Haskell IO function type mismatch -

what problem following code:

nextmatch :: (a -> bool) -> [io a] -> (io a, [io a]) nextmatch f (x:xs) =     s <- x     if f s (return x, xs)         else nextmatch f xs 

the compile error says:

src\main.hs:364:10:     couldn't match expected type `(io a, t0)' actual type `io a'     in stmt of 'do' block: s <- x     in expression:       { s <- x;            if f s (return x, xs) else nextmatch f xs }     in equation `nextmatch':         nextmatch f (x : xs)           = { s <- x;                  if f s (return x, xs) else nextmatch f xs } 

i want function searches list match , returns matched element plus remaining list, tuple.

i still quite new haskell, problem might quite simple...

thanks! chris

as x if of type io a , no need re-return it, contrary tuple need injected monad.

if f s return $ (x, xs) 

but, not issue, io monad , signature doesn't reflect it, return type (io a, [io a]) should io (io a, [io a]). code should follow.

nextmatch :: (a -> bool) -> [io a] -> io (io a, [io a]) nextmatch f (x:xs) =          done <- fmap f x       if done     return $ (x, xs)     else nextmatch f xs  

anyway don't know trying signature of function pretty awkward. should more nextmatch :: (a -> bool) -> [a] -> (a, [a]), using return enough build monadic version of it. nextmatchio = return $ nextmatch , plug computation control flow using other function, provided control.monad.