Wikipedia:Reference desk/Archives/Mathematics/2013 March 24

= March 24 =

The symbol used when defining the alphabet of a language
Syntax (programming languages) uses the  symbol to mean something like "is comprised of the following". However, I wished to know the name of that symbol, but could not find it. Technical restrictions prevent you from searching for it directly, and none of the articles I manually checked (e.g. Syntax (logic), Formal language and Alphabet (computer science)) make use of it. Could someone tell me (a) what it is called, and (b) whether it has its own WP article? Thanks!  It Is Me Here   t / c 18:56, 24 March 2013 (UTC)
 * See also Backus-Naur_form. I don't think that collection of 3 symbols has a name, but just means "is defined as". -- SGBailey (talk) 21:29, 24 March 2013 (UTC)
 * It is really quite difficult at present to find out what that symbol means. Might I suggest adding information on it to more articles, e.g. List of logic symbols?  It Is Me Here   t / c 12:47, 26 March 2013 (UTC)


 * The symbol is listed at list of mathematical symbols, with the definition (mathematical) given by SGBailey as "is defined as" or "equal by definition". However, the usage at the Syntax page is the BNF notation. These are entirely different concepts: from the BNF article "'::=' means that the symbol on the left must be replaced with the expression on the right" -- so, maybe not entirely different, but sufficiently different. They are two uses of the same symbol to mean different things. As for adding to "list of logical symbols", feel free to be bold and add it yourself! But really, the syntax article tells you what notation it uses: "The syntax of textual programming languages is usually defined using a combination of regular expressions (for lexical structure) and Backus–Naur Form" -- so you could have just followed links in the article to get to the symbol definition. SemanticMantis (talk) 18:30, 26 March 2013 (UTC)
 * Hang on, List of mathematical symbols only has  (one colon); are you saying that that is the same thing as  ?  It Is Me Here   t /  c 20:03, 26 March 2013 (UTC)
 * My mistake. You are right, ::= is not := Anyway, I'm not sure where (if) ::= should be added (and I agree that neither one has a formal name; it's just a notation that is usually glossed at first usage). I cannot see any uses of ::= other than the BNF article above. I don't think it is "standard" in any other area of logic. So it probably doesn't belong in list of logic symbols. In my opinion, it is either fine how it is (appearing with def. in BNF article), or could be added to list of computer science symbols. I added some minor some clarification at Syntax (programming languages) SemanticMantis (talk) 21:16, 26 March 2013 (UTC)


 * [Please see User:Giraffedata/comprised of.—Wavelength (talk) 21:25, 26 March 2013 (UTC)]