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How to Generate a Stratego Signature

It is tedious to write a signature that is in sync with a syntax definition. Therefore, Spoofax automatically generates a signature from a syntax definition for the abstract syntax trees that the parser for that syntax definition produces.

Write a Syntax Definition

In the /syntax/ directory in your language project for language lang write a syntax definition in file lang.sdf3.

Possibly write additional syntax defininitions modules and import the in lang.sdf3.

Example. Consider the following SDF3 syntax definition:

module lang
imports Base

lexical sorts IntConst
lexical syntax
  IntConst = [0-9]+
sorts Exp
context-free syntax
  Exp.Int     = IntConst  
  Exp.Plus    = [[Exp] + [Exp]]   {left}
  Exp.Minus   = [[Exp] - [Exp]]   {left}
  Exp         = [([Exp])] {bracket}
context-free priorities
  {left : Exp.Plus Exp.Minus}

Generate Signature

The build for your language project will invoke the SDF3 compiler to generate a signature file for each SDF3 file in your project in directory /src-gen/signatures/ with suffix -sig and extension .str.

The build should be invoked as soon as you save a file.

Thus lang.sdf3 generates /src-gen/signatures/lang-sig.str.

Example. For the SDF3 file above, the following signature is automatically generated:

module signatures/lang-sig
imports signatures/Base-sig

  sorts IntConst
  sorts Exp
                   : string -> IntConst
    Int            : IntConst -> Exp
    Plus           : Exp * Exp -> Exp
    Minus          : Exp * Exp -> Exp
    IntConst-Plhdr : IntConst
    Exp-Plhdr      : Exp
    IntConst-Plhdr : COMPLETION-INSERTION -> IntConst
    Exp-Plhdr      : COMPLETION-INSERTION -> Exp

The injection from strings into the lexical IntConst sort reflects the fact that tokens are represented as strings in ASTs. The placeholder constructors generated for the sorts are used to represent incomplete programs and syntactic code completion1.

Use Signature

To use the signature, import it into the Stratego module that uses its constructors.

Example. To use the signature for the example import it as follows:

module desugar
imports signatures/lang-sig
  // ...


  1. Luis Eduardo de Souza Amorim, Sebastian Erdweg, Guido Wachsmuth, and Eelco Visser. Principled syntactic code completion using placeholders. In Tijs van der Storm, Emilie Balland, and Dániel Varró, editors, Proceedings of the 2016 ACM SIGPLAN International Conference on Software Language Engineering, Amsterdam, The Netherlands, October 31 - November 1, 2016, 163–175. ACM, 2016. URL:, doi:10.1145/2997364.2997374

Last update: 2021-11-15
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