Strange interaction between higher-rank kinds and type synonyms
Here's some code with typechecks with GHC 8.0.1, 8.0.2, 8.2.1, and HEAD:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Bug where
import Data.Kind
data family Sing (a :: k)
data (a :: j) :~~: (b :: k) where
HRefl :: a :~~: a
data instance Sing (z :: a :~~: b) where
SHRefl :: Sing HRefl
(%:~~:->) :: forall (j :: Type) (k :: Type) (a :: j) (b :: k) (r :: a :~~: b) (p :: forall (z :: Type) (y :: z). a :~~: y -> Type).
Sing r
-> p HRefl
-> p r
(%:~~:->) SHRefl pHRefl = pHRefl
type App f x = f x
type HRApp (f :: forall (z :: Type) (y :: z). a :~~: y -> Type) (x :: a :~~: b) = f x
Now I want to refactor this so that I use App
:
(%:~~:->) :: forall (j :: Type) (k :: Type) (a :: j) (b :: k) (r :: a :~~: b) (p :: forall (z :: Type) (y :: z). a :~~: y -> Type).
Sing r
-> App p HRefl
-> App p r
However, GHC doesn't like that:
$ /opt/ghc/8.2.1/bin/ghci Bug.hs -fprint-explicit-kinds -fprint-explicit-foralls
GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Bug.hs:21:20: error:
• Expected kind ‘(:~~:) j z a a’,
but ‘'HRefl j a’ has kind ‘(:~~:) j j a a’
• In the second argument of ‘App’, namely ‘HRefl’
In the type signature:
(%:~~:->) :: forall (j :: Type)
(k :: Type)
(a :: j)
(b :: k)
(r :: a :~~: b)
(p :: forall (z :: Type) (y :: z). a :~~: y -> Type).
Sing r -> App p HRefl -> App p r
|
21 | -> App p HRefl
| ^^^^^
Bug.hs:22:20: error:
• Expected kind ‘(:~~:) j z a b’, but ‘r’ has kind ‘(:~~:) j k a b’
• In the second argument of ‘App’, namely ‘r’
In the type signature:
(%:~~:->) :: forall (j :: Type)
(k :: Type)
(a :: j)
(b :: k)
(r :: a :~~: b)
(p :: forall (z :: Type) (y :: z). a :~~: y -> Type).
Sing r -> App p HRefl -> App p r
|
22 | -> App p r
| ^
These errors are surprising to me, since it appears that the two higher-rank types, z
and y
, are behaving differently here: y
appears to be willing to unify with other types in applications of p
, but z
isn't! I would expect //both// to be willing to unify in applications of p
.
Out of desperation, I tried this other formulation of (%:~~:->)
which uses HRApp
instead of App
:
(%:~~:->) :: forall (j :: Type) (k :: Type) (a :: j) (b :: k) (r :: a :~~: b) (p :: forall (z :: Type) (y :: z). a :~~: y -> Type).
Sing r
-> HRApp p HRefl
-> HRApp p r
But now I get an even stranger error message:
$ /opt/ghc/8.2.1/bin/ghci Bug.hs -fprint-explicit-kinds -fprint-explicit-foralls
GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Bug.hs:21:20: error:
• Expected kind ‘forall z (y :: z). (:~~:) j z a y -> *’,
but ‘p’ has kind ‘forall z (y :: z). (:~~:) j z a y -> *’
• In the first argument of ‘HRApp’, namely ‘p’
In the type signature:
(%:~~:->) :: forall (j :: Type)
(k :: Type)
(a :: j)
(b :: k)
(r :: a :~~: b)
(p :: forall (z :: Type) (y :: z). a :~~: y -> Type).
Sing r -> HRApp p HRefl -> HRApp p r
|
21 | -> HRApp p HRefl
| ^
Bug.hs:21:20: error:
• Expected kind ‘forall z (y :: z). (:~~:) j z a y -> *’,
but ‘p’ has kind ‘forall z (y :: z). (:~~:) j z a y -> *’
• In the first argument of ‘HRApp’, namely ‘p’
In the type signature:
(%:~~:->) :: forall (j :: Type)
(k :: Type)
(a :: j)
(b :: k)
(r :: a :~~: b)
(p :: forall (z :: Type) (y :: z). a :~~: y -> Type).
Sing r -> HRApp p HRefl -> HRApp p r
|
21 | -> HRApp p HRefl
| ^
Bug.hs:22:20: error:
• Expected kind ‘forall z (y :: z). (:~~:) j z a y -> *’,
but ‘p’ has kind ‘forall z (y :: z). (:~~:) j z a y -> *’
• In the first argument of ‘HRApp’, namely ‘p’
In the type signature:
(%:~~:->) :: forall (j :: Type)
(k :: Type)
(a :: j)
(b :: k)
(r :: a :~~: b)
(p :: forall (z :: Type) (y :: z). a :~~: y -> Type).
Sing r -> HRApp p HRefl -> HRApp p r
|
22 | -> HRApp p r
| ^
That's right, it can't match the kinds:
-
forall z (y :: z). (:~~:) j z a y -> *
, and forall z (y :: z). (:~~:) j z a y -> *
Uh... what? Those are the same kind!
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |