Proposal: Make Semigroup as a superclass of Monoid

[Stephen Tetley]

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.
>> >
>> >
>> >
>> > --
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