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

exports
 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

• ASF+SDF Meta Environment provides SDF
• RascalMPL
• Spoofax/IMP [1]
• Stratego/XT
• Strafunski

• GNU bison
• ANTLR

• A Quick Introduction to SDF, Visser, J. & Scheerder, J. (2000) CWI
• The Syntax Definition Formalism SDF, Mark van den Brand, Paul Klint, Jurgen Vinju (2007) CWI

• Grammar Deployment Kit
• SdfMetz computes metrics for SDF grammars
• Download SDF from the ASF+SDF Meta Environment homepage