CODEWITHSUNDEEP
haskell
Rodrigo Stv
Rodrigo Stv

Reputation: 425

How do I read numbers from a file, sum them and print them in Haskell?

How do I read numbers from a file, sum them and print them in Haskell?

I've been trying the following: I can read the numbers, but can't get them to sum.

square x = do
    return x


read_numbers = do
    contents <- readFile "numbers.txt"
    let linesOfFile = map (\x -> read x :: Integer) (lines $ contents)
    return linesOfFile

f = do
    map square read_numbers

Upvotes: 0

Views: 563

Answers (2)

Rodrigo Ribeiro
Rodrigo Ribeiro

Reputation: 3218

Your problem is that you probably don't understand the concept of Monad and how Haskell use them to control side-effects in programs.

Since your objective is just to sum the read numbers, you can do the following:

f = do
      xs <- read_numbers
      return (sum xs)

where sum has its obvious meaning and type:

sum :: (Num a, Foldable t) => t a -> a

EDIT: Answer to Rodrigo Stv commentary.

When you check map type you get: 

 map :: (a -> b) -> [a] -> [b]

When you try to map your square function which has type 

 square :: Int -> IO Int

you got: 

 map square :: [Int] -> [IO Int].

That is a type from list of Int to a list of I/O operations that can return an Int and you cannot sum I/O operations. :)

As I told you, the problem here is to understand how to work with monads.

Upvotes: 2

JP Moresmau
JP Moresmau

Reputation: 7403

There is sum in Data.List and the Prelude (http://hackage.haskell.org/package/base-4.8.2.0/docs/Prelude.html#v:sum)

sum :: (Foldable t, Num a) => t a -> a Source

The sum function computes the sum of the numbers of a structure.

Upvotes: 2

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