The following script seems to disclose a bug in Hugs 1.3c's type
checker. NB it compiles fine with ghc 3.02.
> class (Show a) => Stringable a where
> toStrings :: a -> ShowS
>
> toStrings = shows
>
> class (Stringable a) => With style a where
> with :: style -> a -> ShowS
>
> with _ a = toStrings a
>
> data Align = L Int
>
> instance (Stringable a) => With Align a where
> with (L w) a = showString (ljustify w (toString a))
Some auxiliaries.
> toString :: (Stringable a) => a -> String
> toString a = toStrings a ""
> ljustify :: Int -> String -> String
> ljustify n s = s ++ spaces (n - length s)
> spaces n = replicate n ' '
Hugs tells me
ERROR "Bug.lhs" (line 16): Cannot build superclass instance
*** Instance : With Align a
*** Context supplied : Stringable a (bad type)
*** Required superclass : Stringable a
The message becomes more obscure when I add a further context to the
last instance declaration.
> instance (Show a, Stringable a) => With Align a where
is prompted by
ERROR "Bug.lhs" (line 16): Cannot build superclass instance
*** Instance : With Align a
*** Context supplied : Stringable a ((bad type) a (bad type))
*** Required superclass : Stringable a
As far as I can see both instance declarations are perfectly legal.
Cheers, Ralf
