Read float or string from user input

Question

I want to be able to read numbers from console input, and store them into a list until the user types end, like this:

readN = readNumbers []    

readNumbers nums = do
      n <- readFloatOrString
      case n of
        <<number>> -> return readNumbers (nums ++ [n])
        "end" -> return nums

Is this doable without importing a library?

willeM_ Van Onsem · Accepted Answer · 2020-06-06 17:48:33Z

6

You should not read the value, or at least not immediately. You can first check if the line is "end", if so return the numbers, and otherwise continue reading:

import Text.Read(readMaybe)

readNumbers :: IO [Float]
readNumbers = do
      n <- getLine
      case (n, readMaybe n :: Maybe Float) of
        ("end", _) -> pure []
        (_, Just n) -> (n:) <$> readNumbers
        (_, Nothing) -> …

The … is the part that should handle the case if you did not pass a valid float number.

We thus can process a list with:

Prelude Text.Read> readNumbers 
1
4
end
[1.0,4.0]

(here the boldface part is user input).

edited Jun 6, 2020 at 17:48

answered Jun 6, 2020 at 17:29

willeM_ Van Onsem

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3 Comments

tacofisher Over a year ago

Thanks! what is pure for?

willeM_ Van Onsem Over a year ago

@tacofisher: it "wraps" a value, here in an IO, so pure x, results in an IO that "returns" x.

Redu Over a year ago

@duplode Very nice..!

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