oolong imperative language interpreter

Interpreter is based on a State, Reader and Except monads.

• Reader stores map: variable/function name -> location.
• State stores map: location -> value and a number representing the next free location.
• Except is responsible for error messages.

Static typing module uses Reader and Except monads.

• Reader stores:
  • map: variable/function name -> (type, number representing scope of the declaration)
  • current scope index
  • expected type returned by currently checked function and its name
  • information whether current instruction is inside a loop
• Except is responsible for error messages.

Language description

Program structure

Program is a list of instructions separated by semicolons.

Statements

Statements: empty, composed, if, while, return, break, continue (C/Java-like), declaration, assignment, function definition, post-increment, post-decrement, wrirting to stdout (print).

Types

int, bool, void, string, function type: <([arg_type]) : return_type>

e.g. <(int, string) : bool> is the type of function with 2 arguments (of types int and string) and returning value of type bool

Default types' values when declaring without initializing:

• int => 0
• bool => false
• string => ""

Declaration without initialization of function type variable is forbidden.

Expressions

Mostly C/Java-like, additionally lamda expressions (anonymous functions) with semantics: ([arg_type_and_name]) : return_type -> {[statements]}

e.g.

int x = 5;
< (int) : int > add2  = (int y) : int -> { return y + 2; };

print(add2(x)); // 7

Strings

Standard, can be concatenated by +.

Predefined functions

print function writing expression of a basic type (int, bool, string) to stdout (not void or anonymous functions).

Function parameters

Function parameters can be passed by value or by reference.

By value:

int x = 5;
int mult3 (int y) {
    y = y * 3;
    return y;
}

print(mult3(x)); // 15
print(x); // 5

By reference (in function definition, between the type and the name of the argument there should be an &, like in C++):

int x = 5;
int mult3 (int &y) { // the only difference with example above is '&'
    y = y * 3;
    return y;
}

print(mult3(x)); // 15
print(x); // 15

print(mult3(5)); // ERROR - argument has to be a variable, not a basic type

Variable shadowing

int x = 5;

int f(int y) {
    return x * y;
}

int g(int z) {
    int x = 1;
    return f(x+z);
}

print(g(1)); // g(1) = f(1+1) = 5 * 2 = 10

Function nesting

int x = 5;
int f(int y) {
    int add2(int z) {
        return z + 2;
    }
    return x * add2(y);
}

print(f(1)); // f(1) = 5 * (1 + 2) = 15; 

Closures

<(int) : int> multByPlus1Function(int multiplier) {
    int m = multiplier + 1;
    return (int x) : int -> {
        return x * m;
    };
}

<(int) : int> multBy5 = multByPlus1Function(4);
print(multBy5(7)); // 35

Functions as parameters

bool big(int x) {
    if (x > 50) return true;
    else return false;
}

bool checkCondition(<(int):bool> cond, int x) {
    return cond(x);
}

print(checkCondition(big, 100)); // true
print(checkCondition((int x) : bool -> { return x == 5; }, 10 )); // false

Runtime exceptions

• Division by 0

Static typing

Interpreter perfoms a static type check before program execution.