Syntax Definition Formalism

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The Syntax Definition Formalism (SDF) is a metasyntax used to define context-free grammars: that is, a formal way to describe formal languages. It can express the entire range of context-free grammars. Its current version is SDF3.[1] A parser and parser generator for SDF specifications are provided as part of the free ASF+SDF Meta Environment. These operate using the SGLR (Scannerless GLR parser). An SDF parser outputs parse trees or, in the case of ambiguities, parse forests.


Features of SDF:

  • Supports the entire range of context-free languages
  • Allows modular syntax definitions (grammars can import subgrammars) which enables reuse
  • Supports annotations


The following example defines a simple Boolean expression syntax in SDF2:

module basic/Booleans

  sorts Boolean
  context-free start-symbols Boolean

context-free syntax
   "true"                      -> Boolean
   "false"                     -> Boolean
   lhs:Boolean "|" rhs:Boolean -> Boolean {left}         
   lhs:Boolean "&" rhs:Boolean -> Boolean {left}       
   "not" "(" Boolean ")"       -> Boolean           
   "(" Boolean ")"             -> Boolean

 context-free priorities
   Boolean "&" Boolean -> Boolean >
   Boolean "|" Boolean -> Boolean

Program analysis and transformation systems using SDF[edit]

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