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18 Data Representation in Guile

by Jim Blandy

[Due to the rather non-orthogonal and performance-oriented nature of the SCM interface, you need to understand SCM internals *before* you can use the SCM API. That's why this chapter comes first.]

[NOTE: this is Jim Blandy's essay almost entirely unmodified. It has to be adapted to fit this manual smoothly.]

In order to make sense of Guile's SCM_ functions, or read libguile's source code, it's essential to have a good grasp of how Guile actually represents Scheme values. Otherwise, a lot of the code, and the conventions it follows, won't make very much sense. This essay is meant to provide the background necessary to read and write C code that manipulates Scheme values in a way that is compatible with libguile.

We assume you know both C and Scheme, but we do not assume you are familiar with Guile's implementation.

• Data Representation in Scheme: Why things aren't just totally straightforward, in general terms.
• How Guile does it: How to write C code that manipulates Guile values, with an explanation of Guile's garbage collector.
• Defining New Types (Smobs): How to extend Guile with your own application-specific datatypes.