Hi Olaf, This is the monomorphism restriction. g is a binding without a signature, so it gets specialized. The type annotation is part of the body of g, but if you want to generalize g it should be a separate declaration let g :: forall ... g = ... On 10/5/2021 3:36 PM, Olaf Klinke wrote:

Dear Cafe,

apologies if this is a duplicate question. I want to define an element of a Rank-2 type, use its specialization and then return the general thing. Apparently this is not possible without a newtype.

{-# LANGUAGE RankNTypes #-} newtype General = General {    useGeneral :: forall a. Integral a => a -> Bool    }

doesNotWork :: General doesNotWork = let         g = even :: forall a. Integral a => a -> Bool         b = specializeGeneral g         in General g

doesWork :: General doesWork = let         g = General even         b = specializeGeneral (useGeneral g)         in g

specializeGeneral :: (Int -> Bool) -> Bool specializeGeneral p = p 5

I was under the impression that one can always use a more general type where a more special type is needed. In `doesNotWork` above, despite the explicit Rank-2 type annotation, usage in `specializeGeneral` apparently makes the compiler infer the type of `g` to be (Int -> Bool) and complains that `a` can not me matched with `Bool`. What gets me is that the compiler error is at `General g`, so the compiler must have ignored my Rank-2 type annotation. Should it be allowed to do that?

Olaf _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.