Exhaustivity check should suggest when COMPLETE could be helpful
MacBook-Pro-97:ghc ezyang$ cat A.hs
{-# LANGUAGE PatternSynonyms #-}
module A where
pattern F :: a -> b -> (a, b)
pattern F x y = (x, y)
g :: (a, b) -> (a, b)
g (F x y) = (x, y)
MacBook-Pro-97:ghc ezyang$ inplace/bin/ghc-stage2 -c A.hs -Wall -fforce-recomp
A.hs:8:1: warning: [-Wincomplete-patterns]
Pattern match(es) are non-exhaustive
In an equation for ‘g’: Patterns not matched: _
|
8 | g (F x y) = (x, y)
| ^^^^^^^^^^^^^^^^^^
MacBook-Pro-97:ghc ezyang$ inplace/bin/ghc-stage2 --version
The Glorious Glasgow Haskell Compilation System, version 8.5.20180304Any time the exhaustiveness checker throws up its hands and says that _ is not matched, we ought to give a hint that this may have occurred due to pattern synonyms, and that the author of the pattern synonyms can help out by specifying a COMPLETE pragma.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.5 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |