Fwd: Proposal: Add Data.Semigroup to base, as a superclass of Monoid

Gabriel Gonzalez gabriel439 at gmail.com
Wed Jun 12 23:31:56 CEST 2013

Forgot to copy `libraries` on my answer to your question:

On Wed, Jun 12, 2013 at 3:28 AM, Herbert Valerio Riedel <hvr at gnu.org> wrote:

> On 2013-06-12 at 00:04:04 +0200, Gabriel Gonzalez wrote:
> > I think types that lack an empty element are a misfeature.
> ...so having a data-type for representing non-empty lists (on which
> operation such as head/last/minimum/maximum et. al can be proper
> statically guaranteed total functions as opposed to resorting to
> 'Maybe'-wrapped results which need to be checked dynamically at runtime)
> is a misfeature?
I phrased that poorly.  Non-empty data types are useful, but having a
combining operation on those types of type:

A -> A -> A

... is not.

The very example you gave (non-empty lists) shows why.  If you combine two
non-empty lists you can actually prove a stronger result, that the combined
list has at least two elements.  However, you lose that information if you
use the `mappend` operation.  I'm not saying that non-empty lists shouldn't
have a combining operation, but rather that `mappend` is not the
appropriate operation for the task.

> They usually end up contaminating everything they touch, which is why
> > semigroups forms an entire parallel ecosystem of its own.
> Can you provide a concrete example showing the kind of problematic
> "contamination" that is caused by semigroup-forming types?
The non-empty list example shows what I meant.  The moment you start
including extra information like non-emptiness you have to re-engineer all
downstream operations to preserve that information as faithfully as
possible.  Again, there's nothing wrong with that, but it's not deserving
of its own type class or a special place in the Prelude.

> cheers,
>   hvr
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