Eval (a -> b) instance

[Ralf Hinze]

Dear GHC wizards,

just wanted to let you know that `a -> b' ist not made an instance of
`Eval'. The following piece of code demonstrates the shortcoming.

> instance (Eq b) => Eq (a -> b)        -- cheating
> instance (Num b) => Num (a -> b) where
>     f + g                     =  \a -> f a + g a
>     f - g                     =  \a -> f a - g a
>     f * g                     =  \a -> f a * g a
>     negate f                  =  \a -> negate (f a)
>     abs f                     =  \a -> abs (f a)
>     signum f                  =  \a -> signum (f a)
>     fromInteger n             =  \a -> fromInteger n
>     fromInt n                 =  \a -> fromInt n

Ralf