Summary

The objective of this assignment is to write a parser for a fragment of the C++ programming language. The parser should return an abstract syntax tree at success, and report an error with a line number at failure.

The recommended implementation of the parser is via a BNF grammar processed by the BNF Converter (BNFC) tool. The approximate size of the parser source code should be 100 rules. With BNFC, no more files than the grammar have to be written and submitted.

Method

Build the grammar gradually, so that you can parse the six test files in the given order: as your first goal, parse the first test file. then the second, and so on.

When you have passed one level, just try to parse the next example with your current parser and examine the line at which it fails. In this way, you will find out what new grammar rules you need.

After treating the six tests, run the test script to make your parser perfect.

You can use any code generator (Haskell, Java, C,...) from BNFC. It doesn't need to be the same language as you plan to use in later assignments. However, if you use Haskell, you have the advantage of info files, which tell you exactly where the conflicts are. Assuming your grammar file is CPP.cf and you have called bnfc on it, you can create an info file from the generated Happy file:

    happy -i ParCPP.y

The info file is now in ParCPP.info. Moreover, you can produce a debugging parser by

    happy -da ParCPP.y

Running this parser shows a trace of the LR actions performed during parsing.

Language specification

It is suitable to explain and advisable to implement the parser top-down, starting with the largest units and proceeding to smaller ones.

The specification differs in some places from the official C++ specification.

To help you get going, we have marked with a bold (1) those rules that are needed to parse the first test file.

Programs

A program is a sequence of definitions. (1)

A program may also contain comments and preprocessor directives, which are just ignored by the parser (see below). (1)

Definitions

1. A function definition has a type, a name, an argument list, and a body. (1) A function can optionally be prefixed by inline. Example:

    inline int foo(double x, int y)
    {
      return y + 9 ;
    }

2. Many statements can be used as top-level definitions:

• typedef statements
• variable declarations and initializations
• struct declarations

3. Finally, definitions for using qualified constants are allowed, e.g.

    using std::vector ;

Argument lists, declarations, and function bodies

An argument list is a comma-separated list of argument declarations. It is enclosed in parentheses ( and ). (1)

An argument declaration always has a type. This type is optionally followed by an identifier or an identifier and an initialization, and optionally preceded by the specifier const. The following are examples of argument declarations.

    int
    int x
    int x = 5
    const int& x

Notice that argument declarations with multiple variables (int x, y) are not included. A declaration that occurs as a statement (as shown below), can also have multiple variables. But it must have at least one variable.

A function body is either a list of statements enclosed in curly brackets { and } (1), or an empty body consisting of a semicolon ;.

Statements

Any expression followed by a semicolon ; can be used as a statement. (1)

Any declaration followed by a semicolon ; can be used as a statement. Declarations have the same form as argument declarations in functions, except that they can have more than one variable.

Statements returning an expression (1), for example

    return i + 9 ;

While loops, with an expression in parentheses followed by a statement, for example:

    while (i < 10) ++i ;

Do-while loops, with an expression in parentheses after the loop body, for example:

    do ++i ; while (i < 10) ;

For loops, with a declaration and two expressions in parentheses followed by a statement. For example:

    for (int i = 0 ; i != 10 ; ++i) k = k + i ;

We do not require that any of the fields in parentheses may be empty.

Conditionals: if with an expression in parentheses followed by a statement and optionally by else and a statement. Examples:

    if (x > 0) return x ;
    
    if (x > 0) return x ; else return y ;

Blocks: any list of statement (including empty list) between curly brackets. For instance,

    { 
      int i = 2 ;
      {
      }
      i++ ;
    }

Type definitions: a type and a name for it. Example:

    typedef vector<string> Text ;

Structure definitions: a name and a list of declarations. Example:

    struct Student_info {
      string name;
      double final;
      vector<double> homework;
    } ;  

Notice that the semicolon is obligatory in the end of a structure definition. Otherwise, semicolons are not used after curly brackets - but they are obligatory in all statements and definitions not ending with curly brackets.

Expressions

The following table gives the expressions and their precedence levels. Infix operators are assumed to be left-associative. The arguments in a function call can be expressions of any level. Otherwise, the subexpressions are assumed to be one precedence level above of the main expression.

Note. The table is not exactly the same as in the C++ standard.

Expressions needed for test (1): integer and string literal, qualified constant, left shift.

Notice that it is syntactically permitted to increment and assign a value to any expression, not just variables (and other so-called lvalues).

Qualified constants and template instantiations

Qualified constants are constant names separated by ::. Names can be identifiers but also template instantiations, of the form

    ident < typelist >

where a typelist is a comma-separated list of types. Thus possible qualified constants include

    std::cout
    std::vector<int>::const_iterator

One simple way to implement qualified constants is as nonempty lists separated by ::. The elements of the list are identifiers and template instantiations. Single identifier expressions come out as a special case of these lists.

Qualified constants, but without templates, are needed in (1).

Types

Types are either qualified constants (including plain identifiers) or built-in types, of which we include the following:

    bool
    double
    int
    void

Only the type int is needed in (1).

Literals

We include double-quoted string literals, integer literals, and floating point literals.

A string literal may consist of many can be concatenated strings and in this way divided over lines:

    "hello " "my little "
    "world"
  

Identifiers

An identifier is a letter followed by a list of letters, digits, and underscores.

Comments

There are three kinds of comments.

• anything between tokens /* and */
• anything from token // to the end of a line or the file
• anything from token # to the end of a line or the file (preprocessor directive)
  Comments cannot be nested.
  Comments are needed in (1).

Test programs

These programs are original code from the web page of the book Accelerated C++ (A. Koenig & B. Moo, Addison-Wesley, 2000).

• First: "hello world"

• Second: ask the user's name and say hello to her

• Third: compute the grade of a student

• Fourth: a smarter way to compute the grade

• Fifth: test if a string is a palindrome

• Sixth: random-generate English sentences

Once again: build the grammar gradually so that you can parse these files in this order.

Success criteria

Your grammar must pass the test suite. The test suite contains the example programs, as well as a number of programs which your parser must reject.

The maximum number of 10 shift/reduce conflicts is permitted. The minimum of 2 is almost unavoidable (the "dangling else", and template instantiation vs. less-than operator). But reduce/reduce conflicts are forbidden.

C++ features

Assuming you know some C but not C++, here is a summary of the extra features of C++ involved in this assignment.

Qualified names: s::n, where s is a name space or a class.

using directives: license to use an unqualified name from a name space.

IO streams: cout for output, cin for input. Output is produced by the left shift operator, input is read by the right shift. Example from the second program:

    // read the name
    std::string name;     // define `name'
    std::cin >> name;     // read into `name'
  
    // write a greeting
    std::cout << "Hello, " << name  << "!" << std::endl;

Templates, e.g. generic types: vector<int> is a vector of integers.

Arguments passed by reference (&), with the possibility of forbidding modification (const), e.g. (from the sixth program)

    gen_aux(const Grammar& g, const string& word, vector<string>& ret)