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StringTemplate

StringTemplate

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StringTemplate

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  1. StringTemplate Terence Parr University of San Francisco parrt@cs.usfca.edu

  2. Outline • What is StringTemplate • Operations • Semantics • Example Java/XML Generation • ANTLR 3.0 template

  3. What is StringTemplate? • Template engine designed with Tom Burns (CEO, jGuru.com) while building commercial sites over many years in response to JSP • small: 170k uncompressed binary (w/o test rig) • Evolved from simple “document with holes” into a functional language capable of generating large class of languages • Well suited to generative programming as well as dynamic page generation; being used in new ANTLR parser generator for code generation • Distinguishing characteristic: strictly enforces separation of model and view

  4. Argument for StringTemplate • Car aerodynamics mostly trumps style today • Similarly, the nature of generating text should dominate design decisions: use an output grammar • Don’t have output grammars, we have programs with print statements; template engines arose to encourage separation of logic/display. • Enforcing strict separation also leads to similar grammar-like mechanism • Conclusion: if you’re generating text, you should be using something akin to StringTemplate

  5. Canonical Operations • Attribute reference:<type> • Template references (possibly recursive):<statementList()> • Apply template to multi-valued attribute:<decls:decl()>or<decls:{<it.type> <it.name>;}><items:red(), blue(), green()> • Conditional include:<if(superClass)>extends <superClass><endif><if(name)>Name: <name><else>Guest<endif>

  6. Semantics • Side-effect free expressions • No “state” • No defined order of execution • Lazy evaluation (crucial!) • Dynamically scoped; values inherited • Template inheritance: group to subgroup • Recursive template instantiation

  7. Template Groups • Set of mutually-referential templates with formal arguments group javaTemplates; method(type,name,args,body) ::= << public <type> <name>( <args:arg(); separator=“,”> ) { <body> } >> assign(lhs,expr) ::= “<lhs> = <expr>;” if(expr,stat) ::= “if (<expr>) <stat>” call(name,args) ::= “<name>( <args; separator=“,”> );” …

  8. Example: Dump Java Class • Expected output: class Dump { public int i; public java.lang.String name; public int[] data; public void main(class java.lang.String[] arg1); public void foo(int arg1, float[] arg2); public class java.lang.String bar(); }

  9. Dump Java Class Templates group Java; class(name,fields,methods) ::= << class <name> { <fields:field(); separator="\n"> <methods:method(); separator="\n"> > } >> field() ::= "public <type(t=it.type)> <it.name>;" method() ::= << public <it.returnType> <it.name> (<it.parameterTypes:{<type(t=it)> arg<i>}; separator=", ">); >> type(t) ::= << <if(t.componentType)><t.componentType>[] <else><t.name><endif> >>

  10. Dump Java Class Code public class Dump { public int i; public String name; public int[] data; public static void main(String[] args) throws IOException { StringTemplateGroup group = new StringTemplateGroup(new FileReader("Java.stg"), AngleBracketTemplateLexer.class); Class c = Dump.class; Field[] fields = c.getFields(); Method[] methods = c.getDeclaredMethods(); StringTemplate classST = group.getInstanceOf("class"); classST.setAttribute("name", c.getName()); classST.setAttribute("fields", fields); classST.setAttribute("methods", methods); System.out.println(classST); } public void foo(int x, float[] y) {;} public String bar() {return "";} }

  11. Dump XML Instead group XML; class(name,fields,methods) ::= << <class> <name>$name$</name> $fields:field()$ $methods:method()$ </class> >> field() ::= << <field> <type>$type(t=it.type)$</type><name>$it.name$</name> </field> >> … <class> <name>Dump</name> <field> <type>int</type><name>i</name> </field> … </class>

  12. Sample ANTLR 3.0 Template parser(name, tokens, rules, DFAs) ::= << class <name> extends Parser { <tokens: {public static final int <it.name>=<it.type>;} > public <name>(TokenStream input) { super(input); } <rules; separator="\n"> <DFAs> } >>

  13. ANTLR 3.0 Error Templates • Internationalization: use a template group for each locale; load appropriate templates RULE_REDEFINITION(file,line,col,arg) ::= "<loc()>rule <arg> redefinition” UNDEFINED_RULE_REF(file,line,col,arg) ::= "<loc()>reference to undefined rule: <arg>” loc() ::= "<file>:<line>:<col>: " inherits file, line, col attributes from enclosing template

  14. Language Translation • MVC (parser + “unparser”) • model -- input stream and/or ASTs • view -- templates • controller -- parser • Controller extracts data from model, provides to view without worrying about details of output structure • Maps abstract input concept to output concept like assignment to assignment. • Abstract concepts represented by one or more rules in parser grammar and one or more templates in template file

  15. Translation grammar+ST variable returns [StringTemplate code=null] {StringTemplate t=null,d=null;} : t=type d=declarator SEMI { if ( currentFunctionName==null ) { code = template("globalVariable"); } else { code = template("variable"); } code.setAttribute("type", t); code.setAttribute("name", d); } ; declarator returns [StringTemplate code=null] : id:ID {code=text(id.getText());} ;

  16. Translation Templates • Javavariable(type,name)::="<type> <name>;”globalVariable ::= variable • Pythonvariable(type,name) ::= " ”globalVariable ::= variable • Bytecodesvariable(type,name) ::= ".var is <name> <type>" globalVariable(type,name) ::= ".field <name> <type><\n>"

  17. Summary • The nature of text generation and the enforcement of model-view separation dominate tool design-decisions: • tools should resemble output grammars • StringTemplate is a simple template engine that evolved while building dynamic sites. It is proving exceptionally well suited to code generation tasks including ANTLR 3.0 • Open-source Java, BSD license (also a C# port; python coming) • Links • http://www.stringtemplate.org • http://www.codegeneration.net/tiki-read_article.php?articleId=77