Chapter 7: Syntax-Directed Translation

• Overview
• Studio
• Lab

Lab

Getting Started

• Locate and install the lab7 project into your eclipse workspace as directed by your instructor, following the usual instructions, including those for an ant build.
• Browse this lab document to get an idea of the work you will do for this assignment.

Notes

• In this lab, you are given jmm.cup, which is a CUP grammar for a subset of the Java language jmm , which stands for Java minus minus.

  Browse through the file and familiarize yourself with the structure of this grammar. It is the only file you should modify for this assignment.

• Overall approach:

  You are advised to follow the following strategy as you develop your solution. Failure to follow this advice will increase your development time and could cause anxiety, confusion, and dry flaky skin.

  • The very top part of the AST is already created for you, starting at the rule for CompilationUnit and ending at TypeDeclaration (which is really just a completed class declaration).
  • The build.xml file allows you to run your solution or the reference parser. The targets run a given parser on all of the test files found in the TestFiles directory.
  • Take a look at the ASTs produced by the reference parser by running the appropriate ant task described below.
  • Now look at what happens when you run your parser. You will see the topmost part of the tree only. It's your job to construct the rest of the AST.
  • Proceed by slowly descending down the grammar, implementing a little more, and compiling and testing as you go. For example, the next thing you might do is create nodes for each ClassDeclaration. At first that node could just be a simple TemporaryNode, and you can fill in more detail later, such as the class name, protection bits, and class body.
  • Proceed slowly and test as you go to make sure your AST is developing the direction you desire.
• Debugging with tools like CUP can be confusing, because:
  • Your code in the .cup file is translated into .java code.
  • Exceptions raised in the .java file are reported with respect to the .java line numbers.
  To figure this out, you'll have to trace the exception back to problems in your .cup file:
  1. Go to the error line in the Parser.java file, and look up (the file, not the ceiling) to find the case clause that shows the relevant production in your .cup file.
  2. Now look in your .cup file at the semantic code and try to find the problem.
  3. Failure to create objects in semantic actions will lead to null-pointer exceptions. As described below, the TemporaryNode class is available to act as a place-holder while you decide more customized node-types to instantiate.
• The following ant tasks will help you with building and testing your lab:

  build	Default target, runs appropriate tools and compiles your project to bring it up-to-date. This does not run your parser on any inputs!
  runYourParser	Runs your parser on all files in the TestFiles directory
  runYourParserTrace	Runs your parser in trace mode on all files in the TestFiles directory. The trace shows the actions of the shift-reduce parser so that you can determine where and why the parser detects an error in a test file.
  runReferenceParser	Runs the reference parser on all files in the TestFiles directory, showing the resulting ASTs.
  clean	Deletes the class files and all automatically generated files. Use this if things are behaving weirdly.

• Be certain only to modify the file jmm.cup as you work on this assignment.

Specifics

SelStmt ::= IF:ifterm LP Expr RP Stmt 

AbstractNode ifnode = makeNode(ifterm); 

 AbstractNode n1 = makeNode("SomeKindofNode");
 AbstractNode n2 = makeNode("AnotherKindofNode");
 n1.makeSibling(n2);

 AbstractNode n1 = makeNode("Sib 1");
 AbstractNode n2 = makeNode("Sib 2");
 AbstractNode p  = makeNode("Boss");
 p.adoptChildren(n1.makeSibling(n2));

• The class AbstractNode can be extended to make useful abstract syntax tree nodes.

  Important examples of this are IntegerNode and TemporaryNode which are included in the jmm.cup file in the action code block.

• If you need some auxilliary classes or methods to simplify your semantic actions, these can be placed as methods in the action code block. Several examples are already there for you to see how this can be done.