Proposal: Make Semigroup as a superclass of Monoid

[Stephen Tetley]

> why do you need an mconcat-like operation?

Well, the more operations the better, especially derived ones rather
than ones baked into the class.

For me a class with only one operation wouldn't really meet the
Fairburn threshold. In this case, I'd abstain from voting against as
the consensus is high but I would like to see a proper proposal for
what adding Semigroup includes (and what it leaves out) rather than a
straw in the wind notion to just add it.


On 29 April 2015 at 14:57, David Feuer <david.feuer at gmail.com> wrote:
> Kmett's NonEmpty type (or similar) is the solution to this, but really, why
> do you need an mconcat-like operation?
>
> On Wed, Apr 29, 2015 at 9:51 AM, Stephen Tetley <stephen.tetley at gmail.com>
> wrote:
>>
>> Empty lists are problematic if you want a mconcat like operation.
>>
>> Edward Kmett's semigroups package introduces an EmptyList type to
>> avoid this (the downside being it introduces an EmptyList type).
>>
>> If you don't want an mconcat like operation then Semigroup only has
>> one useful operation (<>).
>>
>>
>>
>> On 29 April 2015 at 14:13, David Feuer <david.feuer at gmail.com> wrote:
>> > What's problematic about empty lists for semigroups?
>> >
>> > On Wed, Apr 29, 2015 at 8:51 AM, Stephen Tetley
>> > <stephen.tetley at gmail.com>
>> > wrote:
>> >>
>> >> The consensus seems largely to favour the high level proposal to add a
>> >> Semigroup class, but there hasn't been much discussion of the
>> >> implementation details i.e. what to do about mappend or a
>> >> Semigroup-like candidate (problematic for empty lists, of course).
>> >>
>> >>
>> >>
>> >> On 29 April 2015 at 12:24, Jeremy <voldermort at hotmail.com> wrote:
>> >> > So what's the conclusion on this? We seem to have a strong consensus
>> >> > on
>> >> > going
>> >> > ahead with the proposal, and a weaker consensus for doing so via the
>> >> > long
>> >> > path.
>> >> >
>> >> >
>> >> >
>> >> > --
>> >> > View this message in context:
>> >> >
>> >> > http://haskell.1045720.n5.nabble.com/Proposal-Make-Semigroup-as-a-superclass-of-Monoid-tp5767835p5808129.html
>> >> > Sent from the Haskell - Libraries mailing list archive at Nabble.com.
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>
>