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 +0000 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- bounces@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- users&data=02%7C01%7Csimonpj%40microsoft.com%7Cc333fbaff2f4406c337e0 8d4c3 3bd1bd%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C6363481090820179 89&sd ata=q%2B8ZSqYfNjC7x6%2Bm9vpsgCmCDo%2FIItppqB5Nwzf6Qj0%3D&reserved=0