I’m learning about the GHC ViewPatterns language extension at the same time as learning about Data.Sequence. I like the the ability to pattern-match on sequences a lot. Unfortunately, it looks like the exhaustiveness checker cannot detect when all possible patterns are covered. Here’s an example:
| #!/usr/bin/env stack | |
| {- stack --resolver=lts-12.6 script -} | |
| {-# LANGUAGE ViewPatterns #-} | |
| {-# OPTIONS_GHC -Wall #-} | |
| import Data.Sequence | |
| import qualified Data.Sequence as Seq | |
| joinShow :: Show a => Seq a -> String | |
| joinShow (viewl -> EmptyL) = "" | |
| joinShow (viewl -> x :< xs) = show x ++ joinShow xs | |
| main :: IO () | |
| main = do | |
| let xs = Seq.fromList [1 :: Int, 2, 3, 4] | |
| print $ joinShow xs | |
| {- | |
| /path/to/ViewPatternDemo.hs:11:1: warning: [-Wincomplete-patterns] | |
| Pattern match(es) are non-exhaustive | |
| In an equation for ‘joinShow’: Patterns not matched: _ | |
| | | |
| 11 | joinShow (viewl -> EmptyL) = "" | |
| | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... | |
| -} |
Running this script results in the following compiler warnings:
/path/to/ViewPatternDemo.hs:11:1: warning: [-Wincomplete-patterns]
Pattern match(es) are non-exhaustive
In an equation for ‘joinShow’: Patterns not matched: _
|
11 | joinShow (viewl -> EmptyL) = ""
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
-}
I’m going to see if there are known GHC bugs related to this issue.
Update: 2019-01-25
It turns out that the PatternSynonyms language extension can do more or less the same thing but with nicer syntax:
| #!/usr/bin/env stack | |
| {- stack --resolver=lts-12.6 script -} | |
| {-# OPTIONS_GHC -Wall #-} | |
| import Data.Sequence | |
| import qualified Data.Sequence as Seq | |
| joinShow :: Show a => Seq a -> String | |
| joinShow Empty = "" | |
| joinShow (x :<| xs) = show x ++ joinShow xs | |
| main :: IO () | |
| main = do | |
| let xs = Seq.fromList [1 :: Int, 2, 3, 4] | |
| print $ joinShow xs |
I think pattern synonyms are strictly less powerful than view patterns which is why (I think) GHC’s exhaustiveness checker works better with them.
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