参考: Developing Applications With Objective Caml より、
たとえば、タプル型でリストを作ったりできる。以下の例では、-rectypesを使って、1の無限リストを直積型で実現する。 Haskellではこれはできない。Nominalな型システムだから?(耳学問、ソース求む)
$ ocaml Objective Caml version 3.09.3 # let rec ones = (1, ones);; This expression has type int * (int * 'a) but is here used with type int * 'a # ^D $ ocaml -rectypes Objective Caml version 3.09.3 # let rec ones = (1,ones);; val ones : int * 'a as 'a = (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (...))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))) #
val x : int * 'a as 'a という型表現に注目して頂きたい。不動点演算子μで書くとμ'a.int*'a という感じ。
Haskellでムリヤリ実現するには。
data Rec t = R (t (Rec t))
という「型レベルの不動点演算子」みたいなものを作る。Haskellでは、不動点演算子Yは y = f (y f)となるのを思い出してほしい。
ones = R (1, ones)
などとして使う。このonesの型は
*Main> :t ones ones :: Rec ((,) Integer)
データ構築子Rを介して再帰を展開する必要がある: printRec (R (a,r)) = "(" ++ show a ++ "," ++ printRec r
*Main> printRec ones (1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,(1,....
(けいご)