Re: Proposal: Make Semigroup as a superclass of Monoid

I'd definitely be -1 on an mconcat-like operation for Semigroup. I think mconcat mostly fails to be useful for monoid, because all it has over fold is knowledge of right and left bias and the theoretical (but practically, I think, rather hard to exploit) ability to group and locally rearrange elements while summing. Kmett has a version of Foldable designed to use with semigroups, and I'd be okay with adding that. On Wed, Apr 29, 2015 at 10:14 AM, Stephen Tetley wrote:

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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.

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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@gmail.com> wrote:

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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 wrote:

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What's problematic about empty lists for semigroups?

On Wed, Apr 29, 2015 at 8:51 AM, Stephen Tetley wrote:

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The consensus seems largely to favour the high level proposal to add

a

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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 wrote:

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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

On 29 April 2015 at 14:57, David Feuer wrote: the

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long path.

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