Re: [xmonad] Re: Why isn't mappend for Query flipped?

On Tue, Jun 09, 2009 at 07:42:58PM -0400, Chung-chieh Shan wrote:

Brent Yorgey wrote in article <20090609164904.GA5859@seas.upenn.edu> in gmane.comp.lang.haskell.xmonad:

...

The upshot is that m1 <+> m2 does m2, and THEN m1.... I propose to change the Monoid instance for Query so that mappend = liftM2 (flip mappend)

When m2 is done followed by m1, sometimes the effect of m1 "takes precedence" (for example, when "doFloat" is pitted against "unfloat = ask >>= \w -> doF (W.sink w)") but sometimes the effect of m2 "takes precedence". I have had to reason about what takes precedence, and this proposed change would make the reasoning harder for me, simply because "flip" is one more level of indirection and I am already used to the match between "mappend" and the mathematical convention for function composition. So, I would say that the proposed change should be made iff this new meaning of <+> can be documented clearly and predictably without reference to the Endo Monoid notion of function composition.

Hmm. I can see how that would be confusing. I suppose another solution would be to just add another combinator which is like <+> but flipped, with some clear documentation. Actually, just adding some clearer documentation would probably go a long way without even changing any code. -Brent