Ah yes, but this forces me to write my instance of Show right away. I cannot write: module A where data X = X module B where import A bar x = show x -- here is the problem moduel C where import A instance Show X where show x = "X" module Main where import A import B -- Show instance used import C -- Show instance defined I am not saying that this is terribly useful, I am just wondering why it is a problem to allow it? Paul 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) On Nov 21, Cale Gibbard wrote:
data X = X deriving Show
bar :: X -> String bar x = show x
There's no need for the class constraint at all. If it's an instance of Show, then you're okay with just applying show to it. There's no need to actually assert that it's actually an instance of Show again.
The only purpose of class constraints is to restrict polymorphism. If a function isn't polymorphic to begin with, you should never need them.
- Cale
On 21/11/05, Paul Govereau <govereau@eecs.harvard.edu> wrote:
Hello,
I was hoping that someone could answer a question I have about the type class system. In Haskell, I cannot write a term with an exact constraint:
data X = X bar :: Show X => X -> String bar x = show x
According to the Haskell 98 report, a qualifier can only be applied to type variables, but I don't see where the trouble is. The term seems to have reasonable type, and I don't see any reason why the dictionary-passing translation shouldn't work out; I am wondering what problems you run into if this restriction is lifted?
Note, with GHC and Glasgow extensions you can write this program:
data Y a = Y foo :: Show (Y a) => Y a -> String foo x = show x
However, the first program is still ruled out. Are there any type-class experts out there that can offer an explanation?
Thanks, Paul _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell