{-# LANGUAGE TypeFamilies, LiberalTypeSynonyms, ImpredicativeTypes #-}
module PolyTypeDecomp where
{- The purpose of this test is to check if decomposition of wanted
equalities in the /constraint solver/ (vs. the unifier) works properly.
Unfortunately most equalities between polymorphic types are converted to
implication constraints early on in the unifier, so we have to make things
a bit more convoluted by introducing the myLength function. The wanted
constraints we get for this program are:
[forall a. Maybe a] ~ Id alpha
[forall a. F [a]] ~ Id alpha
Which, /after reactions/ should create a fresh implication:
forall a. Maybe a ~ F [a]
that is perfectly soluble.
-}
type family F a
type instance F [a] = Maybe a
type family Id a
type instance Id a = a
f :: [forall a. F [a]]
f = undefined
g :: [forall a. Maybe a] -> Int
g x = myLength [x,f]
myLength :: [Id a] -> Int
myLength = undefined