Haskell: monadic takeWhile?

Question

I have some functions written in C that I call from Haskell. These functions return IO (CInt). Sometimes I want to run all of the functions regardless of what

Answer 1

I don't think there is anything like a takeWhileM in the standard library, but you could write it yourself so that only as much IO as needed is executed:

takeWhileM :: (Monad m) => (a -> Bool) -> [m a] -> m [a]
takeWhileM _ [] = return []
takeWhileM p (a:as) =
   do v <- a
      if p v
         then do vs <- takeWhileM p as
                 return (v:vs)
         else return []

The supplied list is only evaluated until an element is found, that doesn't match the predicate:

*Main> takeWhileM (<4) (map f [1..5])
1
2
3
4
[1,2,3]