TypeFamilies.hs

> {-# OPTIONS -fglasgow-exts #-}
>
> module VarArg where
> import Data.FiniteMap   -- for an example below
>
> class BuildList a r  | r-> a where
>     build' :: [a] -> a -> r
>
> instance BuildList a [a] where
>     build' l x = reverse$ x:l
>
> instance BuildList a r => BuildList a (a->r) where
>     build' l x y = build'(x:l) y
>
> --build :: forall r a. (BuildList a r) => a -> r
> build x = build' [] x