TYacc + TLex

Sources: source/classes/tyacc.prg, source/classes/tlex.prg

Standalone classes (TYacc, TLex)

TYacc and TLex implement a complete LALR(1) parser generator and lexical analyzer in pure Harbour. TYacc performs shift-reduce parsing using a generated state machine, while TLex handles tokenization by matching input against a set of token patterns. Together they enable parsing of structured languages, expressions, and configuration files.

TYacc Key DATA Members

DATA	Type	Description
`oLex`	Object	Reference to the associated TLex tokenizer
`lDebug`	Logical	Enable debug trace of parser states and transitions
`nToken`	Numeric	Current look-ahead token identifier
`nState`	Numeric	Current parser state number
`aStates / aValues`	Array	Parser state stack and semantic value stack
`bShift / bReduce`	Code block	Callbacks executed on shift and reduce actions

TYacc Methods

Method	Description
`New( oLex )`	Create a parser associated with the given lexer object
`Parse( cText )`	Parse the input text and return the result value
`Shift( n )`	Shift the next state onto the parser stack
`Reduce( n )`	Reduce by grammar rule number `n`
`Accept()`	Signal successful parse completion
`ProdValue( n )`	Retrieve the semantic value of a production symbol at position `n`

TLex

TLex inherits from TFile and provides tokenization services: it reads input text and returns a stream of tokens with associated semantic values.

DATA	Type	Description
`aTokens`	Array	Array of token pattern strings
`aIds`	Array	Array of numeric token IDs corresponding to each pattern
`cToken`	Character	Current matched token text
`uValue`	Various	Semantic value associated with the current token

Method	Description
`New( cFile, aTokens, aIds, cSeparators )`	Create a lexer with token patterns, IDs, and separator characters
`nGetToken()`	Get the next token from the input; returns the token ID
`SetText( c )`	Set the input text to be tokenized
`Add( cToken, nId )`	Add a new token pattern with its numeric ID

Example: Simple Expression Parser

#include "FiveWin.ch"

function Main()

   local oLex, oYacc
   local aTokens := { "\d+", "\+", "\-", "\*", "/", "\(", "\)" }
   local aIds    := { 1, 2, 3, 4, 5, 6, 7 }  // NUM, PLUS, MINUS, MUL, DIV, LPAR, RPAR

   oLex := TLex():New( , aTokens, aIds, " " )
   oLex:SetText( "12 + 34 * ( 56 - 7 )" )

   oYacc := TYacc():New( oLex )
   oYacc:lDebug := .T.

   // Parse the expression
   oYacc:Parse( oLex:cToken )

return nil

Notes