On Thu, Jan 26, 2023 at 09:05:40AM +0900, Kazu Yamamoto (山本和彦) via Haskell-Cafe wrote:
Hello,
The recent GHC added incomplete-uni-patterns to the -Wall option. So, we have a waning with the following code:
let [addr,port] = args
To avoid this, I changed the code to:
let addr = head args port = head $ tail args
The univeral option type is Maybe, so you leverage that: maybePair :: [a] -> Maybe (a, a) maybePair [a, b] = Just (a, b) maybePair _ = Nothing knownPair :: [a] -> (a, a) knownPair = fromJust . maybePair and then anywhere you need to only match a 2-element list: let (a, b) = knownPair args Or just: {-# OPTIONS_GHC -Wall -Wno-incomplete-uni-patterns #-} let [a, b] knownPair args Perhaps mildly inconvenient at times, but I am inlined to think that overall better than never reporting potential problem patterns. -- Viktor.