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 discussed in the text and 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 do/check statements, as shown in the grammar below
  3. Implement the error recovery mechanism of Niklaus Wirth, described in Section 2.3.5 on the textbook’s companion site (Examples 2.45 through 2.48). 
  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, where every subtree is represented, recursively, by a parenthesized list in which the first element (immediately inside the parentheses) is the root, and the remaining elements are its children, in order. As an example, the tree
              / | \
             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 do/check statements: 

P→  SL $$
SL→  S SL  |  ε
S→  id := R  |  read id  |  write R  |  if R SL fi  |  do SL od  |  check R
R→  E ET
E→  T TT
T→  F FT
F→  ( R )  |  id  |  lit
ET→  ro E  |  ε
TT→  ao T TT  |  ε
FT→  mo F FT  |  ε
ro→  ==  |  <>  |  <  |  >  |  <=  |  >=
ao→  +  |  -
mo→  *  |  /

Here the new nonterminal R is meant to suggest a “relation.”  As in C, a value of 0 is taken to be false; anything else is true.  The relational operators (==, <> [not equal], <, >, <=, and >=) produce either 0 or 1 when evaluated.  A do loop is intended to iterate until some check-ed relation inside it evaluates to false— “check R” is analogous to “if (!R) break” in C. 

As it turns out, if we assume that integers are unbounded, 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
   do check n > 0
       found := 0
       cf1 := 2
       cf1s := cf1 * cf1
       do check cf1s <= cp
           cf2 := 2
           pr := cf1 * cf2
           do check pr <= cp
               if pr == cp
                   found := 1
               cf2 := cf2 + 1
               pr := cf1 * cf2
           cf1 := cf1 + 1
           cf1s := cf1 * cf1
       if found == 0
           write cp
           n := n - 1
       cp := cp + 1


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 for the TAs to reproduce your tests.  Extra credit will be given to students who provide particularly well designed test mechanisms in their submission. 

Note that your code will employ both insertions and deletions: when match sees a token other than the one it expects, it will insert the expected token and continue (presumably after printing an error message).  When a recursive descent routine sees a token that is not in any of its PREDICT sets, it will delete tokens until it finds something in either its FIRST set or its FOLLOW set.

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. 

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 on the csug machines.  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 TAs might not immediately notice.  Note that only one turn-in is required per team, but each student must mail in the trivia separately. 

Extra Credit Suggestions

  1. If you are in CSC 254, complete the extra work for 454. 
  2. Implement a static semantic check to ensure that every check statement appears inside a do statement, and every do statement has at least one check statement that is inside it and not inside any nested do
  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, for loops, or subroutines. 
  6. Generate equivalent output code in some existing language (e.g. C). 

Trivia Assignment

Before the beginning of class on Wednesday, September 21, each student should send e-mail to 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 1.6 in the text in linear parenthesized form. 


Sunday Oct. 2, at 11:59 pm; no extensions. 
Last Change:  28 October 2016 / Michael Scott's email address