Strictness of Semigroup instance for Maybe

[Andrew Martin]

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 <tikhon at jelv.is> 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 <david.feuer at gmail.com> wrote:
>> I think extra laziness here would be a bit surprising.
>> 
>> On Tue, May 22, 2018 at 5:57 PM, Donnacha Oisín Kidney
>> <mail at doisinkidney.com> wrote:
>> > 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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