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  • #14228

Closed
Open
Opened Sep 13, 2017 by Guillaume Bouchard@guibouReporter

PatternSynonyms Non-exhaustive with UnboxedSums

The following implementation of Maybe using UnboxedSums results in Non-exhaustive patterns in case:

(Failure.hs file)

{-# LANGUAGE UnboxedSums #-}
{-# LANGUAGE PatternSynonyms #-}

type Maybe' t = (# t | () #)

pattern Just' :: a -> Maybe' a
pattern Just' x = (# x | #)

pattern Nothing' :: Maybe' a
pattern Nothing' = (# | () #)

foo x = case x of
  Nothing' -> putStrLn "nothing"
  Just' _ -> putStrLn "just"

main = do
  putStrLn "Nothing'"
  foo Nothing'

  putStrLn "Just'"
  foo (Just' "hello")

When executed, it prints:

Nothing'
nothing
Just'
Failure: Failure.hs:10:20-29: Non-exhaustive patterns in case

Compiled with ghc Failure.hs.

Please note that by removing the patterns, and writting foo as following works as expected:

foo x = case x of
  (# | () #) -> putStrLn "nothing"
  (# _ | #) -> putStrLn "just"
Trac metadata
Trac field Value
Version 8.2.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture
Assignee
Assign to
8.2.2
Milestone
8.2.2 (Past due)
Assign milestone
Time tracking
None
Due date
None
Reference: ghc/ghc#14228