readFile.head xs = readFile . (head xs) For that to work, (head xs) must evaluate to a function with which readFile can compose. But that wouldn't be the type you're wanting. readFile $ head xs = readFile (head xs) Graham On 03/04/2015 10:23 PM, Vale Cofer-Shabica wrote:
Could someone please explain why the commented line fails spectacularly while the final line succeeds?
import System.IO (getContents) import System.Environment (getArgs) fileInput :: IO String fileInput = getArgs>>=readFirst where readFirst :: [FilePath] -> IO String readFirst [] = System.IO.getContents --readFirst xs = readFile.head xs readFirst xs = readFile $ head xs
I'm particularly confused given the following typings (from ghci):
readFile.head :: [FilePath] -> IO String readFile.head [] :: a -> IO String
And this is still stranger:
:type readFile.head ["foo", "bar"]
<interactive>:28:16: Couldn't match expected type `a0 -> FilePath' with actual type `[Char]' In the expression: "foo" In the first argument of `head', namely `["foo", "bar"]' In the second argument of `(.)', namely `head ["foo", "bar"]'
<interactive>:28:23: Couldn't match expected type `a0 -> FilePath' with actual type `[Char]' In the expression: "bar" In the first argument of `head', namely `["foo", "bar"]' In the second argument of `(.)', namely `head ["foo", "bar"]'
Many thanks in advance, vale _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners