#14230 closed bug (fixed)
Gruesome kind mismatch errors for associated data family instances
Reported by: | RyanGlScott | Owned by: | goldfire |
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Priority: | high | Milestone: | 8.8.1 |
Component: | Compiler (Type checker) | Version: | 8.3 |
Keywords: | TypeInType | Cc: | Iceland_jack |
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | Poor/confusing error message | Test Case: | indexed-types/should_fail/T14230{,a} |
Blocked By: | Blocking: | ||
Related Tickets: | #14175 | Differential Rev(s): | |
Wiki Page: |
Description
Spun off from https://ghc.haskell.org/trac/ghc/ticket/14175#comment:9. This program, which can only really be compiled on GHC HEAD at the moment:
{-# LANGUAGE GADTs #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} module Bug where class C k where data CD :: k -> k -> * instance C (Maybe a) where data CD :: (k -> *) -> (k -> *) -> *
Gives a heinous error message:
Bug.hs:11:3: error: • Expected kind ‘(k -> *) -> (k -> *) -> *’, but ‘CD :: (k -> *) -> (k -> *) -> *’ has kind ‘Maybe a -> Maybe a -> *’ • In the data instance declaration for ‘CD’ In the instance declaration for ‘C (Maybe a)’ | 11 | data CD :: (k -> *) -> (k -> *) -> * | ^^^^^^^
- We shouldn't be expecting kind
(k -> *) -> (k -> *) -> *
, but rather kindMaybe a -> Maybe a -> *
, due to the instance head itself. - The phrase
‘CD :: (k -> *) -> (k -> *) -> *’ has kind ‘Maybe -> Maybe a -> *’
is similarly confusing. This doesn't point out that the real issue is the use of(k -> *)
.
Another program in a similar vein:
{-# LANGUAGE GADTs #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} module Bug where import Data.Kind class C a where data CD k (a :: k) :: k -> * instance C (Maybe a) where data CD k (a :: k -> *) :: (k -> *) -> *
Bug.hs:13:3: error: • Expected kind ‘(k -> *) -> *’, but ‘CD k (a :: k -> *) :: (k -> *) -> *’ has kind ‘k -> *’ • In the data instance declaration for ‘CD’ In the instance declaration for ‘C (Maybe a)’ | 13 | data CD k (a :: k -> *) :: (k -> *) -> * | ^^^^^^^^^^^^^^^^^^^^^^^
This error message is further muddled by the incorrect use of k
as the first type pattern (instead of k -> *
, as subsequent kind signatures would suggest).
Change History (8)
comment:1 Changed 15 months ago by
Cc: | Iceland_jack added |
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comment:2 Changed 15 months ago by
Milestone: | → 8.4.1 |
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Owner: | set to goldfire |
Priority: | normal → high |
comment:3 Changed 7 months ago by
Milestone: | 8.4.1 → 8.6.1 |
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comment:4 Changed 6 months ago by
Milestone: | 8.6.1 → 8.8.1 |
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This won't be fixed in 8.6. Bumping to 8.8.
comment:5 Changed 2 weeks ago by
By gum, this might just be fixed. On GHC HEAD (commit 2257a86daa72db382eb927df12a718669d5491f8), we get the following error message for the first program:
$ inplace/bin/ghc-stage2 --interactive ../Bug.hs GHCi, version 8.7.20181129: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( ../Bug.hs, interpreted ) ../Bug.hs:11:3: error: • Type indexes must match class instance head Expected: CD @(Maybe a) Actual: CD @(k -> *) -- Defined at ../Bug.hs:11:8 • In the data instance declaration for ‘CD’ In the instance declaration for ‘C (Maybe a)’ | 11 | data CD :: (k -> *) -> (k -> *) -> * | ^^^^^^^
And the following error message for the second program:
GHCi, version 8.7.20181129: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( ../Bug.hs, interpreted ) ../Bug.hs:13:14: error: • Expected kind ‘k -> *’, but ‘a’ has kind ‘*’ • In the second argument of ‘CD’, namely ‘(a :: k -> *)’ In the data instance declaration for ‘CD’ In the instance declaration for ‘C (Maybe a)’ | 13 | data CD k (a :: k -> *) :: (k -> *) -> * | ^
Both of those look like pretty cromulent error messages to me. Should we check in test cases and declare victory?
comment:7 Changed 2 weeks ago by
Resolution: | → fixed |
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Status: | new → closed |
Test Case: | → indexed-types/should_fail/T14230{,a} |
comment:8 Changed 2 weeks ago by
Thanks for identifying this one Ryan. It had completely slipped off my radar.
Richard and I discussed this. It's firmly on his radar.