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

• 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")