[Haskell-cafe] distinguish functions from non-functions in a class/instances

[Philipp N.]

Hello,

i'm trying to wrap functions (a -> b -> ... -> z) of any arity to functions
of type ([String] -> y), where list of strings replaces the typed arguments.

one attempt looks like this (here written with type families, you can
replace it by functional dependencies or what ever):

type family Res x
type instance Res x = x
type instance Res (x->y) = Res y

class Nary x where
    nary :: x -> [String] -> Res x

instance Nary x where
    nary x [] = x

instance Nary (x->y) where
    nary f (x:xs) = nary (f $ read x) xs

i hope you can get the idea.
the problem is, that you cannot distinguish type (x->y) from z, so these
instances are overlapping.
the odd thing is. you can get this to work, if you have a terminating type
as result type (for example (IO x)). then you can work with all types (IO
x), (a -> IO x), (a -> b -> IO x), ...

but i don't want this delimiter IO! any ideas?

greetings
  Philipp N.

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