This module provides SICStus Prolog-compatible :- block BlockSpec, ... declarations for delaying predicate calls if certain arguments are unbound.

See also

- https://sicstus.sics.se/sicstus/docs/3.12.11/html/sicstus/Block-Declarations.html

 block +Heads

Declare predicates to suspend on certain modes. The argument is, like meta_predicate/1, a comma-separated list of modes (BlockSpecs). Calls to the predicate is suspended if at least one of the conditions implies by a blockspec evaluated to true. A blockspec evaluated to true iff all arguments specified as `-' are unbound.

Multiple BlockSpecs for a single predicate can appear in one or more :- block declarations. The predicate is suspended untill all mode patterns that apply to it are satisfied.

The implementation is realised by creating a wrapper that checks the block conditions and either calls the original predicate immediately (if none of the block conditions were true) or uses attributed variables to delay re-evaluating the block condition until any of the arguments in question are bound.

Compatibility

- SICStus Prolog

 block_wrapper_clauses(+Module, +Head, -Clauses) is det[private]

Build a list of clauses that define a block wrapper around predicate Head in Module. If a wrapper for this predicate has already been defined, Clauses is an empty list.

 block_wrapper_clause(+Module, +Name, +HelperHead, -Clause) is nondet[private]

Generate the clauses for the wrapper. The blockspecs are translated into a helper predicate, where each clause checks one block condition. If a block condition is true, attributes are added to all arguments marked as -, so that once any of them are bound, the predicate is called again and the block conditions are re-evaluated. If no block condition was true, the helper predicate fails.

Finally, an initialization clause is generated that sets up the actual wrapper. This wrapper first calls the helper predicate to check all block conditions and delay the call if necessary. If the helper predicate fails (i. e. no block condition was true), the wrapped predicate is called immediately.

The wrapper must be set up in an initialization clause and not as part of the term expansion, because wrap_predicate/4 wrappers are not retained in saved states, which would cause block declarations to break when loading a saved state.

 find_args_to_block_on(+BlockArgs, +HeadArgs, -ArgsToBlockOn) is semidet[private]

Collect into ArgsToBlockOn all arguments from HeadArgs for which the corresponding argument in BlockArgs is -, indicating that the argument is part of the block condition.

 args_to_var_conditions(+ArgsToBlockOn, -Conditions, ?ConditionsTail) is semidet[private]

Convert a list of arguments into a conjunction of var/1 checks that succeeds if all arguments are unbound variables.

This effectively generates an unrolled version of maplist(var, ArgsToBlockOn), ConditionsTail.

 args_to_suspend_calls(+ArgsToBlockOn, -IsAlreadyUnblocked, +BlockedGoal, -SuspendCalls, +Tail) is semidet[private]

Build a sequence of calls that delays BlockedGoal until any variable in ArgsToBlockOn is bound. IsAlreadyUnblocked should be an unbound fresh variable - it is passed directly to unblock/2, which will bind the variable so that the same blocked goal is not called again by another unblock/2 call from the same group.