[Haskell-cafe] Typeclasses and implicit parameters

[ajb at spamcop.net]

{-# OPTIONS -fglasgow-exts #-}

-- G'day everyone.

-- This is okay.
f1 :: (?foo :: String) => String
f1 = ?foo

-- So is this.
f2 :: (Show a, ?foo :: a) => a -> String
f2 _ = show ?foo

-- Hugs allows this.  GHC rejects it on the grounds that "a" is unused
-- on the right-hand side of the (=>).  I think this is arguably a bug
-- in GHC.
f3 :: (Show a, ?foo :: a) => String
f3 = show ?foo

-- GHC rejects this.  Hugs compiles it, but I can't call it as
-- let ?foo = "Hello" in show Foo
--
-- Is there a good reason to disallow this?
data Foo = Foo

instance (?foo :: String) => Show Foo where
    showsPrec _ Foo = showString ?foo . showString "Foo"

-- Cheers,
-- Andrew Bromage