external(+PredSpec)

+PredSpec

Of the form Atom/Integer (predicate name/arity).

Description

This declaration is needed to compile calls to an external predicate before it is actually defined with external/2.

Fail Conditions

Resatisfiable

Exceptions

(4) instantiation fault

PredSpec is not instantiated.

(5) type error

PredSpec is not of the form Atom/Integer.

(62) inconsistent procedure redefinition

A call to PredSpec has already been compiled as a Prolog call or a non-deterministic external call.

Examples

Success:

   % compiling a call to an external before its definition (see description
   % of external/2 for detail on creating external predicates).
      [eclipse]: [user].
       :- import prmsg/1 from msg_lib.
       :- external(prmsg/1). % declare its call_type
       hello :- prmsg("hello").
       user      compiled 216 bytes in 0.03 seconds

   % definition of sines/2 will not raise an inconsistent type definition
   % thanks to the proper declaration above.
      [eclipse]: sh("cat msg_lib.pl").
      :- module(msg_lib).
      :- load('msg.o', "-lm"). % see example in external/2
      :- external(prmsg/1, p_prmsg).
      :- export prmsg/1.

      yes
      [eclipse]: [msg_lib].
       msg_lib.pl      compiled 0 bytes in 0.18 seconds
      yes.
      [eclipse]: hello.
      message: hello
      yes.

Error:
      external(PredSpec).            (Error 4).
      external("p/0").               (Error 5).

      [eclipse]: [user].
       p :- a.
       user   compiled 32 bytes in 0.00 seconds
      yes.
      [eclipse]: external(a/0).        (Error 62).

See Also