Tue, 28 Nov 2000 19:17:32 +0100 (CET), Elke Kasimir <elke.kasimir@catmint.de> pisze:
The implementation I'm interested in (one without constructors) is:
nil fornil forcons = fornil cons x xs fornil forcons = forcons x xs forlist fornil forcons ls = ls fornil forcons
It can be implemented in extended Haskell with local universal quantification (ghc -fglasgow-exts, hugs -98): newtype List a = List (forall b. b -> (a -> List a -> b) -> b) nil = List (\fornil forcons -> fornil) cons x xs = List (\fornil forcons -> forcons x xs) forlist fornil forcons (List ls) = ls fornil forcons map f = forlist nil (\x xs -> cons (f x) (map f xs)) In ghc algebraic types are really implemented in a similar way :-) Pattern matching jumps into the value, giving to it an array of continuations for each constructor. The value evaluates itself and enters the chosen continuation. It should be also directly implentable in OCaml with -rectypes flag which turns off occurs check for types. -- __("< Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK