Proposal #3339: Add (+>) as a synonym for mappend

Conor McBride conor at
Sat Jul 18 10:17:00 EDT 2009

Hi Edward

On 18 Jul 2009, at 14:23, Edward Kmett wrote:

> Yeah, the Monoid instance for Maybe is somewhat unfortunate.
> I can see where they were going as Maybe does provide the natural  
> extension of a semigroup into a monoid by adding a unit element, but  
> it doesn't have a Semigroup class to build on, and so has to require  
> Monoid and in the end you get a definition that conflicts with the  
> MonadPlus/Alternative instances for Maybe, and only really helps if  
> you have broken Monoid instances around that are secretly just  
> Semigroups.


Types should mean more than mere data representations. The
current Monoid instance is inconsistent with the broad
interpretation of Maybe as a monad for exceptional computations.
An isomorphic data representation can and should be used to
attach a unit element to a semigroup,

Data structures are data with structure.

All the best


More information about the Libraries mailing list