4.3.4.5 Block Declarations

The declaration

:- block :BlockSpec, …, :BlockSpec.

where each BlockSpec is a skeletal goal, specifies conditions for blocking goals of the predicate referred to by the skeletal goal (f/3 say). The arguments of the skeletal goal can be:

‘-’

see below

‘?’

‘anything else’

ignored

When a goal for f/3 is to be executed, the mode specs are interpreted as conditions for blocking the goal, and if at least one condition evaluates to true, the goal is blocked.

A block condition evaluates to true if and only if all arguments specified as ‘-’ are uninstantiated, in which case the goal is blocked until at least one of those variables is instantiated. If several conditions evaluate to true, then the implementation picks one of them and blocks the goal accordingly.

The recommended style is to write the block declarations in front of the source code of the predicate they refer to. Indeed, they are part of the source code of the predicate, and must precede the first clause. For example, with the definition:

:- block merge(-,?,-), merge(?,-,-).

merge([], Y, Y).
merge(X, [], X).
merge([H|X], [E|Y], [H|Z]) :- H @< E,  merge(X, [E|Y], Z).
merge([H|X], [E|Y], [E|Z]) :- H @>= E, merge([H|X], Y, Z).

calls to merge/3 having uninstantiated arguments in the first and third position or in the second and third position will suspend.

The behavior of blocking goals for a given predicate on uninstantiated arguments cannot be switched off, except by abolishing or redefining the predicate. See mpg-ref-block.