[Haskell-cafe] [announcement] filtrable: class of filtrable containers

Kosyrev Serge _deepfire at feelingofgreen.ru
Wed Feb 17 12:21:08 UTC 2016

Oleg Grenrus <oleg.grenrus at iki.fi> writes:
> And one more comment:
> The law:
>     filter f = mapMaybe (liftA2 (<$) id (guard ∘ f))
> is very hard to understand.
>     filter f = mapMaybe (\x -> if f x then Just x else Nothing)

Wow, the difference is simply astounding for me.

Does this monster

  liftA2 (<$) id (guard ∘ f)

really stand for

  \x -> if f x then Just x else Nothing


Is there really no simpler "compact" representation for this trivial idea in Haskell?

с уважениeм / respectfully,
Косырев Сергей

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