[Haskell-cafe] isFunction

[john lask]

Oleg provided the following code to test whether a is a function, this code will not type check in hugs, due to the overlapping instances in 

 IsFunction (x->y) f
 IsFunction x f

and the functional dependency | a-> b

ERROR "isfunction.lhs":43 - Instances are not consistent with dependencies
*** This instance    : IsFunction a b
*** Conflicts with   : IsFunction (a -> b) c
*** For class        : IsFunction a b
*** Under dependency : a -> b

is there a way to acheive the same effect in hugs ?


> data HTrue
> data HFalse
> 
> class IsFunction a b | a -> b
> instance TypeCast f HTrue => IsFunction (x->y) f
> instance TypeCast f HFalse => IsFunction x f

> 
> -- literally lifted from the HList library
> class TypeCast   a b   | a -> b, b->a   where typeCast   :: a -> b
> class TypeCast'  t a b | t a -> b, t b -> a where typeCast'  :: t->a->b
> class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b
> instance TypeCast'  () a b => TypeCast a b where typeCast x = typeCast' () x
> instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast''
> instance TypeCast'' () a a where typeCast'' _ x  = x
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