Re: Proposal: Make Semigroup as a superclass of Monoid

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 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 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 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 the long path.

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