Haskell Study Note 7 : Tuple

Tuple

Tuple can have different types of values.

  • Common form : (String, Int)、(Bool, Double, String)

    like : ("Alice", 20)

  • Pair & Triple :

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    -- Pair
    ("Alice", 20)

    -- Triple
    ("Bob", 25, "Engineer")

Basic operations of a Tuple

  • Create a tuple :

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    student :: (String, Int)
    student = ("Tom", 18)

  • Pattern matching & Usage within a function:

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    getStudentInfo :: (String, Int) -> String
    getStudentInfo (name, age) = "Name: " ++ name ++ ", Age: " ++ show age

  • Pair operations :

    fst and snd , like :

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        student = ("Emma", 21)
        
        main = do
            print (fst student)  -- 输出 "Emma"
            print (snd student)  -- 输出 21

    ## ADT

    ADT = Algebraic Data Type 

    To define a ADT : 

    ```haskell
    data Product = Item String Int

Explain :

  • Item is a constructor
  • Product is a new Type name

For instance, if we create a good card :

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notebook :: Product
notebook = Item "Notebook" 50

To read an ADT, we can use pattern matching :

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describeProduct :: Product -> String
describeProduct (Item name price) =
    "Product: " ++ name ++ ", Price: " ++ show price

Practice

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data Product = Item String Int

pen :: Product 
pen = Item "pen" 5

eraser :: Product 
eraser = Item "eraser" 3

notebook :: Product 
notebook = Item "notebook" 20

readTheProduct :: Product -> String
readTheProduct (Item name price) = "Name: " ++ name ++ " Price: " ++ show price ++ "\n"


main :: IO ()
main = do
    putStr (readTheProduct pen)
    putStr (readTheProduct eraser)
    putStr (readTheProduct notebook)

Note:

  1. To define data, you can write outside the main, as it is a type
  2. data as a parameter : you should use () and Item, then you name each elements, just like other common variables
  3. data as return values : you should use Item