Asymptote Proposal - External Modules

# Asymptote Proposal &mdash External Modules

## Overview

External modules allow users to extend Asymptote by calling functions written in another programming language.

Users do this by writing a .asyc file, which contains a mix of Asymptote code and code from another language, say C++. Then, a program is run which produces a .asy file and a C++ source file. The C++ file is compiled to produce a shared library file. Then, the .asy file can be imported in Asymptote to use the externally defined features.

This spec is describes a proposed feature that has not yet been implemented. It is incomplete, and does not address all of the issues involved in implementing the feature.

## Example

Let’s look at a simple example that shows off the main features. Asymptote currently doesn’t offer a way to read the contents of a directory. This would be useful if, say, we wanted to make a series of graphs for every .csv file in a directory.

    /*****
     * dir.asyc
     * Andy Hammerlindl 2007/09/11
     *
     * An example for the proposed external module support in Asymptote.  This reads
     * the contents of a directory via the POSIX commands.
     *
     * Example usage in asymptote:
     *   access dir;
     *   dir.entry[] entries= dir.open('.');
     *   for (dir.entry e : entries)
     *     write(e.name);
     *****/

    // Verbatim code will appear in the c++ or asy file (as specified) interleaved
    // in the same order as it appears here.
    verbatim c++ {
      #include <sys/types.h>
      #include <dirent.h>
      #include <errno.h>

      // asy.h is included by default (needed for hidden code, anyway).
      // Asymptote-specific types, such as array below, are in the asy namespace.
      using namespace asy;
    }

    // Define a new opaque type in asy which is internally represented by struct
    // dirent *.  This is too messy to expose to users of the module, so define
    // everything as private.
    private asytype const struct dirent *entry_t;

    private int entry_d_ino(entry_t e) {
      return (Int)e->d_ino;
    }

    private int entry_d_off(entry_t e) {
      return (Int)e->d_off;
    }

    private int entry_d_reclen(entry_t e) {
      return (Int)e->reclen;
    }

    private string entry_d_type(entry_t e) {
      return string( /*length*/ 1, e->d_type);
    }

    private string entry_d_name(entry_t e) {
      return string(e->d_name);
    }

    // Define an asy structure to expose the information.  These steps are annoying,
    // but straightforward, and not too hard to plow through.
    verbatim asy {
      struct entry {
        restricted int ino;
        restricted int off;
        restricted int reclen;
        restricted int type;
        restricted string name;

        void operator init(entry_t e) {
          ino=entry_d_ino(e);
          off=entry_d_off(e);
          reclen=entry_d_reclen(e);
          type=entry_d_type(e);
          name=entry_d_name(e);
        }
      }
    }

    // Given the name of a directory, return an array of entries.  Return 0
    // (a null array) on error.
    private entry_t[] base_read(string name)
    {
      DIR *dir=opendir(name.c_str());

      // TODO: Add standard style of error reporting.
      if (dir == NULL)
        return 0;

      // Create the array structure.
      // array is derived from gc, so will be automatically memory-managed.
      array *a=new array();

      struct dirent *entry;
      while (entry=readdir(dir))
        a->push<struct dirent *>(entry);

      // The loop has exited, either by error, or after reading the entire
      // directory.  Check before closedir(), in case that call resets errno.
      if (errno != 0) {
        closedir(dir);
        return 0;
      }

      closedir(dir);
      return a;
    }

    verbatim asy {
      private entry[] cleanEntries(entry_t[] raw_entries) {
        if (raw_entries) {
          entry[] entries;
          for (entry_t e : raw_entries)
            entries.push(entry(e));
          return entries;
        }
        return null;
      }

      entry[] read(string name) {
        return cleanEntries(base_read(name));
      }
    }

## Type Mappings

Types in Asymptote do not directly relate to types in C++, but there is a partial mapping between them. The header file `asymptote.h` provides typedefs for the primitive asymptote types. For instance `string` in Asymptote maps to the C++ class `asy::string` which is a variant of `std::string` and `real` to `asy::real` which is a basic floating point type (probably `double`). Because `int` is a reserved word in C++, the Asymptote type `int` is mapped to `asy::Int` which is one of the basic signed numeric types in C++ (currently 64 bit). `asy::pair` is a class that implements complex numbers. In the first version of the external module implementation, these will be the only primitive types with mappings, but eventually all of them will be added.

All Asymptote arrays, regardless of the cell type, are mapped to `asy::array *` where `asy::array` is a C++ class. The cells of the array are of the type `asy::item` which can hold any Asymptote data type. Items can be constructed from any C++ type. Once constructed, the value of an item can be retrieved by the function `template<typename T> T get(const item&)`. Calling `get` on an item using the wrong type generates a runtime error.

    // Examples of using item.
    item x((asy::Int)2);
    item y(3.4);
    item z=new array;
    item w=(asy::real)3.4;

    cout << get<asy::Int>(x);
    cout << get<double>(y);

    x=y;  // x now stores a double.
    cout << get<double>(x);

    cout << get<asy::real>(w);

The `asy::array` class implements, at a minimum, the methods:

- `size_t size()` which returns the number of elements,
- `template <typename T> T read(size_t i) const` which returns the i-th element, interpreted as being of type t.
- `template <typename T> void push(item i)` adds the item to the end of the array.

It allows access to elements of the array as items by `operator[]`. We may specify that `asy::array` be a model of the Random Access Container in the C++ Standard Template Library. It is currently implemented as a subclass of an STL vector.

    // Example of a C++ function that doubles the entries in an array of integers.
    using namespace asy;

    void doubler(array *a) {
      assert(a);
      size_t length=a->size();
      for (size_t i=0; i<length; ++i) {
        Int x=a->read<Int>(i);  // This is shorthand for get<Int>((*a)[i]).
        a[i]=2*x;               // The type of 2*x is also Int, so this will enter
                                // the item as the proper type.
      }
    }

Users can map new Asymptote types to their own custom C++ types using Opaque Type Declarations, explained below.

## Syntactic Features

A .asyc file is neither an asy file with some C++ in it, nor a C++ with some asy code in it. It can only contain a small number of specific constructs:

- Comments
- Function Definitions
- Verbatim Code Block
- Opaque Type Declaration

Each component may produce code for either the .asy file, the .cc file, or both. The pieces of code produced by each construct appears in the output file in the same order as the constructs in the .asyc. For example, if a function definition occurs before a verbatim Asymptote code block, we can be sure that the function is defined and can be used in that block. Similarly, if a verbatim C++ block occurs before a function definition, then the body of the function can use features declared in the verbatim section.

### Comments

C++/Asymptote style comments using `/* */` or `//` are allowed at the top level. These do not affect the definition of the module, but the implementation may copy them into the .asy and .cc to help explain the resulting code.

### Verbatim Code Blocks

Verbatim code, ie. code to be copied directly into the either the output .asy or .cc file can be specified in the .asyc file by enclosing it in a verbatim code block. This starts with the special identifier `verbatim` followed by either `c++` or `asy` to specify into which file the code will be copied, and then a block of code in braces. When the .asyc file is parsed, the parser keeps track of matching open and close braces inside the verbatim code block, so that the brace at the start of the block can be matched with the one at the end. This matching process will ignore braces occuring in comments and string and character literals.

#### Open issue

It may prove to be impractical to walk through the code, matching braces. Also, this plan precludes having a verbatim block with an unbalanced number of braces which might be useful, say to start a namespace at the beginning of the C++ file, and end it at the end of the file. As such, it may be useful to have another technique. A really simple idea (with obvious drawbacks) would be to use the first closing braces that occur at the same indentation level as the verbatim keyword (assuming that the code block itself will be indented). Other alternatives are to use more complicated tokens such as `%{` and `%}`, or the shell style `<<EOF`.

### Function Definitions

A function definition given at the top level of the file (and not inside a verbatim block) looks much like a function definition in Asymptote or C++, but is actually a mix of both. The header of the function is given in Asymptote code, and defines how the function will look in the resulting Asymptote module. The body, on the other hand, is given in C++, and defines how the function is implemented in C++. As a simple example, consider:

    real sum(real x, real y=0.0) {
      return x+y;
    }

#### Header

The header of the definition gives the name, permission, return type, and parameters of the function. Because the function is defined for use in Asymptote, all of the types are given as Asymptote types.

##### Permissions

As in pure Asymptote, the function can optionally be given a `private`, `restricted` or `public` permission. If not specified, the permission is `public` by default. This is the permission that the function will have when it is part of the Asymptote module. The example of `sum` above specifies no permission, so it is public.

Just as public methods such as `plain.draw` can be re-assigned by scripts that import the `plain` module, the current plan is to allow Asymptote code to modify public members of any module, including ones defined using native code. This is in contrast to builtin functions bindings, which cannot be modified.

##### Return Type

This gives the Asymptote return type of the function. This cannot be an arbitrary Asymptote type, but must one which maps to a C++ type as explained in the type mapping section above. Our example of `sum` gives `real` as a return type, which maps to the C++ type `asy::real`.

##### Function Name

This gives the name of the function as it will appear in the Asymptote module. In our example, the Asymptote name is `sum`. The name can be any Asymptote identifier, including operator names, such as `operator +`.

It is important to note that the Asymptote name has no relation to the C++ name of the function, which may be something strange, such as `_asy_func_modulename162`. Also, the actual signature and return type of the C++ function may bear no relation to the Asymptote signature. That said, the C++ name of the function may be defined by giving the function name as `asyname:cname`. Then it can be referred to by other C++ code. The function will be defined with C calling convention, so that its name is not mangled.

##### Formal Parameters

The function header takes a list of formal parameters. Just as in pure Asymptote code, these can include `explicit` keywords, type declarations with array and functional types, and rest parameters. Just as with the return type of the function, the type of each of the parameters must map to a C++ type.

Parameters may be given an optional Asymptote name and an optional C++ name. These may be declared in one of six ways as in the following examples:

    void f(int)
    void f(int name)
    void f(int :)
    void f(int asyname:)
    void f(int :cname)
    void f(int asyname:cname)

If the parameter just contains a type, with no identifier, then it has no Asymptote name and no C++ name. If it contains a single name (with no colon), then that name is both the Asymptote and the C++ name. If it contains a colon in the place of an identifier, with an optional name in front of the colon and an optional name behind the colon, than the name in front (if given) is the Asymptote name, and the name behind (if given) is the C++ name.

The Asymptote name can be any Asymptote identifier, including operator names, but the C++ name must be a valid C++ identifier. For instance `void f(int operator +)` is not allowed, as the parameter would not have a valid C++ name. The examples `void f(int operator +:)` and `void f(int operator +:addop)` are allowed.

When called by Asymptote code, named arguments are only matched to the Asymptote names, so for example a function defined by `void f(int :x, string x:y)` could be called by `f(x="hi mom", 4)`, but one defined by `void f(int x, string x:y)` could not.

Each formal parameter may take a piece of code as a default value. Because the function is implemented in C++, this code must be given as C++ code. More akin to Asymptote than C++, default arguments may occur for any non-rest parameters, not just those at the end of the list, and may refer to earlier parameters in the list. Earlier parameters are refered to by their C++ names. Example:

    void drawbox(pair center, real width, real height=2*width, pen p)

Default arguments are parsed by finding the next comma that is not part of a comment, string literal, or character constant, and is not nested inside parentheses. The C++ code between the equals-sign and the comma is taken as the expression for the default argument.

#### Body

The body of the function is written as C++ code. When the .asyc file is processed, this C++ code is copied verbatim into an actual C++ function providing the implementation. However, the actual body of the resultant C++ function may contain code other than the body provided by the user. This auxiliary code could include instruction to retrieve the arguments of the function from their representation in the Asymptote virtual machine and bind them to local variables with their C++ names. It could also include initialization and finalization code for the function.

In writing code for the function body, one can be assured that all function arguments with C++ names have been bound and are therefore usable in the code. Since all parameters must have Asymptote types that map to C++ types, the types of the paramaters in the body have the type resulting from that mapping.

The `return` keyword can be used to return the result of the function (or without an expression, if the return type was declared as void). The Asymptote return type must map to a C++ type, and the expression given in the return statement will be implicitly cast to that type.

Since the implementation will likely not use an actual return statement to return the value of the function back to the Asymptote virtual machine, the interpreter of the .asyc file may walk through the code converting return expressions into a special format in the actual implementation of the function.

### Opaque Type Declarations

There are a number of mappings between Asymptote and C++ types builtin to the facility. For instance `int` maps to `asy::Int` and `real` to `asy::real`. Users, however, may want to reference other C++ objects in Asymptote code. This done though opaque type declarations.

An opaque type declaration is given by an optional permission modifier, the keyword `asytype`, a C++ type, and an Asymptote identifier; in that order.

    // Examples
    asytype char char;
    public asytype const std::list<asy::Int> *intList;
    private asytype const struct dirert *entry_t;

This declaration mapping the Asymptote identifier to the C++ type within the module. The permission of the Asymptote type is given by the permission modifier (or public if the modifier is omitted). The type is opaque, in that none of its internal structure is revealed in the Asymptote code. Like any other type, however, objects of this new type can be returned from functions, given as an arguments to functions, and stored in variables, structures and arrays.

In many cases, such as the directory listing example at the start, it will be practical to declare the type as private, and use an Asymptote structure as a wrapper hiding the C++ implementation.