Create GCoercible class, analogous to GEq

It could be a superclass of `GEq`, but this would be a breaking change
and interferes with Safe Haskell, so we refrained from making it one.

Author: Ericson2314

some.cabal

@@ -72,6 +72,11 @@ library
     Data.Some.GADT
     Data.Some.Newtype
 
+  -- Proxy for base >= 4.7.0
+  if impl(ghc >= 7.8.2)
+    exposed-modules:
+      Data.GADT.Coerce
+
   other-modules:    Data.GADT.Internal
   build-depends:
       base     >=4.3     && <4.18

src/Data/GADT/Coerce.hs

@@ -0,0 +1,107 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE PolyKinds           #-}
+#if __GLASGOW_HASKELL__ >= 810
+{-# LANGUAGE StandaloneKindSignatures #-}
+#endif
+module Data.GADT.Coerce (
+    GCoercible (..),
+    defaultGcoercible,
+    ) where
+
+import Data.Functor.Product (Product (..))
+import Data.Functor.Sum     (Sum (..))
+import Data.IORef           (IORef)
+import Data.STRef           (STRef)
+import Data.Type.Equality   ((:~:) (..))
+import Data.Type.Coercion
+
+import Unsafe.Coerce        (unsafeCoerce)
+
+import Data.GADT.Compare    (GEq, geq)
+
+#if MIN_VERSION_base(4,10,0)
+import           Data.Type.Equality ((:~~:) (..))
+import qualified Type.Reflection    as TR
+#endif
+
+#if __GLASGOW_HASKELL__ >= 810
+import Data.Kind (Type, Constraint)
+#endif
+
+-- |A class for type-contexts which contain enough information
+-- to (at least in some cases) decide the coercibility of types
+-- occurring within them.
+#if __GLASGOW_HASKELL__ >= 810
+type GCoercible :: (k -> Type) -> Constraint
+#endif
+class GCoercible f where
+    gcoercible :: f a -> f b -> Maybe (Coercion a b)
+
+-- |If 'f' has a 'GEq' instance, this function makes a suitable default
+-- implementation of 'gcoercible'.
+defaultGcoercible :: GEq f => f a -> f b -> Maybe (Coercion a b)
+defaultGcoercible x y = fmap repr $ geq x y
+
+instance GCoercible ((:~:) a) where
+    gcoercible = defaultGcoercible
+
+#if MIN_VERSION_base(4,10,0)
+instance GCoercible ((:~~:) a) where
+    gcoercible = defaultGcoercible
+
+instance GCoercible TR.TypeRep where
+    gcoercible = defaultGcoercible
+#endif
+
+instance (GCoercible a, GCoercible b) => GCoercible (Sum a b) where
+    gcoercible (InL x) (InL y) = gcoercible x y
+    gcoercible (InR x) (InR y) = gcoercible x y
+    gcoercible _ _ = Nothing
+
+instance (GCoercible a, GCoercible b) => GCoercible (Product a b) where
+    gcoercible (Pair x y) (Pair x' y') = do
+        Coercion <- gcoercible x x'
+        Coercion <- gcoercible y y'
+        return Coercion
+
+instance GCoercible IORef where
+    gcoercible x y =
+        if x == unsafeCoerce y
+        then Just $ unsafeCoerce $ repr Refl
+        else Nothing
+
+instance GCoercible (STRef s) where
+    gcoercible x y =
+        if x == unsafeCoerce y
+        then Just $ unsafeCoerce $ repr Refl
+        else Nothing
+
+-- This instance seems nice, but it's simply not right:
+--
+-- > instance GCoercible StableName where
+-- >     gcoercible sn1 sn2
+-- >         | sn1 == unsafeCoerce sn2
+-- >             = Just (unsafeCoerce Refl)
+-- >         | otherwise     = Nothing
+--
+-- Proof:
+--
+-- > x <- makeStableName id :: IO (StableName (Int -> Int))
+-- > y <- makeStableName id :: IO (StableName ((Int -> Int) -> Int -> Int))
+-- >
+-- > let Just boom = gcoercible x y
+-- >
+-- > Data.Type.Coercion.coerceWith boom (const 0) id 0
+-- > let "Illegal Instruction" = "QED."
+--
+-- The core of the problem is that 'makeStableName' only knows the closure it is
+-- passed to, not any type information. Together with the fact that the same
+-- closure has the same 'StableName' each time 'makeStableName' is called on it,
+-- there is potential for abuse when a closure can be given many incompatible
+-- types.
+--
+-- 'GCoericble' gets us closer than GEq, but the problem is Coercions state that
+-- *all* values can be coerced, but due to polymophism it is quite easy to find
+-- situations where some values of a type are safe to coerce and others are not.
+-- We just need one such value to abuse 'GCoercible StableName'.

src/Data/GADT/Internal.hs

@@ -321,32 +321,6 @@ instance GEq TR.TypeRep where
 -- GCompare
 -------------------------------------------------------------------------------
 
--- This instance seems nice, but it's simply not right:
---
--- > instance GEq StableName where
--- >     geq sn1 sn2
--- >         | sn1 == unsafeCoerce sn2
--- >             = Just (unsafeCoerce Refl)
--- >         | otherwise     = Nothing
---
--- Proof:
---
--- > x <- makeStableName id :: IO (StableName (Int -> Int))
--- > y <- makeStableName id :: IO (StableName ((Int -> Int) -> Int -> Int))
--- >
--- > let Just boom = geq x y
--- > let coerce :: (a :~: b) -> a -> b; coerce Refl = id
--- >
--- > coerce boom (const 0) id 0
--- > let "Illegal Instruction" = "QED."
---
--- The core of the problem is that 'makeStableName' only knows the closure
--- it is passed to, not any type information.  Together with the fact that
--- the same closure has the same StableName each time 'makeStableName' is
--- called on it, there is serious potential for abuse when a closure can
--- be given many incompatible types.
-
-
 -- |A type for the result of comparing GADT constructors; the type parameters
 -- of the GADT values being compared are included so that in the case where
 -- they are equal their parameter types can be unified.