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  1. PO
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    <p>It depends on what kind of pattern matching do you mean - it is quite powerful construct and can be used in all sorts of ways. However, I'll try to explain how pattern matching works on lists. You can write for example these patterns:</p> <pre><code>match l with | [1; 2; 3] -&gt; // specific list of 3 elements | 1::rest -&gt; // list starting with 1 followed by more elements | x::xs -&gt; // non-empty list with element 'x' followed by a list | [] -&gt; // empty list (no elements) </code></pre> <p>The F# list is actually a discriminated union containing two cases - <code>[]</code> representing an empty list or <code>x::xs</code> representing a list with first element <code>x</code> followed by some other elements. In C#, this might be represented like this:</p> <pre><code>// Represents any list abstract class List&lt;T&gt; { } // Case '[]' representing an empty list class EmptyList&lt;T&gt; : List&lt;T&gt; { } // Case 'x::xs' representing list with element followed by other list class ConsList&lt;T&gt; : List&lt;T&gt; { public T Value { get; set; } public List&lt;T&gt; Rest { get; set; } } </code></pre> <p>The patterns above would be compiled to the following (I'm using pseudo-code to make this simpler):</p> <pre><code>if (l is ConsList) &amp;&amp; (l.Value == 1) &amp;&amp; (l.Rest is ConsList) &amp;&amp; (l.Rest.Value == 2) &amp;&amp; (l.Rest.Rest is ConsList) &amp;&amp; (l.Rest.Rest.Value == 3) &amp;&amp; (l.Rest.Rest.Rest is EmptyList) then // specific list of 3 elements else if (l is ConsList) &amp;&amp; (l.Value == 1) then var rest = l.Rest; // list starting with 1 followed by more elements else if (l is ConsList) then var x = l.Value, xs = l.Rest; // non-empty list with element 'x' followed by a list else if (l is EmptyList) then // empty list (no elements) </code></pre> <p>As you can see, there is no looping involved. When processing lists in F#, you would use recursion to implement looping, but pattern matching is used on individual elements (<code>ConsList</code>) that together compose the entire list.</p> <p>Pattern matching on lists is a specific case of <em>discriminated union</em> which is discussed by <em>sepp2k</em>. There are other constructs that may appear in pattern matching, but essentially all of them are compiled using some (complicated) <code>if</code> statement.</p>
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    1. POHow does pattern matching work behind the scenes in F#?
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    1. USTomas Petricek
    1. USTomas Petricek
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    1. POHow does pattern matching work behind the scenes in F#?
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    1. COI believe you have forgotten to make EmptyList<T> and ConsList<T> inherit from the abstract List<T>. Might confuse someone...
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      1. USJohan Kullbom
    2. CO@Johan: Yes, indeed! Thanks!
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