Ben Rudiak-Gould writes: | Also, the rule would not be quite as simple as you make it out to be, | since | | forall a. (forall b. Foo a b => a -> b) -> Int | | is a legal type, for example. Is it? GHCi gives me an error if I try typing a function like that.
{-# OPTIONS -fglasgow-exts #-} class Foo a b
f :: forall a. (forall b. Foo a b => a -> b) -> Int f = undefined
No instance for (Foo a b) arising from instantiating a type signature at x.hs:5:4-12 Probable fix: add (Foo a b) to the type signature(s) for `f' Expected type: (forall b1. (Foo a b1) => a -> b1) -> Int Inferred type: (a -> b) -> Int In the definition of `f': f = undefined I think there would need to be a top-level constraint on |a| to guarantee that an instance of |Foo a b| exists, like forall a. (exists c. Foo a c) => (forall b. Foo a b => a -> b) -> Int -- David Menendez <zednenem@psualum.com> | "In this house, we obey the laws <http://www.eyrie.org/~zednenem> | of thermodynamics!"