[Haskell-cafe] About mplus

[Henning Thielemann]

On Wed, 5 Sep 2007, ok wrote:

> On 5 Sep 2007, at 6:16 pm, Henning Thielemann wrote:
>
>> I think it is very sensible to define the generalized function in terms of 
>> the specific one, not vice versa.
>
> The specific point at issue is that I would rather use ++ than
> `mplus`.  In every case where both are defined, they agree, so
> it is rather frustrating to be blocked from using an operator
> which would otherwise have been appropriate.

What is your application, where you need (++) frequently? Today I like 
that (++) points me to the fact, that we are working on lists. Ok, I would 
be fine, if (++) would be a method for all sequence types. But for 
MonadPlus, this is too general for my taste.

> I am a little puzzled that there seems to be no connection between
> MonadPlus and Monoid.  Monoid requires a unit and an associative
> binary operator.  So does MonadPlus.  Unfortunately, they have different
> names.  If only we'd had (Monoid m, Monad m) => MonadPlus m...

Monoid is of kind *
MonadPlus is of kind * -> *