Re: Strictness of Semigroup instance for Maybe

Hello, I think changing the strictness of this function could have potentially dramatic performance effects on a wide range of existing code. Exploring existing code to understand the exact impacts would be a huge challenge, and this is a change that would be hard to phase in. The arbitrariness of decisions like this is part of what makes the Monoid class a mess in the first place. Attaching instances like this to otherwise generic types forces us to make arbitrary choices, which are often not documented on the instances themselves. While the left-bias behavior might make sense in the case of an instance like we have for First, I don't see why it would be considered more correct in this case. I'm -1 on this proposal. Best regards, Eric Mertens On Wed, May 23, 2018 at 4:21 AM Andrew Martin wrote:

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.

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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 &&.)

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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