Explaining pattern matching vs switch

Question

I have been trying to explain the difference between switch statements and pattern matching(F#) to a couple of people but I haven't really been able to explain it well..most of the time they just look at me and say "so why don't you just use if..then..else".

How would you explain it to them?

EDIT! Thanks everyone for the great answers, I really wish I could mark multiple right answers.

Answer 1

Having formerly been one of "those people", I don't know that there's a succinct way to sum up why pattern-matching is such tasty goodness. It's experiential.

Back when I had just glanced at pattern-matching and thought it was a glorified switch statement, I think that I didn't have experience programming with algebraic data types (tuples and discriminated unions) and didn't quite see that pattern matching was both a control construct and a binding construct. Now that I've been programming with F#, I finally "get it". Pattern-matching's coolness is due to a confluence of features found in functional programming languages, and so it's non-trivial for the outsider-looking-in to appreciate.

I tried to sum up one aspect of why pattern-matching is useful in the second of a short two-part blog series on language and API design; check out part one and part two.

Answer 2

Patterns give you a small language to describe the structure of the values you want to match. The structure can be arbitrarily deep and you can bind variables to parts of the structured value.

This allows you to write things extremely succinctly. You can illustrate this with a small example, such as a derivative function for a simple type of mathematical expressions:

type expr =     | Int of int     | Var of string     | Add of expr * expr     | Mul of expr * expr;;  let rec d(f, x) =     match f with     | Var y when x=y -> Int 1     | Int _ | Var _ -> Int 0     | Add(f, g) -> Add(d(f, x), d(g, x))     | Mul(f, g) -> Add(Mul(f, d(g, x)), Mul(g, d(f, x)));; 

Additionally, because pattern matching is a static construct for static types, the compiler can (i) verify that you covered all cases (ii) detect redundant branches that can never match any value (iii) provide a very efficient implementation (with jumps etc.).