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A Scheme object may either be an immediate, i.e. carrying all necessary information by itself, or it may contain a reference to a cell with additional information on the heap. Although in general it should be irrelevant for user code whether an object is an immediate or not, within Guile's own code the distinction is sometimes of importance. Thus, the following low level macro is provided:
A Scheme object is an immediate if it fulfills the
SCM_IMPpredicate, otherwise it holds an encoded reference to a heap cell. The result of the predicate is delivered as a C style boolean value. User code and code that extends Guile should normally not be required to use this macro.
Summary:
SCM_IMP (x) if it is an immediate object.
scm_t_bits value that is delivered by SCM_UNPACK
(x).