第一个Haskell IO程序无法正常工作

Question

Sorry, this is probably really dumb, but can someone explain me why this program doesn't compile? I get Couldn't match expected type 'a1 -> String' with actual type 'IO String' .

import System.Environment

main = do
  [first, last] <- getArgs
  firstnames <- lines . readFile "firstnames_male"
  lastnames <- lines . readFile "lastnames"
  print firstnames

Answer 1

You can't do lines . readFile "lastnames" lines . readFile "lastnames" .

The readFile function returns an IO String , not a String .

You can , however, use the fmap function (or the <$> operator) to achieve this:

main = do
  [first, last] <- argArgs
  firstnames <- lines `fmap` readFile "firstnames_males"
  ...

This works because IO is a functor.