Question

I have such code:

foldl (\a x -> a ++ (f x)) [] list

All seems to be ok, but (f x) :: IO [String], and (++) gives an error ([String] != IO [String]).

How to define empty IO[String] list? I can’t make clean (f x).

Answer 1

The combinator you want here is probably

Prelude Control.Monad> :t foldM
foldM :: Monad m => (a -> b -> m a) -> a -> [b] -> m a

and with that

foldM (\a x -> fmap (a++) (f x)) [] list

to run the actions f x in sequence and accumulate the [String]s they produce.