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
You can't do 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.
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