1.) While I like the appeal to the PVP to export a version of (++) from Data.Monoid and I think this has worked out well for new modules like Control.Category, I'm not sure that with a module that has been around for so long as Data.Monoid can be so flippant about breaking any code that imports it unqualified that also happens to use a list. Lists are everywhere in Haskell, and unqualified imports do exist.

2.) There is also a pretty big caveat in that the choice of which operator should be naturally selected for (++) _is_ ambiguous. Should it be mappend or mplus? Recall that in Haskell 1.4 (++) worked on MonadPlus and it was changed in the great monomorphism revolution of '98.

-Edward Kmett

On Sun, Jul 5, 2009 at 4:41 PM, Henning Thielemann <lemming@henning-thielemann.de> wrote:

On Tue, 30 Jun 2009, Bryan O'Sullivan wrote:

I've thought for a while that it would be very nice indeed if the Monoid class had a more
concise operator for infix appending than "a `mappend` b". I wonder if other people are of a
similar opinion, and if so, whether this is worth submitting a libraries@ proposal over.

We have the package version policy which relies on explicit or qualified imports, such that adding a function like (++) to Data.Monoid cannot harm any package that follow that policy. Thus I vote for not introducing a new operator, in order to keep the set of infix operators to memorize small, but use (++) for the generalized (List.++) aka mappend. The user would however need to hide (++) from Prelude.

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