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

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 19.3 Data types and constants defined by gh
 ===========================================
 
 The following C constants and data types are defined in gh:
 
    `SCM' 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 ( 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 a SCM value that is not the same as any legal Scheme
      value.  It is the value that a Scheme function returns when its
      specification says that its return value is unspecified.
 
  -- Constant: SCM_UNDEFINED
      This is another SCM value that is not the same as any legal Scheme
      value.  It is the value used to mark variables that do not yet
      have a value, and it is also used in C to terminate functions with
      variable numbers of arguments, such as `gh_list()'.
 
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