(guile.info.gz) Unique Values

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(guile.info.gz) Immediate Datatypes

 
 18.2.4.4 Unique Values
 ......................
 
 The immediate values that are neither small integers, characters, nor
 booleans are all unique values -- that is, datatypes with only one
 instance.
 
  -- Macro: SCM SCM_EOL
      The Scheme empty list object, or "End Of List" object, usually
      written in Scheme as `'()'.
 
  -- Macro: SCM SCM_EOF_VAL
      The Scheme end-of-file value.  It has no standard written
      representation, for obvious reasons.
 
  -- Macro: SCM SCM_UNSPECIFIED
      The value returned by expressions which the Scheme standard says
      return an "unspecified" value.
 
      This is sort of a weirdly literal way to take things, but the
      standard read-eval-print loop prints nothing when the expression
      returns this value, so it's not a bad idea to return this when you
      can't think of anything else helpful.
 
  -- Macro: SCM SCM_UNDEFINED
      The "undefined" value.  Its most important property is that is not
      equal to any valid Scheme value.  This is put to various internal
      uses by C code interacting with Guile.
 
      For example, when you write a C function that is callable from
      Scheme and which takes optional arguments, the interpreter passes
      `SCM_UNDEFINED' for any arguments you did not receive.
 
      We also use this to mark unbound variables.
 
  -- Macro: int SCM_UNBNDP (SCM X)
      Return true if X is `SCM_UNDEFINED'.  Apply this to a symbol's
      value to see if it has a binding as a global variable.
 

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