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(guile.info)Data types and constants defined by gh

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Data types and constants defined by gh
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The following C constants and data types are defined in gh:

 - Data type: SCM
     This is a C data type used to store all Scheme data, no matter
     what the Scheme type.  Values are converted between C data types
     and the SCM type with utility functions described below (Note:
     Converting data between C and Scheme).  [FIXME: put in
     references to Jim's essay and so forth.]

 - Constant: SCM_BOOL_T
 - Constant: SCM_BOOL_F
     The _Scheme_ values returned by many boolean procedures in
     libguile.

     This can cause confusion because they are different from 0 and 1.
     In testing a boolean function in libguile programming, you must
     always make sure that you check the spec: `gh_' and `scm_'
     functions will usually return `SCM_BOOL_T' and `SCM_BOOL_F', but
     other C functions usually can be tested against 0 and 1, so
     programmers' fingers tend to just type `if (boolean_function()) {
     ... }'

 - Constant: SCM_UNSPECIFIED
     This is an SCM object which does not correspond to any legal Scheme
     value.  It can be used in C to terminate functions with variable
     numbers of arguments, such as `gh_list()'.