(guile.info.gz) Subrs
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18.2.5.5 Subrs
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[FIXME: this needs to be further subbed, but texinfo has no subsubsub]
A subr is a pointer to a C function, packaged up as a Scheme object
to make it callable by Scheme code. In addition to the function
pointer, the subr also contains a pointer to the name of the function,
and information about the number of arguments accepted by the C
function, for the sake of error checking.
There is no single type predicate macro that recognizes subrs, as
distinct from other kinds of procedures. The closest thing is
`scm_procedure_p'; see Procedures.
-- Macro: char * SCM_SNAME (X)
Return the name of the subr X. The result is undefined if X is
not a subr.
-- Function: SCM scm_c_define_gsubr (char *NAME, int REQ, int OPT, int
REST, SCM (*FUNCTION)())
Create a new subr object named NAME, based on the C function
FUNCTION, make it visible to Scheme the value of as a global
variable named NAME, and return the subr object.
The subr object accepts REQ required arguments, OPT optional
arguments, and a REST argument iff REST is non-zero. The C
function FUNCTION should accept `REQ + OPT' arguments, or `REQ +
OPT + 1' arguments if `rest' is non-zero.
When a subr object is applied, it must be applied to at least REQ
arguments, or else Guile signals an error. FUNCTION receives the
subr's first REQ arguments as its first REQ arguments. If there
are fewer than OPT arguments remaining, then FUNCTION receives the
value `SCM_UNDEFINED' for any missing optional arguments. If RST
is non-zero, then any arguments after the first `REQ + OPT' are
packaged up as a list as passed as FUNCTION's last argument.
Note that subrs can actually only accept a predefined set of
combinations of required, optional, and rest arguments. For
example, a subr can take one required argument, or one required
and one optional argument, but a subr can't take one required and
two optional arguments. It's bizarre, but that's the way the
interpreter was written. If the arguments to `scm_c_define_gsubr'
do not fit one of the predefined patterns, then
`scm_c_define_gsubr' will return a compiled closure object instead
of a subr object.
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