Pattern-match warnings for datatypes with COMPLETE sets break abstraction
Here's a file:
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
module Bug (Boolean(F, TooGoodToBeTrue), catchAll) where
data Boolean = F | T
deriving Eq
pattern TooGoodToBeTrue :: Boolean
pattern TooGoodToBeTrue <- ((== T) -> True)
where
TooGoodToBeTrue = T
{-# COMPLETE F, TooGoodToBeTrue #-}
catchAll :: Boolean -> Int
catchAll F = 0
-- catchAll TooGoodToBeTrue = 1
If you compile this with -Wall
, the warning will warn about T
, not TooGoodToBeTrue
(the other conlike in the COMPLETE
set):
$ /opt/ghc/8.2.1/bin/ghc -fforce-recomp -Wall Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:15:1: warning: [-Wincomplete-patterns]
Pattern match(es) are non-exhaustive
In an equation for ‘catchAll’: Patterns not matched: T
|
15 | catchAll F = 0
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Perhaps this isn't so bad, since it's intra-module. But the problem persists even across modules:
module Foo where
import Bug
catchAll2 :: Boolean -> Int
catchAll2 F = 0
-- catchAll2 TooGoodToBeTrue = 1
$ /opt/ghc/8.2.1/bin/ghc -fforce-recomp -c Bug.hs
$ /opt/ghc/8.2.1/bin/ghc -fforce-recomp -c -Wall Foo.hs
Foo.hs:6:1: warning: [-Wincomplete-patterns]
Pattern match(es) are non-exhaustive
In an equation for ‘catchAll2’: Patterns not matched: Bug.T
|
6 | catchAll2 F = 0
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This one is really bad, since it's warning about Bug.T
, which should be hidden!
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |