On 7/11/07, Stefan O'Rear
Not very nicely.
Option 1. Ignore purity Option 2. Ignore lists
Option 3. Use continuations You maintein purity while keeping flexibility. One possible implementation:
data Result a = Finished (a, [a]) | NeedInput (a -> Result a)
It's on the NeedInput where we hide the input list. If a for some cutoff and some list there were needed n elements, then there will be (n-1) NeedInput's, one for each except for the first. After the last one, the Result will be Finished.
accumUntilCutoff :: (Ord a, Num a) => a -> a -> Result a accumUntilCutoff = acc (0,[]) where acc (s,p) cutoff x | x >= cutoff = Finished (s+x,reverse $ x:p) | otherwise = NeedInput (acc (s+x,x:p) (cutoff-x))
Note how we explicity "traverse" on the "list".
readUntilCutoff :: (Ord a, Num a, Read a) => a -> IO (a,[a]) readUntilCutoff cutoff = get >>= parse . accumUntilCutoff cutoff where get = putStr "Enter an integer: " >> getLine >>= return . read parse (Finished (s,p)) = return (s,p) parse (NeedInput f) = get >>= parse . f
Probably there's a better way of using continuations, but I think this suffices (and works). Cheers, -- Felipe.