[Haskell-cafe] RE: MPTCs and rigid variables

[C Rodrigues]

>{-# OPTIONS_GHC -fglasgow-exts #-}
>
>class Foo a b | a -> b where
>   foo :: Foo b c => a -> Maybe c
>
>instance Foo String () where foo _ = Nothing
>instance Foo Int String where foo 4 = Just (); foo _ = Nothing
>

There appears to be a type-safe way to use unsafeCoerce# for this:

--------
import qualified GHC.Exts

class Foo a b | a -> b where foo :: a -> FooBox b
data FooBox b = forall c. Foo b c => FooBox (Maybe c)

instance Foo () ()
instance Foo String () where foo _ = FooBox Nothing
instance Foo Int String where foo 4 = FooBox $ Just (); foo _ = FooBox 
Nothing

runFoo :: (Foo a b, Foo b c) => a -> Maybe c
runFoo x = case foo x of FooBox x -> GHC.Exts.unsafeCoerce# x
--------

The class constraint check of a,b,c gets moved to the runFoo method instead 
of the instance declarations.  Between the body of foo and the call to foo 
in runFoo, type 'c' is unknown to the compiler, so we have to encapsulate it 
in a FooBox.  The unsafeCoerce# is safe because the type constraint (Foo b 
c) that we've placed both on FooBox and on runFoo permits c to have only the 
type uniquely specified by b.

We actually could remove the Foo constraint on FooBox, which makes the code 
no longer typesafe but it will still work just fine.

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