Project B: The MiniJava Parser

Due: Thursday, February 5, 2009 at 11:00 pm. Use the online dropbox to turn in your files

In this assignment you will extend the initial MiniJava parser and AST representation with the extensions described in the course project description handout.

You should extend MiniJava's syntax to allow the following (all of which are legal in full Java too):

• double is a legal (base) type.

• A floating-point literal constant is a legal expression.

• An array of a base type, e.g., int[], boolean[][][], and in general type[] where type is an arbitrary base type, is a legal (base) type. (Base types are int, boolean, double, and arrays of base types. Only class types are not base types; this restriction is included only because otherwise the language becomes harder to parse! The AST and the rest of the compiler should not depend on this restriction against arrays of class types, however.)

• A one-level array allocation expression, e.g., new int[10], new boolean[20][][], and in general new type[expr]dims where type is an arbitrary non-array base type, expr is an arbitrary expression, and dims is a possibly-empty sequence of []'s, is a legal expression.

• An array dereference, e.g., a[i], b[i][j][k], and in general expr1[expr2] where expr1 is an arbitrary atomic expression and expr2 is an arbitrary expression, is a legal expression. An array dereference is also legal on the left-hand side of an assignment statement. (Atomic expressions exclude unary and binary operator expressions and array allocation expressions.)

• An array length expression, e.g., a.length and in general expr.length where expr is an arbitrary atomic expression, is a legal expression. length is a reserved word in MiniJava (unlike Java).

• An "or" expression (using the || infix operator) is a legal expression.

• if statements do not require else clauses.

• For loops of the restricted form for ( i = expr1 ; expr2 ; i = expr3 ) stmt are allowed, where expr1, expr2, and expr3 are arbitrary expressions, i is an arbitrary variable (but which has to be the same variable in both the initialization and increment clauses - something you might wish to defer checking until the next phase of the project), and stmt is an arbitrary statement.

• break statements are allowed. (You do not need to check in the syntax that break statements only appear inside of loops; semantic checking will enforce this.)

• A class variable declaration may be preceded by the static reserved word to declare a static class variable.

You should follow the precedence and associativity rules of regular Java for these extensions. It's OK to use CUP's precedence declarations to achieve this. It's OK to have one shift/reduce conflict in your CUP grammar, for the "dangling else" problem. Add the -expect 1 option before the minijava.cup argument in the Makefile to build Parser/parser.java if you decide to accept this shift/reduce conflict. (FYI, in one sample solution, the author couldn't find a way to revise the CUP grammar specification to avoid this conflict.)

You should add new AST classes and/or modify existing AST classes so that you can represent the new MiniJava constructs. You should define the appropriate toString operations on these classes so that they can be pretty-printed in a form that is syntactically legal and produces the same AST if it is parsed again. The other operations required of AST nodes, e.g. typechecking, evaluating, and lowering, you should implement by throwing UnimplementedError exceptions (for now).

You only need to get the parser to work (and keep the extended scanner working). You do not need to do anything to enforce type checking rules or other semantic-analysis constraints on the input program.

Do the following:

1. Extend the specification of MiniJava's syntactic structure to describe the extended language, in the same style. (You can assume precedence and associativity is specified separately, and it is OK to define a grammar that is ambiguous with respect to the "dangling else" problem.)
   
2. Add and/or modify classes in the AST subdirectory to represent the extended language.
   
3. Extend Parser/minijava.cup to parse the extended language and construct the abstract syntax tree representing the parsed program.
   
4. Develop test cases that demonstrate that your extended parser and AST classes work, both in cases that should now be syntactically legal and in cases that should still be syntactically illegal. (Since the parser quits at the first error, you'll likely need several illegal test case files to test the different illegal cases.) You do not need to check for lexical errors, just syntactic errors. The SamplePrograms directory contains some files that should parse after you make your changes; some of the files should also parse successfully with the initial version of the MiniJava compiler.

You can use the -parse -printAST options to the MiniJava compiler to just run the parsing phase and print out the AST that it builds. See the test_parser target in the Makefile for an example, and feel free to make your own target(s) to make running the tests you like easier and more mechanical.

Turn in the following:

1. Your extended MiniJava syntax specification.
   
2. Your modified minijava.cup file. Clearly identify your changes using comments.
   
3. Your new and/or modified AST/*.java files. Clearly identify any modifications using comments.
   
4. Your test cases, with names of the form name.legal.java for test cases that should parse successfully and name.illegal.java for test cases that should trigger syntax errors.
   
5. A transcript of running your parser and printing out the resulting AST on each of your test cases (at least).

Create a single archive containing these files, and submit it using the online dropbox by the due date.