16 May
2011
16 May
'11
5:18 a.m.
On Monday 16 May 2011 11:07:15, Michael Vanier wrote:
Usually in monad tutorials, the >>= operator for the list monad is defined as:
m >>= k = concat (map k m) -- or concatMap k m
but in the GHC sources it's defined as:
m >>= k = foldr ((++) . k) [] m
As far as I can tell, this definition is equivalent to the previous one
It is indeed, otherwise at least one of them would be wrong.
(correct me if I'm wrong), so I was wondering why this definition was chosen instead of the other one. Does anybody know?
I don't *know*, but I suspect it's for efficiency, writing concat (map k m) might not be unfolded enough to make foldr/build fusion fire in cases where it applies. I'm just guessing, though.
Thanks in advance,
Mike