Heterogeneous equality
Hi! The base package contains the module Data.Type.Equality, which contains the type (:~:) for homogeneous equality. I was a bit surprised that there is no type for heterogeneous equality there. Is there such a type somewhere else in the standard library? I tried to define a type for heterogeneous equality myself as follows: > {-# LANGUAGE GADTs, PolyKinds, TypeOperators #-} > > data a :~~: b where > > HRefl :: a :~~: a To my surprise, the kind of (:~~:) defined this way is k -> k -> *, not k -> l -> *. Why is this? Apparently, the latter, more general, kind is possible, since we can define (:~~:) :: k -> l -> * as follows: > {-# LANGUAGE GADTs, PolyKinds, TypeOperators #-} > > data (a :: k) :~~: (b :: l) where > > HRefl :: a :~~: a All the best, Wolfgang ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
Re: Trouble with injective type families
I'd like to add that the reason we never extended System FC with support for injectivity is that the proof of soundness of doing so has remained elusive. A similar unsoundness in type improvement may cause unpredictable behavior in type inference, but it can't ever launch the rockets. So we keep it in type inference but out of FC. Richard > On Jul 5, 2017, at 9:37 AM, Wolfgang Jeltschwrote: > > Dear Simon, > > thank you very much for this elaborate explanation. > > I stumbled on this issue when using functional dependencies years ago. > The solution at that time was to use type families. > > I did not know that injectivity is handled analogously to functional > dependencies. Given that it is, the syntax for injectivity makes a lot > more sense. > > All the best, > Wolfgang > > Am Mittwoch, den 05.07.2017, 06:45 + schrieb Simon Peyton Jones: >> Functional dependencies and type-family dependencies only induce extra >> "improvement" constraints, not evidence. For example >> >> class C a b | a -> b where foo :: a -> b >> instance C Bool Int where ... >> >> f :: C Bool b => b -> Int >> f x = x -- Rejected >> >> Does the fundep on 'b' allow us to deduce (b ~ Int), GADT-like, in the >> body of 'f', and hence accept the definition. No, it does not. Think >> of the translation into System F. We get >> >> f = /\b \(d :: C Bool b). \(x::b). x |> ??? >> >> What evidence can I used to cast 'x' by to get it from type 'b' to >> Int? >> >> Rather, fundeps resolve ambiguity. Consider >> >> g x = foo True + x >> >> The call to 'foo True' gives rise to a "wanted" constraint (C Bool >> beta), where beta is a fresh unification variable. Then by the fundep >> we get an "improvement" constraint (also "wanted") (beta ~ Int). So we >> can infer g :: Int -> Int. >> >> >> In your example we have >> >>x :: forall a b. (T Int ~ b) => a >>x = False >> >> Think of the System F translation: >> >>x = /\a b. \(d :: T Int ~ b). False |> ?? >> >> Again, what evidence can we use to cast False to 'a'. >> >> >> In short, fundeps and type family dependencies only add extra >> unification constraints, which may help to resolve ambiguous >> types. They don’t provide evidence. That's not to say that they >> couldn't. But you'd need to extend System FC, GHC's core language, to >> do so. >> >> Simon >> >> >>> >>> -Original Message- >>> From: Glasgow-haskell-users [mailto:glasgow-haskell-users- >>> boun...@haskell.org] On Behalf Of Wolfgang Jeltsch >>> Sent: 05 July 2017 01:21 >>> To: glasgow-haskell-users@haskell.org >>> Subject: Trouble with injective type families >>> >>> Hi! >>> >>> Injective type families as supported by GHC 8.0.1 do not behave like >>> I >>> would expect them to behave from my intuitive understanding. >>> >>> Let us consider the following example: >>> {-# LANGUAGE RankNTypes, TypeFamilyDependencies #-} class C a where type T a = b | b -> a instance C Bool where type T Bool = Int type X b = forall a . T a ~ b => a x :: X Int x = False >>> I would expect this code to be accepted. However, I get the >>> following >>> error message: >>> A.hs:14:5: error: • Could not deduce: a ~ Bool from the context: T a ~ Int bound by the type signature for: x :: T a ~ Int => a at A.hs:13:1-10 ‘a’ is a rigid type variable bound by the type signature for: x :: forall a. T a ~ Int => a at A.hs:11:19 • In the expression: False In an equation for ‘x’: x = False • Relevant bindings include x :: a (bound at A.hs:14:1) >>> This is strange, since injectivity should exactly make it possible >>> to >>> deduce a ~ Bool from T a ~ Int. >>> >>> Another example is this: >>> {-# LANGUAGE GADTs, TypeFamilyDependencies #-} class C a where type T a = b | b -> a instance C Bool where type T Bool = Int data G b where G :: Eq a => a -> G (T a) instance Eq (G b) where G a1 == G a2 = a1 == a2a >>> I would also expect this code to be accepted. However, I get the >>> following error message: >>> B.hs:17:26: error: • Could not deduce: a1 ~ a from the context: (b ~ T a, Eq a) bound by a pattern with constructor: G :: forall a. Eq a => a -> G (T a), in an equation for ‘==’ at B.hs:17:5-8 or from: (b ~ T a1, Eq a1) bound by a pattern with constructor: G :: forall a. Eq a => a -> G (T a), in an equation for ‘==’ at B.hs:17:13-16 ‘a1’ is a rigid type variable bound by a pattern with
Re: Trouble with injective type families
Dear Simon, thank you very much for this elaborate explanation. I stumbled on this issue when using functional dependencies years ago. The solution at that time was to use type families. I did not know that injectivity is handled analogously to functional dependencies. Given that it is, the syntax for injectivity makes a lot more sense. All the best, Wolfgang Am Mittwoch, den 05.07.2017, 06:45 + schrieb Simon Peyton Jones: > Functional dependencies and type-family dependencies only induce extra > "improvement" constraints, not evidence. For example > > class C a b | a -> b where foo :: a -> b > instance C Bool Int where ... > > f :: C Bool b => b -> Int > f x = x -- Rejected > > Does the fundep on 'b' allow us to deduce (b ~ Int), GADT-like, in the > body of 'f', and hence accept the definition. No, it does not. Think > of the translation into System F. We get > > f = /\b \(d :: C Bool b). \(x::b). x |> ??? > > What evidence can I used to cast 'x' by to get it from type 'b' to > Int? > > Rather, fundeps resolve ambiguity. Consider > > g x = foo True + x > > The call to 'foo True' gives rise to a "wanted" constraint (C Bool > beta), where beta is a fresh unification variable. Then by the fundep > we get an "improvement" constraint (also "wanted") (beta ~ Int). So we > can infer g :: Int -> Int. > > > In your example we have > > x :: forall a b. (T Int ~ b) => a > x = False > > Think of the System F translation: > > x = /\a b. \(d :: T Int ~ b). False |> ?? > > Again, what evidence can we use to cast False to 'a'. > > > In short, fundeps and type family dependencies only add extra > unification constraints, which may help to resolve ambiguous > types. They don’t provide evidence. That's not to say that they > couldn't. But you'd need to extend System FC, GHC's core language, to > do so. > > Simon > > > > > > -Original Message- > > From: Glasgow-haskell-users [mailto:glasgow-haskell-users- > > boun...@haskell.org] On Behalf Of Wolfgang Jeltsch > > Sent: 05 July 2017 01:21 > > To: glasgow-haskell-users@haskell.org > > Subject: Trouble with injective type families > > > > Hi! > > > > Injective type families as supported by GHC 8.0.1 do not behave like > > I > > would expect them to behave from my intuitive understanding. > > > > Let us consider the following example: > > > > > > > > {-# LANGUAGE RankNTypes, TypeFamilyDependencies #-} > > > > > > class C a where > > > > > > type T a = b | b -> a > > > > > > instance C Bool where > > > > > > type T Bool = Int > > > > > > type X b = forall a . T a ~ b => a > > > > > > x :: X Int > > > x = False > > I would expect this code to be accepted. However, I get the > > following > > error message: > > > > > > > > A.hs:14:5: error: > > > • Could not deduce: a ~ Bool > > > from the context: T a ~ Int > > > bound by the type signature for: > > > x :: T a ~ Int => a > > > at A.hs:13:1-10 > > > ‘a’ is a rigid type variable bound by > > > the type signature for: > > > x :: forall a. T a ~ Int => a > > > at A.hs:11:19 > > > • In the expression: False > > > In an equation for ‘x’: x = False > > > • Relevant bindings include x :: a (bound at A.hs:14:1) > > This is strange, since injectivity should exactly make it possible > > to > > deduce a ~ Bool from T a ~ Int. > > > > Another example is this: > > > > > > > > {-# LANGUAGE GADTs, TypeFamilyDependencies #-} > > > > > > class C a where > > > > > > type T a = b | b -> a > > > > > > instance C Bool where > > > > > > type T Bool = Int > > > > > > data G b where > > > > > > G :: Eq a => a -> G (T a) > > > > > > instance Eq (G b) where > > > > > > G a1 == G a2 = a1 == a2a > > I would also expect this code to be accepted. However, I get the > > following error message: > > > > > > > > B.hs:17:26: error: > > > • Could not deduce: a1 ~ a > > > from the context: (b ~ T a, Eq a) > > > bound by a pattern with constructor: > > > G :: forall a. Eq a => a -> G (T a), > > > in an equation for ‘==’ > > > at B.hs:17:5-8 > > > or from: (b ~ T a1, Eq a1) > > > bound by a pattern with constructor: > > > G :: forall a. Eq a => a -> G (T a), > > > in an equation for ‘==’ > > > at B.hs:17:13-16 > > > ‘a1’ is a rigid type variable bound by > > > a pattern with constructor: G :: forall a. Eq a => a -> G > > > (T > > > a), > > > in an equation for ‘==’ > > > at B.hs:17:13 > > > ‘a’ is a rigid type variable bound by > > > a pattern with constructor: G :: forall a. Eq a => a -> G > > > (T > > > a), > > > in an equation for ‘==’ > > > at B.hs:17:5 > > > • In the second argument of ‘(==)’, namely ‘a2’ > > > In the expression: a1 == a2 > > >
RE: Trouble with injective type families
Functional dependencies and type-family dependencies only induce extra "improvement" constraints, not evidence. For example class C a b | a -> b where foo :: a -> b instance C Bool Int where ... f :: C Bool b => b -> Int f x = x -- Rejected Does the fundep on 'b' allow us to deduce (b ~ Int), GADT-like, in the body of 'f', and hence accept the definition. No, it does not. Think of the translation into System F. We get f = /\b \(d :: C Bool b). \(x::b). x |> ??? What evidence can I used to cast 'x' by to get it from type 'b' to Int? Rather, fundeps resolve ambiguity. Consider g x = foo True + x The call to 'foo True' gives rise to a "wanted" constraint (C Bool beta), where beta is a fresh unification variable. Then by the fundep we get an "improvement" constraint (also "wanted") (beta ~ Int). So we can infer g :: Int -> Int. In your example we have x :: forall a b. (T Int ~ b) => a x = False Think of the System F translation: x = /\a b. \(d :: T Int ~ b). False |> ?? Again, what evidence can we use to cast False to 'a'. In short, fundeps and type family dependencies only add extra unification constraints, which may help to resolve ambiguous types. They don’t provide evidence. That's not to say that they couldn't. But you'd need to extend System FC, GHC's core language, to do so. Simon | -Original Message- | From: Glasgow-haskell-users [mailto:glasgow-haskell-users- | boun...@haskell.org] On Behalf Of Wolfgang Jeltsch | Sent: 05 July 2017 01:21 | To: glasgow-haskell-users@haskell.org | Subject: Trouble with injective type families | | Hi! | | Injective type families as supported by GHC 8.0.1 do not behave like I | would expect them to behave from my intuitive understanding. | | Let us consider the following example: | | > {-# LANGUAGE RankNTypes, TypeFamilyDependencies #-} | > | > class C a where | > | > type T a = b | b -> a | > | > instance C Bool where | > | > type T Bool = Int | > | > type X b = forall a . T a ~ b => a | > | > x :: X Int | > x = False | | I would expect this code to be accepted. However, I get the following | error message: | | > A.hs:14:5: error: | > • Could not deduce: a ~ Bool | > from the context: T a ~ Int | > bound by the type signature for: | > x :: T a ~ Int => a | > at A.hs:13:1-10 | > ‘a’ is a rigid type variable bound by | > the type signature for: | > x :: forall a. T a ~ Int => a | > at A.hs:11:19 | > • In the expression: False | > In an equation for ‘x’: x = False | > • Relevant bindings include x :: a (bound at A.hs:14:1) | | This is strange, since injectivity should exactly make it possible to | deduce a ~ Bool from T a ~ Int. | | Another example is this: | | > {-# LANGUAGE GADTs, TypeFamilyDependencies #-} | > | > class C a where | > | > type T a = b | b -> a | > | > instance C Bool where | > | > type T Bool = Int | > | > data G b where | > | > G :: Eq a => a -> G (T a) | > | > instance Eq (G b) where | > | > G a1 == G a2 = a1 == a2a | | I would also expect this code to be accepted. However, I get the | following error message: | | > B.hs:17:26: error: | > • Could not deduce: a1 ~ a | > from the context: (b ~ T a, Eq a) | > bound by a pattern with constructor: | > G :: forall a. Eq a => a -> G (T a), | > in an equation for ‘==’ | > at B.hs:17:5-8 | > or from: (b ~ T a1, Eq a1) | > bound by a pattern with constructor: | > G :: forall a. Eq a => a -> G (T a), | > in an equation for ‘==’ | > at B.hs:17:13-16 | > ‘a1’ is a rigid type variable bound by | > a pattern with constructor: G :: forall a. Eq a => a -> G (T | > a), | > in an equation for ‘==’ | > at B.hs:17:13 | > ‘a’ is a rigid type variable bound by | > a pattern with constructor: G :: forall a. Eq a => a -> G (T | > a), | > in an equation for ‘==’ | > at B.hs:17:5 | > • In the second argument of ‘(==)’, namely ‘a2’ | > In the expression: a1 == a2 | > In an equation for ‘==’: (G a1) == (G a2) = a1 == a2 | > • Relevant bindings include | > a2 :: a1 (bound at B.hs:17:15) | > a1 :: a (bound at B.hs:17:7) | | If b ~ T a and b ~ T a1, then T a ~ T a1 and subsequently a ~ a1, because | of injectivity. Unfortunately, GHC does not join the two contexts (b ~ T | a, Eq a) and (b ~ T a1, Eq a1). | | Are these behaviors really intended, or are these bugs showing up? | | All the best, | Wolfgang | ___ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.hask | ell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fglasgow-haskell- |