ImpliedSymbolTerminal.cs

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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Irony.Parsing {
  //In some grammars there is a situation when some operator symbol can be skipped in source text and should be implied by parser.
  // In arithmetics, we often imply "*" operator in formulas:
  //  x y => x * y.
  // The SearchGrammar in Samples provides another example: two consequtive terms imply "and" operator and should be treated as such:
  //   x y   => x AND y 
  // We could use a simple nullable Non-terminal terminal in this case, but the problem is that we cannot associate precedence
  // and associativity with non-terminal, only with terminals. Precedence is important here because the implied symbol identifies binary
  // operation, so parser should be able to use precedence value(s) when resolving shift/reduce ambiguity. 
  // So here comes ImpliedSymbolTerminal - it is a terminal that produces a token with empty text. 
  // It relies on scanner-parser link enabled - so the implied symbol token is created ONLY 
  // when the current parser state allows it and there are no other alternatives (hence lowest priority value).
  // See SearchGrammar as an example of use of this terminal. 
  public class ImpliedSymbolTerminal : Terminal {
    public ImpliedSymbolTerminal(string name)  : base(name) { 
      this.Priority = Terminal.LowestPriority; //This terminal should be tried after all candidate terminals failed. 
    }

    public override void Init(Irony.Parsing.GrammarData grammarData) {
      base.Init(grammarData);
      //Check that Parser-scanner link is enabled - this terminal can be used only if this link is enabled
      if (Grammar.FlagIsSet(LanguageFlags.DisableScannerParserLink))
        grammarData.Language.Errors.Add(GrammarErrorLevel.Error, null, Resources.ErrImpliedOpUseParserLink, this.Name); 
           //"ImpliedSymbolTerminal cannot be used in grammar with DisableScannerParserLink flag set"
    }

    public override Token TryMatch(ParsingContext context, ISourceStream source) {
      return source.CreateToken(this); //Create an empty token representing an implied symbol.
    }

  }//class
}//namespace