# Interpreters

It is pretty common for applications to offer extension capabilities by means of a simple "extension language". Usually this language is invented for the particular application, rather than simply picking one of the many available interpreters. This forces users to learn a new language just to be able to use the program.

In contrast, early versions of Siag used Scheme as its interpreter. The implementation chosen was SIOD by George Carrette. This concept has now been taken even further by adding support for multiple interpreters.

The supporting code for each language resides in its own module. As an example, Siag can currently be linked with support for these interpreters:

SIOD
  An extremely small Scheme interpreter which cooperates exceptionally well with C.
Guile
  Another Scheme interpreter. This one follows the R4RS more closely than SIOD, which can be important if external Scheme programs are to be used in Siag.
Tcl
  Tcl is another popular embeddable scripting language.
C
  Expressions in Siag can also be written in a simple C-like syntax.

Pathetic Writer and Egon Animator can be linked with SIOD, Guile and Tcl.

## Cross-calling

One interpreter can call another using a string-based interface. In SIOD, a call to make Tcl beep can look like this:

    (exec-expr 'Tcl "puts \\a")

And Tcl can tell Guile to collect garbage in the following fashion:

    exec-expr Guile "(gc)"