Haskell，如何用 case-of 语句替换嵌套的 if？

Question

I'm starting to learn haskell and I'm solving some simple programming challenges. So my current problem right now is just printing some text based on the value being passed:

main = do
  n <- readLn :: IO Int
  if n `mod` 2 > 0
      then putStrLn "Weird"
      else if n > 1 && n < 6
          then putStrLn "Not Weird"
          else if n > 5 && n < 21
              then putStrLn "Weird"
              else putStrLn "Not Weird"

However I think this is fairly ugly, is there someway I could replace the nested if structure with a more elegant pattern matching?

I've tried:

main :: IO()
main = do
    n <- readLn :: IO Int
    case n of
        n `mod` 2 /= 0 -> putStrLn "Weird"
        n >= 2 && N <= 5 -> putStrLn "Not Weird"
        n >= 6 && N <= 20 -> putStrLn "Weird"
        n > 20 -> putStrLn "Not Weird"

but I get a compile error.

Edit: Final solution

check :: Int -> String
check n
    | (n `mod` 2 /= 0) = "Weird"
    | (n >= 2) && (n <= 5) = "Not Weird"
    | (n >= 6) && (n <= 20) = "Weird"
    | otherwise = "Not Weird"

main :: IO()
main = do
    n <- readLn :: IO Int
    putStrLn (check n)

Answer 1

You need to use guards if you have boolean expressions (conditions), instead of actual patterns.

main :: IO()
main = do
    n <- readLn :: IO Int
    case () of
       _ | n `mod` 2 /= 0 -> putStrLn "Weird"
         | n >= 2 && n <= 5 -> putStrLn "Not Weird"
         | n >= 6 && n <= 20 -> putStrLn "Weird"
         | n > 20 -> putStrLn "Not Weird"

Consider using otherwise in the last case: assuming you want to catch everything else, it's faster and suppresses a GHC warning about non-exhaustiveness.

A "multiway if" can also be used as an alternative, but it requires an extension.

Answer 2

A case expression [Haskell-report] deals with patterns. An expression like n `mod` 2 /= 0 is not a pattern.

I propose to define a helper function, and use guards :

tmp :: Int -> String
tmp n | n `mod` 2 /= 0 = "Weird"
      | n >= 2 && N <= 5 = "Not Weird"
      | n >= 6 && N <= 20 = "Weird"
      | n > 20 = "Not Weird"
      | otherwise = …

You will still need to define a value for the otherwise case, in case n <= 0 .

We can then use this with:

main = do
    n <- readLn
    putStrLn (tmp n)

Answer 3

"multiway if" was mentioned in chi's answer but not explained and I think it's useful to know about for such cases. Enabling the MultiWayIf extension (eg by writing {-# LANGUAGE MultiWayIf #-} at the top of the file) allows you to write

if | (n `mod` 2 /= 0) -> putStrLn "Weird"
   | (n >= 2) && (n <= 5) -> putStrLn "Not Weird"
   | (n >= 6) && (n <= 20) -> putStrLn "Weird"
   | otherwise -> putStrLn "Not Weird"