[Haskell-cafe] package for algebraic structures

[Wolfgang Jeltsch]

Hello,

for Grapefruit’s incremental list signal support, I needed a type class of 
semigroups. A semigroup is similar to a monoid. The difference is that a 
semigroup doesn’t need to have a neutral element. So a semigroup type class 
would make a perfect superclass of Monoid, by the way.

Since a semigroup class could be of more general interest, I decided to not 
include it in Grapefruit. Since I couldn’t quickly find an implementation of 
semigroups in Haskell, I put my own into a new package.

Now, a package only for one class with one method seems like overkill. 
However, it could serve as a start for a package of all kinds of algebraic 
structures. So I called the package “algebra” and put it on Hackage.

So if someone wants to implement rings, ideals, fields or whatever, I’d be 
happy to receive patches.

Best wishes,
Wolfgang