[Haskell-cafe] Typeclasses and implicit parameters

[ajb at spamcop.net]

G'day all.

Quoting Simon Peyton-Jones <simonpj at microsoft.com>:

> | -- 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