Patternp

Patternp is a term used in discussions of pattern matching to denote a boolean predicate that tests whether a value conforms to a specified pattern. In formal terms, patternp(P, v) returns true if the value v matches the structural and literal constraints encoded by the pattern P, and false otherwise. Some presentations allow currying, where patternp(P)(v) performs the same test.

Patterns considered by patternp can be simple or complex. They may include literals, constructors, tuples, lists, and records, as well as wildcards that ignore certain components. Unlike a full match operation, patternp is typically described as a test rather than a binding operation, meaning it answers yes/no without producing variable bindings. Some systems allow guards or type predicates to be attached to a pattern, and patternp must decide whether those additional conditions hold.

Examples (in pseudo-syntax): patternp([x, _, 3], [1, 2, 3]) yields true, because the first element can be bound and the third element matches 3. patternp([x, _, 3], [1, 4, 5]) yields false, since the third element is not 3. Note that patternp’s purpose is verification; it is distinct from an actual match that would bind variables or produce a result.

Patternp is primarily a theoretical concept used to reason about pattern matching, and is also referenced in libraries and tooling that implement runtime checks or static analysis. See also: pattern matching, predicates, destructuring.