G'day all.
Quoting Simon Peyton-Jones
| -- 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"
This should be illegal. The way in which implicit parameters are bound depends on the call site of teh overloaded function. E.g. the call site of f3 above affects the value of ?foo.
Yes, I've read the manual section on this. For completeness, here's the final solution, courtesy of int-e (whose real name I don't know; sorry), which is much more elegant than I expected: -- Type hackery import GHC.Exts (unsafeCoerce#) newtype Mark m a = Mark { unMark :: a } toDummy :: Mark n t -> (n -> t) toDummy (Mark x) _ = x fromDummy :: (n -> t) -> Mark n t fromDummy f = Mark (f undefined) -- And now, the real code class StringAsType s where reifiedString' :: Mark s String withString :: (StringAsType s) => s -> (String -> a) -> a withString s k = k (toDummy reifiedString' s) getString :: (StringAsType s) => s -> String getString s = withString s id bindString :: (forall s. StringAsType s => Mark s a) -> String -> a bindString = unsafeCoerce# mkStringAsType :: String -> (forall s. StringAsType s => s -> a) -> a mkStringAsType s f = bindString (fromDummy f) s -- ReifiedString can now be used as an instance context That higher-rank type makes all the difference. Cheers, Andrew Bromage