prolog_code.pl -- Utilities for reasoning about code

This library collects utilities to reason about terms commonly needed for reasoning about Prolog code. Note that many related facilities can be found in the core as well as other libraries:

comma_list(?CommaList, ?List)

semicolon_list(?SemicolonList, ?List)

True if CommaList is a nested term over the ','/2 (';'/2) functor and List is a list expressing the elements of the conjunction. The predicate is deterministic if at least CommaList or List is sufficiently instantiated. If both are partial structures it enumerates ever growing conjunctions and lists. CommaList may be left or right associative on input. When generated, the CommaList is always right associative.

This predicate is typically used to reason about Prolog conjunctions (disjunctions) as many operations are easier on lists than on binary trees over some operator.

mkconj(A, B, Conj) is det

mkdisj(A, B, Disj) is det

Create a conjunction or disjunction from two terms. Reduces on true (mkconj/2) and false (mkdisj/2). Note that a false encountered in a conjunction does not cause the conjunction to be false, i.e. semantics under side effects are preserved.

The Prolog `, and ;` operators are of type xfy, i.e. right associative. These predicates preserve this grouping. For example,

?- mkconj((a,b), c, Conj)
Conj = (a,b,c)

is_predicate_indicator(@Term) is semidet

True when Term is a predicate indicator

pi_head(?PredicateIndicator, ?Goal) is det

Translate between a PredicateIndicator and a Goal term. The terms may have a module qualification.

Errors: - type_error(predicate_indicator, PredicateIndicator)

head_name_arity(?Goal, ?Name, ?Arity) is det

Similar to functor/3, but deals with SWI-Prolog's zero-argument callable terms and avoids creating a non-callable term if Name is not an atom and Arity is zero.

most_general_goal(+Goal, -General) is det

General is the most general version of Goal. Goal can be qualified.

See also: - is_most_general_term/1.

extend_goal(:Goal0, +Extra, -Goal) is det

Extend the possibly qualified Goal0 with additional arguments from Extra. If Goal0 is insufficiantly instantiated (i.e., a variable), a term call(Goal0, ...) is returned.

predicate_label(++PI, -Label) is det

Create a human-readable label for the given predicate indicator. This notably hides the module qualification from user and built-in predicates. This predicate is intended for reporting predicate information to the user, for example in the profiler.

First PI is converted to a head and the hook user:prolog_predicate_name/2 is tried.

predicate_sort_key(+PI, -Key) is det

Key is the (module-free) name of the predicate for sorting purposes.

is_control_goal(@Goal)

True if Goal is a compiled Prolog control structure. The difference between control structures and meta-predicates is rather unclear. The constructs below are recognised by the compiler and cannot be redefined. Note that (if->then;else) is recognised as ((if->then);else).

body_term_calls(:BodyTerm, -Goal) is nondet

True when BodyTerm calls Goal. This predicate looks into control structures as well as meta predicates based on predicate_property/2.

When a variable is called, this is normally returned in Goal. Currently if a variable is called with additional arguments, e.g., call(Var, a1), this call is reported as call(Var, a1).

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

mkdisj(Arg1, Arg2, Arg3)
semicolon_list(Arg1, Arg2)