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

Here is a really short example that shows pattern matching usefulness:

Let's say you want to sort up an element in a list:

["Venice","Paris","New York","Amsterdam"] 

to (I've sorted up "New York")

["Venice","New York","Paris","Amsterdam"] 

in an more imperative language you would write:

function up(city, cities){  
    for(var i = 0; i < cities.length; i++){
        if(cities[i] === city && i > 0){
            var prev = cities[i-1];
            cities[i-1] = city;
            cities[i] = prev;
        }
    }
    return cities;
}

In a functional language you would instead write:

let up list value =  
    match list with
        | [] -> []
        | previous::current::tail when current = value ->  current::previous::tail
        | current::tail -> current::(up tail value)

As you can see the pattern matched solution has less noise, you can clearly see what are the different cases and how easy it's to travel and de-structure our list.

I've written a more detailed blog post about it here.