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|
{-# LANGUAGE QuantifiedConstraints, TypeFamilies #-}
module T15244 where
class (forall t . Eq (c t)) => Blah c
-- Unquantified works
foo :: (Eq (a t), Eq (b t), a ~ b) => a t -> b t -> Bool
foo a b = a == b
-- Works
-- Two quantified instances fail with double ambiguity check errors
bar :: (forall t . Eq (a t), forall t . Eq (b t), a ~ b) => a t -> b t -> Bool
bar a b = a == b
-- Minimal.hs:11:8: error:
-- • Could not deduce (Eq (b t1))
-- from the context: (forall t1. Eq (a t1), forall t1. Eq (b t1),
-- a ~ b)
-- bound by the type signature for:
-- bar :: forall (a :: * -> *) (b :: * -> *) t.
-- (forall t1. Eq (a t1), forall t1. Eq (b t1), a ~ b) =>
-- a t -> b t -> Bool
-- at Minimal.hs:11:8-78
-- • In the ambiguity check for ‘bar’
-- To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
-- In the type signature:
-- bar :: (forall t. Eq (a t), forall t. Eq (b t), a ~ b) =>
-- a t -> b t -> Bool
-- |
-- 11 | bar :: (forall t . Eq (a t), forall t . Eq (b t), a ~ b) => a t -> b t -> Bool
-- |
-- [And then another copy of the same error]
-- Two copies from superclass instances fail
baz :: (Blah a, Blah b, a ~ b) => a t -> b t -> Bool
baz a b = a == b
-- Minimal.hs:34:11: error:
-- • Could not deduce (Eq (b t)) arising from a use of ‘==’
-- from the context: (Blah a, Blah b, a ~ b)
-- bound by the type signature for:
-- baz :: forall (a :: * -> *) (b :: * -> *) t.
-- (Blah a, Blah b, a ~ b) =>
-- a t -> b t -> Bool
-- at Minimal.hs:33:1-52
-- • In the expression: a == b
-- In an equation for ‘baz’: baz a b = a == b
-- |
-- 34 | baz a b = a == b
-- |
-- Two copies from superclass from same declaration also fail
mugga :: (Blah a, Blah a) => a t -> a t -> Bool
mugga a b = a == b
-- • Could not deduce (Eq (a t)) arising from a use of ‘==’
-- from the context: (Blah a, Blah a)
-- bound by the type signature for:
-- mugga :: forall (a :: * -> *) t.
-- (Blah a, Blah a) =>
-- a t -> a t -> Bool
-- at Minimal.hs:50:1-47
-- • In the expression: a == b
-- In an equation for ‘mugga’: mugga a b = a == b
-- |
-- 51 | mugga a b = a == b
-- |
-- One copy works
quux :: (Blah a, a ~ b) => a t -> b t -> Bool
quux a b = a == b
-- Works
|
