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9.2.5 The SCM Type in Guile

Guile classifies Scheme objects into two kinds: those that fit entirely within an SCM, and those that require heap storage.

The former class are called immediates. The class of immediates includes small integers, characters, boolean values, the empty list, the mysterious end-of-file object, and some others.

The remaining types are called, not surprisingly, non-immediates. They include pairs, procedures, strings, vectors, and all other data types in Guile. For non-immediates, the SCM word contains a pointer to data on the heap, with further information about the object in question is stored in that data.

This section describes how the SCM type is actually represented and used at the C level. Interested readers should see libguile/tags.h for an exposition of how Guile stores type information.

In fact, there are two basic C data types to represent objects in Guile: SCM and scm_t_bits.