[SOLVED] GHC Warning: Non-exhaustive Pattern Match for List Despite Explicit Match on Empty List

GHC Warning: Non-exhaustive Pattern Match for List Despite Explicit Match on Empty List

I’m trying to write a simple Haskell function that distinguishes between an empty list and a non-empty list. Here’s my code:

{-# LANGUAGE OverloadedLists, RebindableSyntax #-}

import Prelude (Int, IO, putStrLn)
import GHC.Exts (IsList (..))

doneVsItems :: [Int] -> IO ()
doneVsItems [] = putStrLn "Done"
doneVsItems (p:ps) = putStrLn "Items"

main :: IO ()
main = doneVsItems []

However, when I compile the code with -Werror=incomplete-patterns enabled, I get the following error:

error: [GHC-62161] [-Wincomplete-patterns, Werror=incomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘doneVsItems’:
        Patterns of type ‘[Int]’ not matched: []
  |
7 | doneVsItems [] = putStrLn "Done"
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

I explicitly pattern match on both the empty list ([]) and the non-empty list ((p:ps)), so I don’t understand why GHC is saying the pattern match is non-exhaustive. What am I missing here?

It looks like OverloadedLists, and RebindableSyntax no longer work together. I've created an issue here.

Solution

This is a bug with GHC. The extensions OverloadedLists, and RebindableSyntax no longer work together. See steps to reproduce.