CS 301 Homework 4 - due 3:30pm, Wednesday, March 10, 1999

Typechecking

In this assignment, you will build a type-checker for the PCAT abstract syntax trees generated in assignment 3. To do this, you'll process declarations to extract type information about identifiers and process statements and expressions to make sure that identifiers and literals are used correctly.

For this assignment, assume that any use of real numbers or records in PCAT programs is illegal: that is, issue an error if either of these features is used. In addition, assume that type definitions are not recursive, i.e.,

should be handled as completely equivalent to

However, you should handle recursive procedure declarations correctly.

Your typechecker should detect all semantic errors, including:

• incompatible or illegal operand types in expressions, statements, or declarations;
• uses of undefined symbols;
• multiple declarations of a symbol within a single scope;
• attempting to assign to a constant;
• incorrect number or types of arguments in procedure call;
• EXIT statements in inappropriate contexts;
• RETURN statements in inappropriate contexts, or lacking a return value when one is required or vice-versa;
• (for purposes of this assignment only) use of constructs involving real numbers or records. (Any such use should produce some error message; the message text need not specifically complain about reals or records, however.)

Do not attempt to check that function procedures always execute a RETURN, or that array references are within bounds. Consult the PCAT Language Reference Manual for detailed rules about type compatibility.

Your typechecker should be an executable called check, which reads input from stdin. The input consists of a PCAT program in externalized AST format, as produced by the parser of assignment 3. (Thus, the output of parser can be piped into check.) When it detects an error, check should issue a suitable message to stderr citing the nature of the error and the line number associated with the AST construct where the error occurred, and then exit gracefully. It should not attempt error recovery. If there are no errors, check should produce no output at all. As usual, the ``correct'' behavior of the the typechecker is specified by the behavior of /u/cs301acc/4/check. The precise text of your error messages does not have to match the ``official'' executable, however.

Implementation and Program Submission

Several support files are provided to simplify the implementation:

• As in assignment 3, ast.[hc] contain suitable type and constructor definitions for AST trees.
• parse_ast.y defines a function parse_Program that reads externalized AST and builds an internal tree using the constructor functions.
• types.[ch] defines suitable type and constructor definitions for describing PCAT types.

You will need to use a global symbol table or an environment parameter (as in assignment 1's check.c) to keep track of the definitions currently in scope. There is no need to engineer this mechanism to handle large numbers of symbols, nor to worry about deallocation of symbol information.

Prepare a makefile that builds your checker and produces an executable check.

Submit your program by mailing a shar ``bundle'' containing the relevant files to cs301acc.