Weird return in Haskell

Question

checkstring :: [String] -> Int -> [String]
checkstring p n = do    z <- doesFileExist (p !! n)
    if z
    then p
    else error $ "'" ++ (p !! n) ++ "' file path does not exist"

It checks for a element in the string by looking at "n"(so if n = 2 it will check if the second string in the list) then see if it exists. If it does exist it will return the original string list, if not it will error.Why does it do this? :

Couldn't match expected type `[t0]' with actual type `IO Bool'
    In the return type of a call of `doesFileExist'
    In a stmt of a 'do' expression: z <- doesFileExist (p !! n)

Answer 1

The type of doesFileExist is String -> IO Bool. If your program wants to know whether a file exists, it has to interact with the file system, which is an IO action. If you want your checkString function to do that, it will also have to have some kind of IO-based type. For example, I think something like this would work, though I haven't tried it:

checkstring :: [String] -> Int -> IO [String]
checkstring p n = do    z <- doesFileExist (p !! n)
    if z
    then return p
    else error $ "'" ++ (p !! n) ++ "' file path does not exist"