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Disambiguation

rascal-0.40.17

Synopsis​

Disambiguation is the definition of filters on the parse trees that Syntax Definitions define. There are several ways of defining Disambiguation in Rascal.

Description​

There are generally three ways of removing ambiguity from parse forests that are produced by parsers generated from Syntax Definitions.

  • The first way is to add disambiguation declarations to the Syntax Definition. You can choose from:
    • Priorities, which can be used to define the relative priority in expression languages
    • Associativity, which can be used to define relative associativity between operators of expression languages
    • Except constraints, which can be used to eliminate specific rules from non-terminal instances.
    • Follow constraints, which can be used to implement longest match using lookahead
    • Precede constraints, which can be used to implement first match using look behind
    • Reserve constraints, which allow you to remove a finite sets of strings from a Syntax Definition to implement keyword reservation
  • The second way is to add Actions that will be triggered just after parsing and allow you to trim a parse forest using any information necessary.
  • The third way is use the Visit statement on a parse tree and implement your own filter post-parsing time, or any other kind of program that processes Parse Trees.