Opened 5 months ago
Last modified 4 months ago
#13262 new feature request
Allow type synonym family application in instance head if it has no free variables
Reported by: | ezyang | Owned by: | |
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Priority: | low | Milestone: | |
Component: | Compiler (Type checker) | Version: | 8.0.1 |
Keywords: | backpack | Cc: | |
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | None/Unknown | Test Case: | |
Blocked By: | Blocking: | #12680 | |
Related Tickets: | Differential Rev(s): | ||
Wiki Page: |
Description
GHC rejects the following program:
{-# LANGUAGE TypeFamilies, FlexibleInstances #-} module F where type family F a type instance F Int = Int -> Int instance Show (F Int) where show _ = "fun"
But this doesn't seem fundamentally problematic to me: at the instance declaration, we know that F Int will reduce, and after that reduction there isn't any problem with the instance.
Here's a less restrictive instance head test: first, we reduce type families as much as possible. Then, if there are still leftover non-reducing type families, reject the instance.
After having typed this up, I have realized that I don't actually need this feature. But I'm filing it here for posterity.
Change History (11)
comment:1 Changed 4 months ago by
comment:2 Changed 4 months ago by
This has been proposed before (though I can't find where), and I find the idea quite distasteful. Consider what this would mean at the term level:
f :: Bool -> Int f (not True) = 5 f True = 6
Parsing challenges aside, there's nothing at all wrong with the above declaration. But do we really want programmers to be able to do this? It's the same at the type level.
comment:3 Changed 4 months ago by
By analogy, though, TypeSynonymInstances
are also distasteful, because it's effectively letting you write myfalse = False; f myfalse = 5
(well, that doesn't actually do what you want in Haskell today :o)
comment:4 Changed 4 months ago by
Agreed. But vanilla type synonyms are much simpler than type families. It's all a judgment call.
comment:5 Changed 4 months ago by
Priority: | lowest → low |
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Actually, I realized that something stronger would be OK here: it's OK for a type family synonym to occur in an instance, even if it doesn't reduce away, AS LONG AS the expression has no free variables.
This is pretty useless for normal programmers but actually it is pretty useful in Backpack. Consider:
-- there's some type family F a :: * -> * data T instance Monad (F T) where ...
This is something that I'd really quite like to write in a signature, because what I am saying is that there is some existing type family F, and whatever it is the abstract type you picked, F T better reduce to a monad. Yes I could replace the type family with another abstract type for the monad, but if I want Backpack and the type family to coexist, I get better backwards compatibility by reusing the type family.
The analogy is how we handle type families in constraints:
f :: Monad (F a) => ....
This is OK because a
is a skolem variable, so we aren't ever going to instantiate it to some variable while we're typechecking the body of f. What would be wrong, and is impossible to write in Haskell's type language today, is f :: (forall a. Monad (F a)) => ...
!
comment:6 Changed 4 months ago by
Blocking: | 12680 added |
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comment:7 Changed 4 months ago by
Summary: | Allow type synonym family application in instance head if it reduces away → Allow type synonym family application in instance head if it has no free variables |
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comment:8 Changed 4 months ago by
Keywords: | backpack added |
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comment:9 Changed 4 months ago by
This seems to be more complicated than just lifting the restriction, because whenever we have a wanted Show (F T) (where F is the type family), we always flatten this into Show a, a ~ F T; and of course, there is never an instance of Show for a in the top-level. I'm not entirely sure how the solver solves this when Show (F T) is a local constraint; some guidance here would be appreciated.
comment:10 Changed 4 months ago by
Let's not do this unless there is a really compelling reason. More complication. And soon you'll want
type instance F (G [a]) = blah
to work, assuming that G [a]
can reduce to (say) [Tree a]
.
This way lies madness. Let's not complicate the language of type patterns. Yet anyway.
comment:11 Changed 4 months ago by
Oh all right. We can wait till someone else pipes in and requests this :)
Perhaps we should guard the "first, we reduce type families as much as possible" step with the
TypeSynonymInstances
extension (which is implied byFlexibleInstances
, apparently).