[Haskell-cafe] List instance of Alternative: why (++)?

Fiddling around, I found myself wanting: coalesce :: [a] -> [a] -> [a] -- or -- :: (Foldable t) => t a -> t a -> t a coalesce a b = if null a then b else a I expected this to be (<|>) (it is for Maybe!) but instead I find no canonical implementation of it anywhere, and what seems like a useless instance Alternative []. What's the rationale?