Re: Strictness of Semigroup instance for Maybe

I second this expectation. I've recently had a hard time ensuring laziness in my work and it's been comforting in rooting out failures to know that the standard libraries, at least, are concerned about it. On Tue, May 22, 2018, 16:57 Tikhon Jelvis wrote:

I think the extra laziness makes sense here—it matches the behavior of common functions like &&. My general expectation is that functions are as lazy as they can be and, in the case of operators with two arguments, that evaluation goes left-to-right. (Again like &&.)

On Tue, May 22, 2018 at 4:37 PM, David Feuer wrote:

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I think extra laziness here would be a bit surprising.

On Tue, May 22, 2018 at 5:57 PM, Donnacha Oisín Kidney wrote:

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The current semigroup instance for Maybe looks like this:

instance Semigroup a => Semigroup (Maybe a) where Nothing <> b = b a <> Nothing = a Just a <> Just b = Just (a <> b)

However, it could be lazier:

instance Semigroup a => Semigroup (Maybe a) where Nothing <> b = b Just a <> b = Just (maybe a (a<>) b)

This causes different behaviour for Data.Semigroup.First and Data.Monoid.First:

>>> Data.Monoid.getFirst . foldMap pure $ [1..] Just 1 >>> fmap Data.Semigroup.getFirst . Data.Semigroup.getOption . foldMap (pure.pure) $ [1..] _|_

A different definition for `Option` gets back the old behaviour:

newtype LeftOption a = LeftOption { getLeftOption :: Maybe a }

instance Semigroup a => Semigroup (LeftOption a) where LeftOption Nothing <> ys = ys LeftOption (Just x) <> LeftOption ys = LeftOption (Just (maybe x (x<>) ys))

instance Semigroup a => Monoid (LeftOption a) where mempty = LeftOption Nothing mappend = (<>)

>>> fmap Data.Semigroup.getFirst . getLeftOption . foldMap (LeftOption . Just . Data.Semigroup.First) $ [1..] Just 1

Is there any benefit to the extra strictness? Should this be changed?

Another consideration is that the definition could equivalently be right-strict, to get the desired behaviour for Last, but I think the left-strict definition probably follows the conventions more.

I originally posted this to reddit ( https://www.reddit.com/r/haskell/comments/8lbzan/semigroup_maybe_too_strict/ ) and was encouraged to post it here.

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