3.10 Advanced control-structures: blocks

The predicates of this section form a tightly related set for realising premature successful or failing exits from a block. These predicates are first of all useful for error-recovery. They were primarily implemented for compatibility reasons.

block(+Label, +Goal, -ExitValue)

Execute Goal in a block. Label is the name of the block. Label is normally an atom, but the system imposes no type constraints and may even be a variable. ExitValue is normally unified to the second argument of an exit/2 call invoked by Goal.

exit(+Label, +Value)

Calling exit/2 makes the innermost block which Label unifies exit. The block's ExitValue is unified with Value. If this unification fails the block fails.

fail(+Label)

Calling fail/1 makes the innermost block which Label unifies fail immediately. Implemented as

fail(Label) :- !(Label), fail.

!(+Label)

Cut all choice-points created since the entry of the innermost block which Label unifies.

The example below illustrate these constructs to immediately report a syntax-error from a `deep-down' procedure to the outside world without passing it as an argument `all-over-the-place'.

parse(RuleSet, InputList, Rest) :- block(syntaxerror, phrase(RuleSet, InputList, Rest), Error), ( var(Error) -> true ; format('Syntax-error: ~w~n', Error), fail ). integer(N) --> digit(D1), !, digits(Ds), { name(N, [D1|Ds]) }. digits([D|R]) --> digit(D), digits(R). digits(_) --> letter(_), !, { exit(syntaxerror, 'Illegal number') }. digits([]) --> []. digit(D, [D|R], R) :- between(0'0, 0'9, D). letter(D, [D|R], R) :- between(0'a, 0'z, D).