I made a small program that tests numbers against some properties:

Welcome to Numbers!
Is it odd? 4
Is it a multiple of 18? 38
Is it big? 100

That's basically it :)

My code is:

import System.IO
import Text.Read

askUser :: String -> IO Double
askUser str = do
  putStr str
  num <- fmap readMaybe getLine

  case num of
    Nothing -> putStrLn "That is not a number!" >> askUser str
    Just num -> return num

numberCheck :: [Double] -> String -> IO ()
numberCheck list text = do
  num <- askUser ("Is it " ++ text ++ "? ")

  if num `elem` list
    then putStrLn "Yes"
    else putStrLn "No"

main :: IO ()
main = do
  hSetBuffering stdout NoBuffering
  putStrLn "Welcome to Numbers!"
  let alot = 99999

  let oddNum = take alot [x | x <- [0..], odd (round x)]
  numberCheck oddNum "odd"

  let multiples18 = take alot [x | x <- [0..], round x `mod` 18 == 0]
  numberCheck multiples18 "a multiple of 18"

  let bigNumbers = take alot [99..]
  numberCheck bigNumbers "big"

Is there anything that can be improved? I was especially thinking of the alot number, as it is not a "clean" way of solving this. But if I don't, Haskell will continue checking the list infinitely.

Is there any nicer way of achieving this?

Note: I used lists instead of comparisons because that's what I just learned, and so you can ignore me using lists if you want to :)


1 Answer 1


Integer not Double

Double is not a reasonable type because checking if a number is big does not work with Doubles, you should use Integer

First class functions

Using a list is not a clean procedure, it does not work for very big numbers and it is slow for big numbers.

It is better to pass in a validation function:

numberCheck :: (Integer -> Bool) -> String -> IO ()
numberCheck predicate text = do
  num <- askUser ("Is it " ++ text ++ "? ")

  if predicate num
    then putStrLn "Yes"
    else putStrLn "No"

Usage is:

main = do
  hSetBuffering stdout NoBuffering
  putStrLn "Welcome to Numbers!"

  numberCheck odd "odd"
  numberCheck (\x -> x `mod` 18 == 0) "multiple of 18"
  numberCheck (> 100) "big (bigger than 100)"
  • \$\begingroup\$ Thanks :) I actually only used list comprehensions because that's what I just learned ;). But why doesn't it work with Doubles? \$\endgroup\$
    – Rakete1111
    Sep 1, 2016 at 12:22
  • \$\begingroup\$ @Rakete1111 Is it big? 400.321 No Because a non-integer number is never contained in an integer range. \$\endgroup\$
    – Caridorc
    Sep 1, 2016 at 12:41

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