SWI-Prolog -- Manual

>12.4.3Analysing Terms via the Foreign Interface

Each argument of a foreign function (except for the control argument) is of type term_t, an opaque handle to a Prolog term. Three groups of functions are available for the analysis of terms. The first just validates the type, like the Prolog predicates var/1, atom/1, etc., and are called PL_is_*(). The second group attempts to translate the argument into a C primitive type. These predicates take a term_t and a pointer to the appropriate C type and return TRUE or FALSE depending on successful or unsuccessful translation. If the translation fails, the pointed-to data is never modified.

12.4.3.1 Testing the type of a term

int PL_term_type(term_t)

Obtain the type of a term, which should be a term returned by one of the other interface predicates or passed as an argument. The function returns the type of the Prolog term. The type identifiers are listed below. Note that the extraction functions PL_get_*() also validate the type and thus the two sections below are equivalent.

        if ( PL_is_atom(t) )
        { char *s;

          PL_get_atom_chars(t, &s);
          ...;
        }

or

        char *s;
        if ( PL_get_atom_chars(t, &s) )
        { ...;
        }

Version 7 added PL_NIL, PL_BLOB, PL_LIST_PAIR and PL_DICT. Older versions classify PL_NIL and PL_BLOB as PL_ATOM, PL_LIST_PAIR as PL_TERM and do not have dicts.

`PL_VARIABLE`	A variable or attributed variable
`PL_ATOM`	A Prolog atom
`PL_NIL`	The constant `[]`
`PL_BLOB`	A blob (see section 12.4.9.2)
`PL_STRING`	A string (see section 5.2)
`PL_INTEGER`	A integer
`PL_RATIONAL`	A rational number
`PL_FLOAT`	A floating point number
`PL_TERM`	A compound term
`PL_LIST_PAIR`	A list cell (`[H\|T]`)
`PL_DICT`	A dict (see section 5.4))

The functions PL_is_<type> are an alternative to PL_term_type(). The test PL_is_variable(term) is equivalent to PL_term_type(term) == PL_VARIABLE, but the first is considerably faster. On the other hand, using a switch over PL_term_type() is faster and more readable then using an if-then-else using the functions below. All these functions return either TRUE or FALSE.

int PL_is_variable(term_t): Returns non-zero if term is a variable.
int PL_is_ground(term_t): Returns non-zero if term is a ground term. See also ground/1. This function is cycle-safe.
int PL_is_atom(term_t): Returns non-zero if term is an atom.
int PL_is_string(term_t): Returns non-zero if term is a string.
int PL_is_integer(term_t): Returns non-zero if term is an integer.
int PL_is_rational(term_t): Returns non-zero if term is a rational number (P/Q). Note that all integers are considered rational and this test thus succeeds for any term for which PL_is_integer() succeeds. See also PL_get_mpq() and PL_unify_mpq().
int PL_is_float(term_t): Returns non-zero if term is a float. Note that the corresponding PL_get_float() converts rationals (and thus integers).
int PL_is_callable(term_t): Returns non-zero if term is a callable term. See callable/1 for details.
int PL_is_compound(term_t): Returns non-zero if term is a compound term.
int PL_is_functor(term_t, functor_t): Returns non-zero if term is compound and its functor is functor. This test is equivalent to PL_get_functor(), followed by testing the functor, but easier to write and faster.
int PL_is_list(term_t): Returns non-zero if term is a compound term using the list constructor or the list terminator. See also PL_is_pair() and PL_skip_list().
int PL_is_pair(term_t): Returns non-zero if term is a compound term using the list constructor. See also PL_is_list() and PL_skip_list().