On 16/05/2011 10:07 AM, 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
(correct me if I'm wrong), so I was wondering why this definition was
chosen instead of the other one. Does anybody know?
Any time you see a more convoluted definition which ought to be
equivilent to a simpler one, the answer is usually "because this way
makes some important compiler optimisation fire". It's even possible
that the optimisation in question would fire anyway now, but way back
when the code was written, the compiler wasn't as smart.
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