Question

Hey guys, I need some help, how can I sum a list from a file using the sum builtin function?

example:
text.txt contains [1,2,3,4]

and i want to get this list from the file and sum these numbers with the sum builtin. Is it possible?

Thanks!

Answer 1

Sure.

main = do
    contents <- readFile "text.txt"
    print . sum . read $ contents

This uses some standard functions from the Prelude:

• readFile :: FilePath -> IO String reads the file into a string.

• read :: Read a => String -> a converts from a String to any type in the Read typeclass. You might have to add a type annotation if the compiler couldn’t figure out which type you wanted. In this case the compiler can figure it out since we used sum on the result, so it infers that a must be [Integer]. (It actually infers Num a => [a], but this defaults to [Integer] due to type defaulting).

  read expects the same format generated by show on the same type.

Note that I had to use do-notation to extract the String from IO String in order to apply read to it.