Haskell Study Note (Lesson 4)

Lesson 4 Conditionals & Branching

Basic if - else Expressions

The basic structure of if-else is :

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if conidition
then resultWhenTrue
else resultWhenfalse

For example, to judge whether a number is positive :

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isPossive :: Int -> String
isPossive n = 
    if n > 0
    then "Yes"
    else "No"

Guard in Depth

Basic grammar :

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functionName args
  | condition1 = result1
  | condition2 = result2
  | otherwise  = defaultResult

Note that the otherwise (Default Branch) is neccesary.

An example :

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isPossive :: Int -> String
isPossive n 
    | n > 0  = "Positive"
    | n < 0  = "Negative"
    | otherwise = "Zeor"

Practice : Smart Shopping Assitant

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shoppingAssistant :: Double -> (Double, String)
shoppingAssistant amount 
    | amount >= 500 = 
        let final = amount * 0.8
        in (final, "Free shipping + 20 % off")
    | amount >= 300 = 
        let final = amount * 0.9 + 30 
        in (final, "Shipping fee added + 10 % off")
    | otherwise = 
        let final = amount + 30
        in (final, "No discount, shipping fee added")



main :: IO ()
main = do
    let a = 400.0
    print (shoppingAssistant(a))

Note :

  1. To use guard, note the place of your =
  2. To output Triple, you can use print instead of putStrLn