Pattern Matching

Pattern Matching

Haskell functions can have multiple equations, each matching a different pattern. Let's see this with factorial — a classic math function where factorial(n) means n * (n-1) * ... * 1, and factorial(0) is defined as 1. For example, factorial(5) = 5 * 4 * 3 * 2 * 1 = 120.

In Haskell, we can express this naturally with pattern matching — one equation for the base case (0) and one for everything else:

factorial :: Int -> Int
factorial 0 = 1
factorial n = n * factorial (n - 1)

Patterns are tried top-to-bottom. The first matching equation is used. When we call factorial 3, it doesn't match 0, so the second equation fires: 3 * factorial 2, which expands to 3 * 2 * factorial 1, then 3 * 2 * 1 * factorial 0, and finally factorial 0 matches the first equation and returns 1.

Wildcard Pattern

Use _ to match anything without binding it:

isZero :: Int -> Bool
isZero 0 = True
isZero _ = False

List Patterns

You can pattern match on list structure with [] (empty) and (x:xs) (head:tail):

myHead :: [a] -> a
myHead (x:_) = x

Your Task

Define factorial using pattern matching (base case 0 = 1, recursive case). Print factorial 10.