Maurício wrote:
Hi,
Why is this wrong?
---- class MyClass r where function :: r -> s
data MyData u = MyData u
instance MyClass (MyData v) where function (MyData a) = a ----
GHC says that the type of the result of 'function' is both determined by the "rigid type" from MyClass and the "rigid type" from MyData. But why can't both be the same?
particular instances can't add extra restrictions (eg: "both types are the same") to the interface declared by the class (eg: "both types are arbitrary"). Compare this version: ----8<---- {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} module Test where class MyClass r s where function :: r -> s data MyData u = MyData u instance MyClass (MyData v) v where function (MyData a) = a ----8<---- And ghci session: ----8<---- *Test> function (MyData "hello") <interactive>:1:0: No instance for (MyClass (MyData [Char]) s) arising from a use of `function' at <interactive>:1:0-24 Possible fix: add an instance declaration for (MyClass (MyData [Char]) s) In the expression: function (MyData "hello") In the definition of `it': it = function (MyData "hello") *Test> :t function (MyData "hello") function (MyData "hello") :: (MyClass (MyData [Char]) s) => s *Test> function (MyData "hello") :: String "hello" ----8<---- I don't know how evil those language extensions are, though - I just fiddled until it worked...
What am I doing wrong?
Claude -- http://claudiusmaximus.goto10.org/