Re: Strictness of Semigroup instance for Maybe

I feel the the way concerning being lazy as possible and being left-strict where there is a symmetric choice to be made. This seems to be a common theme is base, although I’ve never seen it officially endorsed. I have seen Edward Kmett talk about this on reddit (contrasting it with the Monoid classes in strict-by-default languages), but I cannot find the thread. Sent from my iPhone

On May 22, 2018, at 7:57 PM, 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 &&.)

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