Let us consider the following simple code
{-# OPTIONS -fglasgow-exts #-}
module Foo where
data Term a where B :: Bool -> Term Bool C :: Term Bool -> Term t -> Term t I :: Int -> Term Int
shw (I t) = ("I "++) . shows t shw (B t) = ("B "++) . shows t shw (C p q) = ("Cnd "++) . (shw p) . (shw q)
It loads in GHCi 6.4 and 6.4.1: Prelude> :l /tmp/g.hs Compiling Foo ( /tmp/g.hs, interpreted ) Ok, modules loaded: Foo. *Foo> :t I 1 I 1 :: Term Int *Foo> :t B True B True :: Term Bool However, when we do *Foo> :t shw shw :: Term Bool -> String -> [Char] The inferred type of shw shows that it takes the values of Term *Bool*. And yet the very first clause of shw mentions (I t), which GHCi correctly reports to be of the type Term Int... It seems the last clause of shw confuses GHCi: if we remove it, the inferred type of shw is "Term a -> String -> [Char]", as one would expect. That is not a type expression printing problem: *Foo> shw (I 1) <interactive>:1:5: Couldn't match `Bool' against `Int' Expected type: Term Bool Inferred type: Term Int In the application `I 1' the first clause of shw notwithstanding...