Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>What you're writing is called a pushdown automaton. This is usually more power than you need to write a lexer, it's certainly excessive if you're writing a lexer for a modern language like CSS. A recursive descent parser is close in power to a pushdown automaton, but recursive descent parsers are much easier to write and to understand. Most parser generators generate pushdown automatons.</p> <p>Lexers are almost always written as finite state machines, i.e., like your code except get rid of the "stack" object. Finite state machines are closely related to regular expressions (actually, they're provably equivalent to one another). When designing such a parser, one usually starts with the regular expressions and uses them to create a deterministic finite automaton, with some extra code in the transitions to record the beginning and end of each token.</p> <p>There are tools to do this. The <code>lex</code> tool and its descendants are well known and have been translated into many languages. The <code>ANTLR</code> toolchain also has a lexer component. My preferred tool is <code>ragel</code> on platforms that support it. There is little benefit to writing a lexer by hand <em>most</em> of the time, and the code generated by these tools will probably be faster and more reliable.</p> <p>If you do want to write your own lexer by hand, good ones often look something like this:</p> <pre><code>function readToken() // note: returns only one token each time while !eof c = peekChar() if c in A-Za-z return readIdentifier() else if c in 0-9 return readInteger() else if c in ' \n\r\t\v\f' nextChar() ... return EOF function readIdentifier() ident = "" while !eof c = nextChar() if c in A-Za-z0-9 ident.append(c) else return Token(Identifier, ident) // or maybe... return Identifier(ident) </code></pre> <p>Then you can write your parser as a recursive descent parser. Don't try to combine lexer / parser stages into one, it leads to a total mess of code. (According to the Parsec author, it's slower, too).</p>
    singulars
    1. This table or related slice is empty.
    1. POhand coding a parser
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USDietrich Epp
    1. USDietrich Epp
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POhand coding a parser
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTAcceptedByOriginator
    1. COYou know, If I had this kind of help when I did my thesis things would have been some much more understandable. Thanks for a great answer! No combining of the lexer and parser will ever take place, that much I've learned the hard way.
      singulars
      1. PO
      1. USJohn Leidegren
    2. COThanks for going into detail about lexers! +1
      singulars
      1. PO
      1. USVivin Paliath
    3. COI have but 1 question left for you, where does `c` go? When I say return `IDENTIFIER` where does the token value reside? This is a trivial question but I have to ask! If I just put the character in a string builder when do I flush that and do I ever run the risk of leaving junk in my string builder?
      singulars
      1. PO
      1. USJohn Leidegren
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload