What is 'Pattern Matching' in functional languages?

Question

I\'m reading about functional programming and I\'ve noticed that Pattern Matching is mentioned in many articles as one of the core features of functional languages.

Answer 1

It means that instead of writing

double f(int x, int y) {
  if (y == 0) {
    if (x == 0)
      return NaN;
    else if (x > 0)
      return Infinity;
    else
      return -Infinity;
  } else
     return (double)x / y;
}

You can write

f(0, 0) = NaN;
f(x, 0) | x > 0 = Infinity;
        | else  = -Infinity;
f(x, y) = (double)x / y;

---

Hey, C++ supports pattern matching too.

static const int PositiveInfinity = -1;
static const int NegativeInfinity = -2;
static const int NaN = -3;

template  struct Divide {
  enum { value = x / y };
};
template  struct aux { enum { value = PositiveInfinity }; };
template <> struct aux { enum { value = NegativeInfinity }; };
template  struct Divide {
  enum { value = aux<(x>0)>::value };
};
template <> struct Divide<0, 0> {
  enum { value = NaN };
};

#include 

int main () {
    printf("%d %d %d %d\n", Divide<7,2>::value, Divide<1,0>::value, Divide<0,0>::value, Divide<-1,0>::value);
    return 0;
};