Commit d4a926ba authored by Simon Peyton Jones's avatar Simon Peyton Jones

Test Trac #10226

Fixed by the patch for #10009
parent a1275a76
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE AllowAmbiguousTypes #-} -- only necessary in 7.10
{-# LANGUAGE FlexibleContexts #-} -- necessary for showFromF' example
{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
module T10226 where
type family F a
type family FInv a
-- This definition is accepted in 7.8 without anything extra, but requires
-- AllowAmbiguousTypes in 7.10 (this, by itself, is not a problem):
showFromF :: (Show a, FInv (F a) ~ a) => F a -> String
showFromF fa = undefined
-- Consider what happens when we attempt to call `showFromF` at some type b.
-- In order to check that this is valid, we have to find an a such that
--
-- > b ~ F a /\ Show a /\ FInv (F a) ~ a
--
-- Introducing an intermeidate variable `x` for the result of `F a` gives us
--
-- > b ~ F a /\ Show a /\ FInv x ~ a /\ F a ~ x
--
-- Simplifying
--
-- > b ~ x /\ Show a /\ FInv x ~ a /\ F a ~ x
--
-- Set x := b
--
-- > Show a /\ FInv b ~ a /\ F a ~ b
--
-- Set a := FInv b
--
-- > Show (FInv b) /\ FInv b ~ FInv b /\ F (FInv b) ~ b
--
-- Simplifying
--
-- > Show (FInv b) /\ F (FInv b) ~ b
--
-- Indeed, we can give this definition in 7.8, but not in 7.10:
showFromF' :: (Show (FInv b), F (FInv b) ~ b) => b -> String
showFromF' = showFromF
{-------------------------------------------------------------------------------
In 7.10 the definition of showFromF' is not accepted, but it gets stranger.
In 7.10 we cannot _call_ showFromF at all all, even at a concrete type. Below
we try to call it at type b ~ Int. It would need to show
> Show (FInv Int) /\ F (FInt Int) ~ Int
all of which should easily get resolved, but somehow don't.
-------------------------------------------------------------------------------}
type instance F Int = Int
type instance FInv Int = Int
test :: String
test = showFromF (0 :: Int)
......@@ -254,4 +254,4 @@ test('T10020', normal, compile, [''])
test('T10079', normal, compile, [''])
test('T10139', normal, compile, [''])
test('T10340', normal, compile, [''])
test('T10226', normal, compile, [''])
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