Question

something like

getFirstError :: [Either a b] -> a
getFirstError (x:y:...:Left w:z) = w

wrt Haskell but it might be interesting to know how other languages with pattern matching accomplish this.

Answer 1

You can, despite the other answers, do this using view patterns extension in GHC:

   getFirstError ((msum . map test) -> Just x) = x
        where test (Left x) = Just x
              test (Right x) = Nothing

Alternatively using pattern guards:

   getFirstError (xs) | Just x <- (msum $ map test xs) = x
        where test (Left x) = Just x
              test (Right) x = Nothing