On Wed, 2009-04-08 at 16:57 +0200, Thomas Davie wrote:
We have two possible definitions of an "iterateM" function:
iterateM 0 _ _ = return [] iterateM n f i = (i:) <$> (iterateM (n-1) f =<< f i)
iterateM n f i = sequence . scanl (>>=) (return i) $ replicate n f
The former uses primitive recursion, and I get the feeling it should be better written without it. The latter is quadratic time – it builds up a list of monadic actions, and then runs them each in turn.
It's also quadratic in invocations of f, no? If your monad's (>>=) doesn't object to being left-associated (which is *not* the case for free monads), then I think iterateM n f i = foldl (>>=) (return i) $ replicate n f would be both correct and linear. If you're monad's (>>=) is itsef quadratic in time when left-associated, you can try applying a CPS transformation to fix the problem.[1] jcc [1] http://wwwtcs.inf.tu-dresden.de/~voigt/mpc08.pdf