Paul Govereau wrote:
BTW, The above program is a translation of an idiomatic use of functors in ML (pardon my syntax):
module A : sig type t = ... end module B : funsig(X:SHOW where t = A.t) sig bar : A.t -> string end module C : SHOW where t = A.t open A open B(C)
ML modules poorly correspond to Haskell modules. Rather, they correspond to Haskell typeclasses: {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} module Foo where -- module A : sig type t = ... end -- A is a constraint on type t class A t -- different instances of A will define what t actually means -- module B : funsig(X:SHOW where t = A.t) sig bar : A.t -> string end class (A t, Show t) => B t where bar :: t -> String -- module C : SHOW where t = A.t -- A restriction on Show such that its type parameter is constrained to -- be in A class (A t, Show t) => C t instance (A t, Show t) => C t -- open A -- Perhaps some implementation of A must be provided somewhere... -- open B(C) instance C t => B t where bar = show -- Eventually we need to give a structure that implements signature A data X = X instance A X instance Show X where show _ = "X" test = bar X