I’ve wanted this before as well. Maybe we should throw a newtype at it?
newtype LeftBiased a = LeftBiased [a]
instance Alternative (LeftBiased a) where
empty = []
[] <|> b = b
a <|> _ = a
newtype RightBiased a = RightBiased [a]
instance Alternative (RightBiased a) where
empty = []
a <|> [] = a
_ <|> b = b
This could be generalised to work on any Foldable, actually, if that’s
desirable. And of course the bikeshed could be a different colour as for
the names.
It’s unfortunate that the instance for Maybe is already biased differently;
I wonder if this instance would be useful, or if it’s already available
somewhere?
newtype Unbiased a = Unbiased (Maybe a)
instance (Monoid m) => Alternative (Unbiased m) where
empty = Nothing
Just a <|> Just b = Just (a <> b)
_ <|> Just b = Just b
Just a <|> _ = Just a
_ <|> _ = Nothing
On Fri, May 5, 2017 at 11:12 PM, 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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