Hi Mark, Yes I would favour Rebecca's approach in her book to use Semigroup. Personally I never think of lists as Monads, as I see this instance useful only to power list comprehension. Concatenation with (<>) is more meaningful and holds better semantics than (<|>), described as just "An associative binary operation". It's so vague it could be the documentation for a Magma typeclass, and the name "Alternative" is quite loaded (we think especially of parsers when thinking/seeing Alternative). Cheers, Hécate Le 12/05/2024 à 21:41, Mark McConnell via Haskell-Cafe a écrit :
We know that lists have map. Sometimes we want to take the higher perspective of Functor, replacing map with fmap, then replacing fmap with <$> . This post asks the corresponding question about append.
The notation for append within List itself is ++ . Often we think of lists as Monads. Applicative generalizes Monad, and Alternative specializes Applicative. Alternative has <|> which behaves as ++ on List. I have been using <|> for this purpose.
Recently, I have begun to read and enjoy "Effective Haskell". In Chapter 1, this book uses <> for append. Clearly they are thinking of List as a Semigroup.
I'm curious about your thoughts on ++ versus <|> versus <> .
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