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block(+Goal, ?Tag, +Recovery)

Similar to call(Goal) if Goal succeeds or fails. If an exit_block/1 is executed inside Goal, whose argument unifies with Tag, then Recovery is executed.
+Goal
An atom or a compound term.
?Tag
An atom, integer or variable.
+Recovery
An atom or a compound term.

Description

First Goal is called from the current module and if this succeeds then block/3 succeeds. If Goal fails then so does the call of block/3. If, however, during the execution of Goal there is a call of exit_block(TagExit) such that Tag unifies with TagExit, then block/3 calls the goal Recovery, and succeeds or fails according to whether Recovery succeeds or fails. If Tag does not unify with TagExit, the system continues looking for an earlier invocation of block/3.

Fail Conditions

Fails if Goal fails, or if Recovery fails.

Resatisfiable

No.

Exceptions

(4) instantiation fault
Either Goal or Recovery is not instantiated.
(5) type error
Goal is neither an atom nor a compound term.
(5) type error
Tag is not an atomic type.
(5) type error
Recovery is neither an atom nor a compound term.
(68) calling an undefined procedure
Either Goal or Recovery is an undefined predicate.

Examples

Success:
      block(a_goal, label, true).
        % similar to a label for a 'goto'
      exit_block(label).
        % the associated 'goto' statement.
      [eclipse]: [user].
       go:-
             getval(i,2) ->
             exit_block(hello).
       user compiled 100 bytes in 0.02 seconds
      yes.              % go/0 exits with Tag 'hello' if
                        % the global variable i is 2.
      [eclipse]: setval(i,1), block(go, hello, write(hello)).
      no.
      [eclipse]: setval(i,2),block(go,hello,writeln(hello)).
      hello
      yes.

Fail:
      block(exit_block(t), t, 3 = 2).

Error:
      block(go, hello, Recovery).      (Error 4).
      block(Goal, any, thing).         (Error 4).
      block(go, hello(X), problem).    (Error 5).
      block(go, hello, "a").           (Error 5).
      block(nonex, t, write(bye)).     (Error 68).

See Also

exit_block / 1