[Haskell-cafe] Re: How do I do conditional tail recursion in a monad?

21 Mar 2007

      DavidA wrote:
...
I would like to write a function untilM, which would be to until as mapM is to 
map.
An example use would be if I had a function
dice :: State Int Int
which returns random dice throws within a state monad.
Then I'd like to be able to do something like
untilM (\(s,p)-> s>=100) f (0,1)
    where f (s,p) = do d <- dice
                       return (s+d, p*d)
This should throw dice until the sum exceeds 100, and keeping track of the 
product as we go along.
How about going with an infinite list of random numbers with no monads
involved

    head
  . dropUntil ((>= 100) . fst)
  . scanl' (\(s,p) d -> (d+s,p*d)) (0,1)
  . randomRs (1,6)

Of course, this has the drawback that you cannot take further random
numbers afterwards.

Regards,
apfelmus