Editing XPL (section)

===ANALYZER===
The XCOM compiler has a hand-written [[Lexical analysis|lexical scanner]] and a mechanically-generated parser.  The syntax of the compiler's input language (in this case, XPL) is described by a simplified [[BNF grammar]].  XPL's grammar analyzer tool '''ANALYZER''' or '''XA''' turns this into a set of large data tables describing all legal combinations of the syntax rules and how to discern them.  This table generation step is re-done only when the language is changed.  When the compiler runs, those data tables are used by a small, language-independent parsing algorithm to parse and respond to the input language.  This style of table-driven parser is generally easier to write than an entirely hand-written [[recursive descent]] parser.  XCOM uses a [[bottom-up parsing]] method, in which the compiler can delay its decision about which syntax rule it has encountered until it has seen the rightmost end of that phrase.  This handles a wider range of programming languages than [[Top-down parsing|top-down]] methods, in which the compiler must guess or commit to a specific syntax rule early, when it has only seen the left end of a phrase.