There was already some pattern matching explained, in the context of bindings here. Here we explain pattern matching in other contexts, which are:

• function definitions
• list comprehensions
• do notations

Before we start with that, we provide more details about pattern matching, regarding:

• lists
• algebraic data types
• tuples

Additionally, even before that we explain the wildcard _.

• wildcard _
  • stands-in for any value
• example
  • code:

    main :: IO ()
    main = 
        do
            print (firstFromTuple3 ("A",'b','c'))
            print (secondFromList "Hallo!")
            print (getZipFromPerson (PersonNameZipAndEMail "Haskell" "Brooks" "Curry" 16802 "hbc@psu.edu"))
     
    firstFromTuple3 :: (a,b,c) -> a
    firstFromTuple3 (x,_,_) = x
     
    secondFromList :: [a] -> a
    secondFromList (_:x:_) = x
     
    data Person = PersonNameAndZip String String String Int | PersonNameZipAndEMail String String String Int String
     
    getZipFromPerson :: Person -> Int
    getZipFromPerson (PersonNameAndZip _ _ _ nZip) = nZip
    getZipFromPerson (PersonNameZipAndEMail _ _ _ nZip _) = nZip

  • compiles, with warning: Pattern match(es) are non-exhaustive
  • executes, with output:

    "A"
    'a'
    16802

• NOTE: The function secondFromList will execute with runtine error when there is less than two elements in the list.
• example
  • code:

    main :: IO ()
    main = 
        do
            print (secondFromList "H")
     
    secondFromList :: [a] -> a
    secondFromList (_:x:_) = x

  • compiles, with warning: Pattern match(es) are non-exhaustive
  • executes, with output:

    Test3-exe.exe: app\Main.hs:7:1-26: Non-exhaustive patterns in function secondFromList

• binding actual parameters (“caller perspective”) to parameters (“inner function perspective”)
• select the structure of data, to pick cases
• binds parameter names, to define the function itself
• example:
  • code:

    import Prelude hiding (length)
     
    main :: IO ()
    main = 
        do
            print (length s)                            -- "caller perspective" for first parameter of function length
     
    s :: String
    s = ('H':'e':['l', 'l', 'o']) ++ (' ':"World!")
     
    length :: [a] -> Integer
    length [] = 0                                       -- "inner function perspective" - selected when list is empty
    length (_:lrx) = 1 + (length lrx)                   -- "inner function perspective" - selected when at least one element
                                                          -- using the rest (lrx) for recursive call of length (again "caller perspective")