Composing monad actions with folds

Question

Let\'s take a function of type (Monad m) => a -> m a. For example:

ghci> let f x = Just (x+1)

I\'d like to be able to

Answer 1

If you make f strict as in

f x = let y = x+1 in y `seq` Just y

or

-- remember to enable -XBangPatterns
f !x = Just (x+1)

and leave the rest alone, your code runs in constant space (albeit slowly) even with very large n:

ghci> times 4000000000 f 3
Just 4000000003