Global variables

A FunC program is a list of function declarations, function definitions, and global variable declarations. This section focuses on global variables.

A global variable is declared using the global keyword, followed by the variable's type and name. For example:

global ((int, int) -> int) op;

Here's a simple program demonstrating how to use a global functional variable:

int check_assoc(int a, int b, int c) {
  return op(op(a, b), c) == op(a, op(b, c));
}

int main() {
  op = _+_;
  return check_assoc(2, 3, 9);
}

In this example, the global variable op is assigned the addition operator _+_. The program then verifies the associativity of addition using three sample integers: 2, 3, and 9.

Under the hood, global variables in FunC are stored in the c7 control register of the TVM, with a maximum limit of 31 variables.

In FunC, you can omit the type of a global variable. In this case, the compiler determines the type based on how the variable is used. For example, you can rewrite the previous program like this:

global op;

int check_assoc(int a, int b, int c) {
  return op(op(a, b), c) == op(a, op(b, c));
}

int main() {
  op = _+_;
  return check_assoc(2, 3, 9);
}

Declaring multiple global variables

FunC allows you to declare multiple global variables using a single global keyword. The following examples are equivalent:

global int A;
global cell B;
global C;

global int A, cell B, C;

Restrictions on global and local variable names

A local variable cannot have the same name as a previously declared global variable. The following example is invalid and will not compile:

global cell C;

int main() {
  int C = 3; ;; Error: cannot declare a local variable with the same name as a global variable
  return C;
}

However, the following example is valid:

global int C;
int main() {
  int C = 3;
  return C;
}

In this case, int C = 3; is not declaring a new local variable but instead assigning value 3 to the global variable C.

This behavior is explained in more detail in the section on statements.

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