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Processing option lists and dicts |
Below is an example. While PL_option_t
is a struct, its
members are initialised using the PL_OPTION() macro. The data
structure is not constant because PL_scan_options()
adds the option names as
atoms to speed up option processing. The macro PL_OPTIONS_END
terminates the option list.
static PL_option_t mypred_options[] = { PL_OPTION("quoted", OPT_BOOL), PL_OPTION("length", OPT_SIZE), PL_OPTION("callback", OPT_TERM), PL_OPTIONS_END }; static foreign_t mypred(term_t a1, term_t options) { int quoted = FALSE; size_t length = 10; term_t callback = 0; if ( !PL_scan_options(options, 0, "mypred_options", mypred_options, "ed, &length, &callback) ) return FALSE; <implement mypred> }
The only defined value for flags is currently OPT_ALL
,
which causes this function to raise a domain error if an option is
passed that is not in specs. Default in SWI-Prolog is to
silently ignore unknown options, unless the Prolog flag iso
is true
. The opttype argument defines the type
(group) of the options, e.g., "write_option"
. Option types
are defined by the ISO standard. SWI-Prolog only uses this if OPT_ALL
is specified, to raise a domain_error
of the indicated type
if some option is unused. The type name is normally the name of the
predicate followed by
_option
or the name of a representative of a group of
predicates to which the options apply.
Defined option types and their corresponding pointer type are described below.
OPT_BOOL
int
TRUE
.OPT_INT
int
OPT_INT64
int64_t
OPT_UINT64
uint64_t
OPT_SIZE
size_t
OPT_DOUBLE
double
OPT_STRING
char*
CVT_ALL|REP_UTF8|BUF_STACK|CVT_EXCEPTION
. The buffered
string must be guarded using PL_STRINGS_MARK()
and PL_STRINGS_RELEASE().OPT_ATOM
atom_t
OPT_TERM
term_t
The ISO standard demands that if an option is repeated the last occurance holds. This implies that PL_scan_options() must scan the option list to the end.