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    <p>I think <em>sepp2k</em> already answered most of the question, but I'd like to add a couple of points that may clarify how F#/OCaml compiler interprets the code and explain some common uses. </p> <p><strong>Regarding the <code>'</code> symbol</strong> - this is just a part of a name (a valid identifier starts with a letter and then contains one or more letters, numbers or <code>'</code> symbols). It is usually used if you have a function or value that is very similar to some other, but is in some way <em>new</em> or <em>modified</em>.</p> <ul> <li><p>In your example, <code>xs</code> is a list that should be summed and the pattern matching decomposes the list and gives you a new list (without the first element) that you need to sum, so it is called <code>xs'</code></p></li> <li><p>Another frequent use is when declaring a local utility function that implements the functionality and takes an additional parameter (typically, when writing tail-recursive code):</p> <pre><code>let sum list = let rec sum' list res = match list with | [] -&gt; res | x::xs -&gt; sum' xs (res + x) sum' list 0 </code></pre></li> </ul> <p>However, I think there is usually a better name for the function/value, so I try to avoid using <code>'</code> when writing code (I think it isn't particularly readable and moreover, it doesn't colorize correctly on StackOverflow!)</p> <p><strong>Regarding the <code>::</code> symbol</strong> - as already mentioned, it is used to create lists from a single element and a list (<code>1::[2;3]</code> creates a list <code>[1;2;3]</code>). It is however worth noting that the symbol can be used in two different ways and it is also interpreted in two different ways by the compiler.</p> <p>When creating a list, you use it as an operator that constructs a list (just like when you use <code>+</code> to add two numbers). However, when you use it in the <code>match</code> construct, it is used as a <em>pattern</em>, which is a different syntactic category - the pattern is used to <em>decompose</em> the list into an element and the remainder and it succeeds for any non-empty list:</p> <pre><code>// operator let x = 0 let xs = [1;2;3] let list = x::xs // pattern match list with | y::ys -&gt; // ... </code></pre>
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    1. POWhat does :: and ' mean in oCaml?
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    1. USTomas Petricek
    1. USTomas Petricek
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    1. POWhat does :: and ' mean in oCaml?
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    1. COThe :: is called the cons operator, if I'm not mistaken.
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    2. COYou forgot to declare `sum'` function recursive. Also, there should be a `in` keyword after the match.
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      1. USMikael S
    3. COThanks - I added the `rec` keyword. I wrote the code in F#, which is whitespace-sensitive and doesn't require `in` keywords.
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