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

[Theodore Lief Gannon]

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?
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