Octave interface working.

## Basic Usage

Import this module into your Pure scripts as follows:

    using octave;

This will add an embedded instance of the Octave interpreter to your program. (You can import this module as often as you want, but there’s always only one instance of Octave in each process.)

`octave_eval` s

  Executes arbitrary Octave code.

    > octave_eval "eig([1 2;3 4])";
    ans =

      -0.37228
       5.37228

    0

This prints the result on stdout and returns a result code (zero if everything went fine). To suppress the printing of the result, simply terminate the Octave statement with a semicolon:

    > octave_eval "eig([1 2;3 4]);";
    0

`octave_set` var val
`octave_get` var

  Set and get global variables in the Octave interpreter.

This allows you to define values to be used when evaluating Octave code, and to transfer results back to Pure. However, before such globals can be accessed in Octave, you must explicitly declare them as globals:

    > octave_eval "global x y ans";
    0

Now you can use `octave_set` and `octave_get` to transfer values between Pure and Octave as follows:

    > octave_set "x" {1.0,2.0;3.0,4.0};
    {1.0,2.0;3.0,4.0}
    > octave_eval "eig(x);";
    0
    > octave_get "ans";
    {-0.372281323269014;5.37228132326901}

Note that the most recent result can always be accessed through Octave’s `ans` variable. You can also use an explicit variable definition as follows:

    > octave_eval "y = eig(x);";
    0
    > octave_get "y";
    {-0.372281323269014;5.37228132326901}

## Direct Function Calls

`octave_call` fun n args

  Call an octave function in a direct fashion. `fun` denotes the name of the function, `n` the number of function results and `args` the function arguments.

    > let x = {1.0,2.0;3.0,4.0};
    > octave_call "eig" 1 x;
    {-0.372281323269014;5.37228132326901}

Note the second argument, which denotes the desired number of return values. This will usually be 1 (or 0 if you don’t care about the result), but some Octave functions may return a variable number of results, depending on how they’re called. Multiple values are returned as tuples in Pure:

    > octave_call "eig" 2 x;
    {-0.824564840132394,-0.415973557919284;0.565767464968992,-0.909376709132124},
    {-0.372281323269014,0.0;0.0,5.37228132326901}

If there are multiple arguments, you can specify them either as a tuple or a list:

    > octave_call "norm" 1 (x, 2);
    5.46498570421904
    > octave_call "norm" 1 [x, 1];
    6.0

Instead of a function name, you can also specify the function to be called using a special kind of Octave object, a function value. These are returned, e.g., by Octave’s `str2func` and `inline` builtins. For your convenience, pure-octave provides a frontend to these builtins, the `octave_func` function, which lets you specify an Octave function in one of two ways:

`octave_func` name

  Returns the Octave function with the given name. This works like Octave’s `str2func` builtin.

`octave_func` expr

  Returns an “inline” function, where `expr` is an Octave expression (as a string) describing the function value. This works like Octave’s `inline` builtin. Instead of just an Octave expression, you can also specify a tuple or a list consisting of the inline expression and the parameter names. (Otherwise the parameters are determined automatically, see the description of the `inline` function in the Octave manual for details.)

Note that inline functions allow you to call stuff that is not an Octave function and hence cannot be specified directly in `octave_call`, such as an operator. Examples:

    > let eig = octave_func "eig";
    > let mul = octave_func "x*y";
    > octave_call eig 1 (octave_call mul 1 (x,x));
    {0.138593383654928;28.8614066163451}
    > let add = octave_func ("x+y","x","y");
    > octave_call add 1 (x,x);
    {2.0,4.0;6.0,8.0}

## Data Conversions

As shown above, the `octave_set`, `octave_get` and `octave_call` functions convert Pure data to corresponding Octave values and vice versa. Octave scalars and matrices of boolean, integer, double, complex and character data are all supported by this interface, and are mapped to the corresponding Pure data types in a straightforward manner (scalars to scalars, matrices to matrices and strings to strings). Note that in any case these conversions create copies of the data, so modifying, say, an Octave matrix received via `octave_get` in Pure only affects the Pure copy of the matrix and leaves the original Octave matrix unchanged.

Any other kind of Octave object (including Octave function objects, see Direct Function Calls) is just passed through as is, in the form of a cooked pointer to an Octave value which frees itself when garbage-collected. You can check for such objects with the `octave_valuep` predicate:

`octave_valuep` x

  Check for Octave value pointers.

    > let eig = octave_func "eig";
    > eig; octave_valuep eig;
    #<pointer 0x230dba0>
    1

Such Octave value pointers can be used freely whereever an Octave value is needed (i.e., in invocations of `octave_set` and `octave_call`).

You should also note the following:

- Octave’s cell arrays and structures are not supported at this time, so they will show up as opaque Octave value pointers in Pure land. This allows these objects to be passed around freely, but you can’t inspect or modify them in Pure.

- Scalars and 1x1 matrices are indistinguishable in Octave, thus any 1x1 matrix will be returned as a scalar from Octave to Pure.

- All types of boolean and integer matrices are returned from Octave to Pure as (machine) integer matrices. When converted back to Octave, these all become Octave `int32` matrices, but you can easily convert them to boolean or other types of matrices in Octave as needed. For instance:

      > octave_set "a" {1,2;3,4};
      {1,2;3,4}
      > octave_eval "global a ans";
      0
      > octave_eval "eig(a)";
      error: eig: wrong type argument `int32 matrix'
      1
      > octave_eval "eig(double(a))";
      ans =

        -0.37228
         5.37228

      0
      > octave_eval "a>2";
      ans =

         0   0
         1   1

      0
      > octave_get "ans";
      {0,0;1,1}

- Octave strings are mapped to Pure strings, and character matrices with more than one row are mapped to (symbolic) column vectors of Pure strings. Example:

      > octave_set "a" "Hello, world!";
      "Hello, world!"
      > octave_eval "a";
      a = Hello, world!
      0
      > octave_eval "[a;'abc']";
      ans =

      Hello, world!
      abc

      0
      > octave_get "ans";
      {"Hello, world!";"abc          "}

## Calling Back Into Pure

The embedded Octave interpreter provides one special builtin, the `pure_call` function which can be used to call any function defined in the executing Pure script from Octave. For instance:

    > even m::matrix = {~(int x mod 2) | x=m};
    > octave_eval "pure_call('even', 1:12)";
    ans =

      0  1  0  1  0  1  0  1  0  1  0  1

    0

Here’s the description of the `pure_call` function, as it is printed with Octave’s `help` command:

    `pure_call' is a built-in function

      RES = pure_call(NAME, ARG, ...)
      [RES, ...] = pure_call(NAME, ARG, ...)

      Execute the Pure function named NAME (a string) with the given arguments.
      Arguments and result types may be scalars and matrices of boolean, integer,
      double, complex and character data. The Pure function may return multiple
      results as a tuple. Example: pure_call('succ', 99) => 100.

## Caveats and Notes

Directly embedding Octave in Pure programs is convenient because it allows easy exchange of data between Pure and Octave, and you can also call Octave functions directly from Pure and vice versa. However, it also comes at a cost. A default build of Octave pulls in quite a few dependencies of its own which might conflict with other modules loaded in a Pure script. Specifically, we have found that older Octave versions may give problems with third-party graphics libraries such as VTK, if used in the same program as Octave. (These seem to be fixed in the latest Octave and VTK versions, however.)

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