---
title: "Syntax Definition Formalism"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Syntax_Definition_Formalism"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T11:39:36.670441+00:00"
instance: "kb-cron"
---

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. 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.


== Overview ==
Features of SDF:

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


== Examples ==
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


== Program analysis and transformation systems using SDF ==
ASF+SDF Meta Environment provides SDF
RascalMPL
Spoofax/IMP [1]
Stratego/XT
Strafunski


== See also ==
GNU bison
ANTLR


== References ==


== Further reading ==
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


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