Assignment 2:  Syntax Error Recovery

During the last assignment you probably encountered a wide variety of error messages.  The nature of these messages depends on both the language definition and the compiler or interpreter.  You may have noticed that across languages and implementations these messages differ greatly in their usefulness and specificity.  One feature common to all of the languages you used is syntax error recovery.  In the simplest sense, syntax error recovery is the mechanism by which a compiler or interpreter continues to parse a program (and find more syntax errors) after it encounters an instance of invalid syntax. 

Your task in this assignment is to implement syntax error recovery for an extended version of the calculator language we have been discussing in class.  We provide a basic scanner and parser (written in C).  Given this initial code base, you must:

  1. Translate the code we provide into C++.  Obviously, you must make any changes needed for the code to compile without errors under g++.  In addition, you must replace any calls to C libraries (e.g. for I/O) with the standard C++ equivalents (no printf!). 
  2. Extend the language with if and while statements, as shown in the grammar below
  3. Implement exception-based syntax error recovery, as described in Section 2.3.4 on the CD.  At the least, you should attach handlers to statements, conditions, and expressions. 
  4. Output a syntax tree with the structure suggested (for a slightly different language) in Example 4.15 and Figure 4.12.  Your output should be in linear, parenthesized form reminiscent of (but not the same as) what you generated in Assignment 1.  More specifically, every parent and its children in the tree should be represented by a parenthesized list, recursively.  For example, the tree
                a
              / | \
             b  c  d
            /|  |
           e f  g
    would be represented by the string (a (b e f) (c g) d).  If you are familiar with Lisp or Scheme, this is the standard notation for trees in these languages. 

When run, your program should read a calculator program from standard input, and then output either syntax error messages or a correct syntax tree. 

The initial source code for this assignment is available HERE.  As currently written, it prints a trace of predictions and matches.  You should disable that. 

Extended Language

Here is an LL(1) grammar for the calculator language, extended with if and while statements: 

P→  SL $$
SL→  S SL  |  ε
S→  id := E  |  read id  |  write E  |  if C SL end  |  while C SL end
C→  E ro E
E→  T TT
T→  F FT
F→  ( E )  |  id  |  lit
TT→  ao T TT  |  ε
FT→  mo F FT  |  ε
ro→  ==  |  !=  |  <  |  >  |  <=  |  >=
ao→  +  |  -
mo→  *  |  /

As it turns out, the extensions make the calculator language Turing complete (if still quite impractical).  As an illustration, here is a program that calculates the first n primes:

   read n
   cp := 2
   while n > 0
       found := 0
       cf1 := 2
       cf1s := cf1 * cf1
       while cf1s <= cp
           cf2 := 2
           pr := cf1 * cf2
           while pr <= cp
               if pr == cp
                   found := 1
               end
               cf2 := cf2 + 1
               pr := cf1 * cf2
           end
           cf1 := cf1 + 1
           cf1s := cf1 * cf1
       end
       if found == 0
           write cp
           n := n - 1
       end
       cp := cp + 1
   end
   $$

Suggestions

You do not have to build the syntax tree as an explicit data structure in your program in order to generate the right output.  You are welcome to build it if you want to, though, and extra credit options 3 and 4 (realized as separate, post-parsing traversals of the tree) will be easier if you do. 

We’ve given you a trivial Makefile.  You should add to it a target test that causes make to pipe sample calculator programs (of your choosing) into your parser.  This will make it easier to reproduce your tests.  Extra credit will be given to students who provide particularly well designed test mechanisms in their submission. 

When match sees a token other than the one it expects, it could simply throw a syntax_error exception.  The resulting algorithm would recover by deletion only.  An attractive alternative is to mirror Wirth’s recovery algorithm and have match insert what it expects and continue (presumably after printing an error message).  You may implement either strategy.  For extra credit, try both and compare the results (see below). 

Division of labor and writeup

As in most assignments this semester, you may work alone or in teams of two.  Be sure to follow all the rules on the Grading page.  As with all assignments, use the turn-in script:  ~cs254/bin/TURN_IN.  Put your write-up in a README.txt or README.pdf file in the directory in which you run the script.  Be sure to describe any features of your code that the TA might not immediately notice. 

Extra Work for CSC 454

Students in 454 must implement immediate error detection:  epsilon productions should be predicted only when the upcoming token is in the context-specific FOLLOW set. 

Extra Credit Suggestions

  1. If you are in CSC 254, complete the extra work for 454. 
  2. Compare deletion-only recovery (in which match throws syntax_error when it sees a token it does not expect) with a mixed strategy, in which match inserts what it expects.  Which approach seems to result in better recovery?  Why? 
  3. Extend the language with typed variable declarations, as described in Section 4.6, and implement type checking. 
  4. After parsing and checking, execute (interpret) the calculator program.
  5. Extend the calculator language in other interesting ways.  You might, for instance, add arrays, strings, loops, or subroutines. 
  6. Generate equivalent output code in some existing language (e.g. C). 
  7. Implement locally least cost error recovery using the FMQ algorithm (hard). 

Trivia Assignment

Before the beginning of class on Tuesday, September 18, each student should send e-mail to cs254 containing answers to the following questions: 

  1. Are you working alone or in a team?  If a team, who is your partner? 
  2. Lines 3–4 of file scan.h define an enum type named token.  You will need to change this definition for the current assignment:  show how. 
  3. Give the idiomatic C++ replacement for
        printf("%d + %d = %d\n", a, b, a+b);
  4. Write the tree from Figure 1.5 in linear parenthesized form. 

MAIN DUE DATE: 

Wednesday September 26, at 11:59 pm; no extensions. 
Last Change:  12 September 2012 / Michael Scott's email address