Wildcard patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings
Consider this program:
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
{-# OPTIONS_GHC -Wincomplete-patterns #-}
module Bug 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
This compiles with no warnings with -Wall
. But if you tweak catchAll
to add a catch-all case at the end:
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
{-# OPTIONS_GHC -Wincomplete-patterns #-}
module Bug 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
catchAll _ = error "impossible"
Then if you compile it with -Wall
, you'll get a very misleading warning:
$ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive Bug.hs -Wall
GHCi, version 8.1.20170228: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Bug.hs:17:1: warning: [-Woverlapping-patterns]
Pattern match is redundant
In an equation for ‘catchAll’: catchAll TooGoodToBeTrue = ...
|
17 | catchAll TooGoodToBeTrue = 1
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I would have expected the warning to be on the catchAll _ = error "impossible"
case!
Trac metadata
Trac field | Value |
---|---|
Version | 8.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | mpickering |
Operating system | |
Architecture |