Lab 2: Type checker and interpreter for C/C++

Programming Language Technology, 2015


The objective of this lab is to write a type checker and an interpreter for a fragment of the C++ programming language. The type checker should check the program and send it to the interpreter at success. The interpreter should run the program and correctly perform all its input and output actions. At type checking failure, a type error should be reported.

Before the lab can be submitted, the program has to pass some tests, which are given on the course web page via links later in this document.

The recommended implementation is via a BNF grammar processed by the BNF Converter (BNFC) tool. The syntax tree created by the parser should then be processed further by a program using the skeleton generated by BNFC.

The fragment of C++ covered is smaller than in Laboration 1, and does not really include any C++ specific features not available in C. You can use the grammar given here, also explained in the Lecture notes, Chapter 2.

The type checker and the interpreter code will be roughly 300-800 lines together, depending on the programming language used.

All BNFC supported languages can be used, but guidance is guaranteed only for Haskell and Java 1.5.

The type system and the interpreter are partially characterized by formal rules in in the course lecture notes, chapters 4 and 5.


In the type checker, the recommended procedure is two passes:

  1. build a symbol table with all function types, including the built-in functions;
  2. type check and annotate the code by using this symbol table.

In the interpreter, do a similar thing:

  1. build a symbol table that provides for every function its source code syntax tree; the built-in functions can be left out and treated separately in the rule for evaluating function calls;
  2. interpret the program by evaluating the expression main().

The type checker


Only the four built-in types


are taken into account. Every expression has one of these types.

Types of functions in the symbol table can be represented in any way that stores their argument and return types. For instance, the function header

    int f (double x, bool b)

can create a symbol table entry (not actual code):

    f → ([double, bool], int)

mapping the name f to a pair whose first component is the list of argument types and the second component is the return type.


A program is a sequence of definitions.

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

An interpretable program must have a function main of type int that takes no arguments. It may or may not have a return statement.

    int main () {


A function definition has a type, a name, an argument list, and a body. Example:

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

The language has four built-in functions dealing with input and output:

    void   printInt(int x)        // print an integer and a newline in standard output
    void   printDouble(double x)  // print a double and a newline in standard output
    int    readInt()              // read an integer from standard input
    double readDouble()           // read a double from standard input

Typing rules

The same function name may be used in at most one function definition.

All return statements in a function body must return an expression whose type is the return type of the function.

You don't need to check that there actually is a return statement.

Argument lists, declarations, and function bodies

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

An argument declaration has a type and an identifier, for instance

    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 a list of statements enclosed in curly brackets { and } .

Typing rules

An argument list may only use each variable once.



Any declaration followed by a semicolon ; can be used as a statement. Declarations have one of the following formats:

Typing. The initializing expression must have the declared type.

Return Statement

Statements returning an expression, for example

    return i + 9 ;

Typing. The type of the returned expression must be the same as the return type of the function in which it occurs.

While Loop

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

    while (i < 10) ++i ;

Typing. The expression must have type bool.


if with an expression in parentheses followed by a statement, else, and another statement. Examples:

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

Typing. The expression must have type bool.

(No else-less if statements.)


Any list of statements (including empty list) between curly brackets. For instance,

      int i = 2 ;
      i++ ;

Typing rules

A variable may only be declared once on the same block level.

The parameters of a function have the same level as the top-level block in the body.


The following table gives the expressions constructs, their precedence levels, and their associativity. The associativity of operators is given as left, right, or none. For binary operators, in general any of the three associativity is meaningful. For pre-, post-, and mixfix operators, at most one of left or right associativity makes sense, and the alternative is non-associative. As they are bracketed, the arguments in a function call can be expressions of any level. Otherwise, some subexpressions have to be one precedence level above of the main expression to implement the required associativity.

Note. The table is not exactly the same as in the C++ standard. Also note that these precedence levels are already implemented in the provided grammar.

level expression forms assoc explanation type
15 literal - literal literal type
15 identifier - variable declared type
15 f(e,...,e) none function call return type
14 v++, v-- none in/decrement (sugar)
13 ++v, --v none in/decrement (sugar)
12 e*e, e/e left mult, div operand type (int or double)
11 e+e, e-e left add, sub operand type (int or double)
9 e<e, e>e, e>=e, e<=e none comparison bool
8 e==e, e!=e none (in)equality bool
4 e&&e left conjunction bool
3 e||e left disjunction bool
2 v=e right assignment type of LHS

Typing rules

Integer, double, and boolean literals have their usual types,

Variables have the type declared in the nearest enclosing block. A variable must be declared before it is used in an expression.

The arguments of a function call must have types corresponding to the argument types of the called function. The number of arguments must be the same as in the function declaration (thus the C++ default argument rule is not applied). Notice that only identifiers are used as functions.

Increments and decrements only apply to variables. (This can be covered in the parser.)

Comparison and (in)equality apply to two operands of the same type, which is int, double, or bool.

Conjunction and disjunction apply to operands of type bool only.

Assignments have the same type as the left-hand-side variable. Notice that only variables are used as left-hand-sides.

(There are no qualified constants or template instantiations.)


We include integer literals and floating point literals.

There are also two boolean literals, true and false. Notice that the names true and false were not specified as literals in Lab 1, so you probably treated them as identifiers.


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


There are three kinds of comments.

Comments cannot be nested.

The interpreter

The top-level interpreter is run by by evaluating the expression main(). The return value is ignored.


There are four types of values:

Instead of boolean values, you may use integers. Then true can be interpreted as 1 and false as 0.

Values can be seen as a special case of expressions: as expressions that contain no variables and cannot be evaluated further. But it is recommended to have a separate datatype of values, in order to guarantee that evaluation always results in a value.

Thus, the evaluation of an expression in an environment should always result in a value.


A program is a sequence of function definitions. Each function has a parameter list and a body, which is a sequence of statements.

The evaluation of a function call starts by evaluating the arguments and building an environment where the received values are assigned to the argument variables (a.k.a. parameters) of the function.

The statements in the body are then executed in the order defined by their textual order as altered by while loops and if conditions.

The function returns a value, which is obtained from the return statement. After encountering a return, all the following statements in the function body should be ignored (not evaluated). If the return type is void, no return statement is required.


A declaration, e.g.

    int i ;

adds a variable to the current environment. Its value is initialized if and only if the declaration includes an initializing expression, e.g.

    int i = 9 ;

An expression statement, e.g.

    i++ ;

is evaluated, and its value is ignored.

A block of statements, e.g.

      int i = 3 ;
      i++ ;

is interpreted in an environment where a new context is pushed on the context stack at entrance, and popped at exit.

A while statement, e.g.

    while (1 < 10){
      i++ ;

is interpreted so that the condition expression is first evaluated. If the value is true, the body is interpreted in the resulting environment, and the while statement is executed again. If the value is false, the statement after the while statement is interpreted.

An if-else statement, e.g.

    if (1 < 10) i++ ; else j++ ;

is interpreted so that the condition expression is first evaluated. If the value is true, the statement before else is interpreted. If the value is false, the statement after else is interpreted.

A return statement is executed by evaluating its expression argument. The value is returned to the caller of the function, and no more statements in the function body are executed.


The interpretation of an expression, also called evaluation, returns a value whose type is determined by the type of the expression.

A literal, e.g.


is not evaluated further but just converted to the corresponding value.

A variable, e.g.


is evaluated by looking up its value in the innermost context where it occurs. If the variable is not in the context, or has no value there, the interpreter terminates with an error message

    uninitialized variable x

A function call, e.g.


is interpreted by first evaluating its arguments from left to right. The environment is then looked up to find out how the function is interpreted on the resulting values.

Calls of the four built-in functions can be hard-coded as special cases in the expression evaluation code.

A postincrement,


has the same value as its body initially has (here i). The value of the variable i is then incremented by 1. i-- correspondingly decrements i by 1. If i is of type double, 1.0 is used instead.

A preincrement,


has the same value as i plus 1. This incremented value replaces the old value of i. The decrement and double variants are analogous.

The arithmetic operations addition, subtraction, multiplication, and division,

    a + b
    a - b
    a * b
    a / b

are interpreted by evaluating their operands from left to right. The resulting values are then added, subtracted, etc., by using appropriate operations of the implementation language. We are not picky about the precision chosen, but suggest for simplicity that int should be int and double should be double.


    a <  b
    a >  b
    a >= b
    a <= b
    a == b
    a != b

are treated similarly to the arithmetic operations, using comparisons of the implementation language. The returned value is an integer.


    a && b

is evaluated lazily: first a is evaluated. If the result is true (1), also b is evaluated, and the value of b is returned. However, if a evaluates to false (0), then false is returned without evaluating b.


    a || b

is also evaluated lazily: first a is evaluated. If the result is false (0), also b is evaluated, and the value of b is returned. However, if a evaluates to true (1), then true is returned without evaluating b.


    x = a

is evaluated by first evaluating a. The resulting value is returned, but also the context is changed by assigning this value to the innermost occurrence of x.

Lab format

Input and output

The interpreter must be a program called lab2, which is executed by the command

    lab2 <SourceFile>

and prints its output to the standard output. The output at success must be just the output defined by the interpreter.

The output at failure is an interpreter error, or a TYPE ERROR or a SYNTAX ERROR, depending on the phase at which the error occurs. These error messages should also give some useful explanation, but we don't specify what exactly its format.

The input can be read not only from user typing on the terminal, but also from standard input redirected from a file or by echo. For instance,

    ./lab2 <test-input
    echo 20 | ./lab2

The easiest way to produce the proper format is to use the ready-made files in either of

Example of success

Source file

  int main ()
    int i = readInt() ; //5
    printInt(i) ;   //5
    printInt(i++) ; //5
    printInt(i) ;   //6
    printInt(++i) ; //7
    printInt(i) ;   //7

Running the interpreter:

    % echo 3 | ./lab2

Examples of failure

Type error

Source file

  int f (double c)
    int n = 1 ;
    while(c) ++n ;

Running the type checker

    % lab2
    condition c in while: expected bool, found double

Interpreter error

Source file

  int main ()
    int i ;
    printInt(i) ;
    printInt(i++) ;
    printInt(i) ;
    printInt(++i) ;
    printInt(i) ;

Running the interpreter

    % lab2
    uninitialized variable x

Thus it is assumed that the type checker does not detect uninitialized variables.

Compiling the interpreter

The interpreter is submitted as source files that can be compiled by typing make. The file names must match the ready-made files in either the Haskell package or the Java 1.5 package. The simplest solution is to copy the contents of these files and replace the grammar, the type checker, and the interpreter by your own files.

If you want to write the interpreter in another language, the procedure is the same: send a tar package and make sure the interpreter can be compiled in a normal Unix enviroment by typing make.

Test programs

Run the testsuite before submitting the lab.

Success criteria

Your interpreter must pass the testsuite. The test suite contains both good and bad programs. The good programs must be executed correctly, whereas the bad ones must fail in appropriate ways in either the type checker or the interpreter.

The solution must be written in an easily readable and maintainable way. For instance, tailoring it for the programs in the test suite is not maintainable!


Submit your lab by using Fire. Please include exactly all the files that are required for building your solution, including a Makefile. Do not however submit any generated files, and kindly avoid using archives (upload each file individually).

If you have any problems getting the test program to run, or if you think that there is an error in the test suite, contact the teachers via the course Google group.

A hint on file structure

The easiest way to get started is to extend from one of the following templates:

One way to extend the files to the full language is to use the BNFC-generated skeleton as a template. You can also bootstrap them from the files in the mini interpreter.

In these packages, you only have to change three files: