Stefan O'Rear
On Thu, Sep 20, 2007 at 03:23:05PM +0100, Jon Fairbairn wrote:
Another example would be mplus. I happen to think that ++ is a good name for mplus, [...]
The reason ++ and mplus is so similar is that they used to be the same; (++) *was* the MonadPlus class member.
I haven't forgotten this (having been there for the old version)! I'm somewhat surprised that I can find no record of my objecting to the change for H98, but I wasn't on that committee and was pretty ill at the time...
Cale Gibbard explains this as {{{Quite a few people on the Haskell 98 committee had an irrational fear of polymorphism.}}}
It wasn't irrational (and not all of the committe agreed, see http://www.cs.chalmers.se/~rjmh/Haskell/Messages/Decision.cgi?id=273); the motivation was didactic. But the point of my proposal here is that we can have it both ways: beginners importing just List and using ++ get error messages about Lists, but advanced programmers importing List and MonadPlus can use ++ without accidentally constraining their code to the wrong Monad. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk