模式匹配功能结果

Question

I am trying to do match on function result on scala it will look like this

validate(number) match {
case true => "true"
case false => "false"
}

I am trying to do something similar in haskell

main = do    
    number <- getLine  
    res <- case (validate (read number::Int)) of 
        (True) -> "True"
        (False) -> "False"
    putStrLn res

Also I understand I can do with out the do notation, but not sure where to start.

Answer 1

In Haskell read throws an exception if there is a parse error.

But there is readMaybe which will return a Maybe value.

import Text.Read

main = do
  number <- getLine
  let res = case (readMaybe number :: Maybe Int) of
              Just x -> "True"
              Nothing -> "False"
  putStrLn res

Note the use of let instead of monadic bind operator <- since readMaybe is a pure function.

You might even be able to avoid the type hint if you use the parsed value in a way which allows the compiler to infer its type, eg:

fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
  -- any list will do

main = do
  number <- getLine
  case readMaybe number of
    Nothing -> putStrLn "bad number"
    Just n  -> print (fibs !! n)

No type annotation is needed here because !! take an Int argument, and so n must have type Int which means readMaybe number must have type Maybe Int .

Update

Stylistically I would recommend that you use let here - using bind is completely unnecessary as you are computing a pure value.

However, you can write this with the bind operator by using return :

main = do
  number <- getLine
  res <- return $ case (readMaybe number :: Maybe Int) of
                    Just _ -> "True"
                    Nothing -> "False"
  putStrLn res