Predicate dispatch

In computer programming, predicate dispatch is a generalisation of multiple dispatch ("multimethods") that allows the method to call to be selected at runtime based on arbitrary decidable logical predicates and/or pattern matching attached to a method declaration.[1][2]

Polymorphism
Ad hoc polymorphism
Parametric polymorphism
Subtyping

Raku supports predicate dispatch using "where" clauses that can execute arbitrary code against any function or method parameter.[3]

Julia has a package for it with PatternDispatch.jl but otherwise natively supports multiple dispatch.

Experimental implementations have been created for Common LISP,[4][5] and for Java (JPred[2]).

It allows open extension of previously declared methods at a fine-grained level, but multiple extensions with identical or overlapping predicates created by different developers may interfere with each other in unanticipated ways. In this respect it is similar to aspect-oriented programming.

References
  1. Millstein, T. "Practical Predicate Dispatch" (PDF). Cite journal requires |journal= (help)
  2. Millstein, T.; Frost, C.; Ryder, J.; Warth, A. (2009). "Expressive and modular predicate dispatch for Java". ACM Transactions on Programming Languages and Systems. 31 (2): 1. CiteSeerX 10.1.1.212.4268. doi:10.1145/1462166.1462168.
  3. "class Signature". Retrieved 2020-07-07.
  4. "Predicate Dispatching in Common Lisp Object System" (PDF).
  5. "pcostanza/filtered-functions". GitHub. Retrieved 2016-04-07.


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