(guile.info.gz) Unique Values
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(guile.info.gz) Boolean Data
(guile.info.gz) Immediate Datatypes
18.2.4.4 Unique Values
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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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