Statements
This section briefly discusses FunC statements, constituting the code of ordinary function bodies.
Expression statements
The most common type of a statement is the expression statement. It's an expression followed by ;. Expression's description would be quite complicated, so only a sketch is presented here. As a rule all sub-expressions are computed left to right with one exception of asm stack rearrangement which may define order manually.
Variable declaration
It is not possible to declare a local variable without defining its initial value.
Here are some examples of variables declarations:
int x = 2;
var x = 2;
(int, int) p = (1, 2);
(int, var) p = (1, 2);
(int, int, int) (x, y, z) = (1, 2, 3);
(int x, int y, int z) = (1, 2, 3);
var (x, y, z) = (1, 2, 3);
(int x = 1, int y = 2, int z = 3);
[int, int, int] [x, y, z] = [1, 2, 3];
[int x, int y, int z] = [1, 2, 3];
var [x, y, z] = [1, 2, 3];
Variable can be "redeclared" in the same scope. For example, this is a correct code:
int x = 2;
int y = x + 1;
int x = 3;
In fact, the second occurrence of int x is not a declaration, but just a compile-time insurance that x has type int. So the third line is essentially equivalent to a simple assignment x = 3;.
In nested scopes, a variable can be truly redeclared just like in the C language. For example, consider the code:
int x = 0;
int i = 0;
while (i < 10) {
(int, int) x = (i, i + 1);
;; here x is a variable of type (int, int)
i += 1;
}
;; here x is a (different) variable of type int
But as mentioned in the global variables section, a global variable cannot be redeclared.
Note that a variable declaration is an expression statement, so actually constructions like int x = 2 are full-fledged expressions. For example, this is a correct code:
int y = (int x = 3) + 1;
It is a declaration of two variables x and y equal to 3 and 4 correspondingly.
Underscore
Underscore _ is used when a value is not needed. For example, suppose a function foo has type int -> (int, int, int). We can get the first returned value and ignore the second and third like this:
(int fst, _, _) = foo(42);
Function application
A call of a function looks like as such in a conventional language. The arguments of the function call are listed after the function name, separated by commas.
;; suppose foo has type (int, int, int) -> int
int x = foo(1, 2, 3);
But notice that foo is actually a function of one argument of type (int, int, int). To see the difference, suppose bar is a function of type int -> (int, int, int). Unlike in conventional languages, you can compose the functions like that:
int x = foo(bar(42));
instead of the similar but longer form:
(int a, int b, int c) = bar(42);
int x = foo(a, b, c);
Also Haskell-style calls are possible, but not always (to be fixed later):
;; suppose foo has type int -> int -> int -> int
;; i.e. it's carried
(int a, int b, int c) = (1, 2, 3);
int x = foo a b c; ;; ok
;; int y = foo 1 2 3; wouldn't compile
int y = foo (1) (2) (3); ;; ok
Lambda expressions
Lambda expressions are not supported yet.
Methods calls
Non-modifying methods
If a function has at least one argument, it can be called as a non-modifying method. For example, store_uint has type (builder, int, int) -> builder (the second argument is the value to store, and the third is the bit length). begin_cell is a function that creates a new builder. The following codes are equivalent:
builder b = begin_cell();
b = store_uint(b, 239, 8);
builder b = begin_cell();
b = b.store_uint(239, 8);
So the first argument of a function can be passed to it being located before the function name, if separated by .. The code can be further simplified:
builder b = begin_cell().store_uint(239, 8);
Multiple calls of methods are also possible:
builder b = begin_cell().store_uint(239, 8)
.store_int(-1, 16)
.store_uint(0xff, 10);
Modifying methods
If the first argument of a function has type A and the return value of the function has the shape of (A, B) where B is some arbitrary type, then the function can be called as a modifying method. Modifying method calls may take some arguments and return some values, but they modify their first argument, that is, assign the first component of the returned value to the variable from the first argument. For example, suppose cs is a cell slice and load_uint has type (slice, int) -> (slice, int): it takes a cell slice and number of bits to load and returns the remainder of the slice and the loaded value. The following codes are equivalent:
(cs, int x) = load_uint(cs, 8);
(cs, int x) = cs.load_uint(8);
int x = cs~load_uint(8);
In some cases we want to use a function as a modifying method that doesn't return any value and only modifies the first argument. It can be done using unit types as follows: Suppose we want to define function inc of type int -> int, which increments an integer, and use it as a modifying method. Then we should define inc as a function of type int -> (int, ()):
(int, ()) inc(int x) {
return (x + 1, ());
}
When defined like that, it can be used as a modifying method. The following will increment x.
x~inc();
. and ~ in function names
Suppose we want to use inc as a non-modifying method too. We can write something like that:
(int y, _) = inc(x);
But it is possible to override the definition of inc as a modifying method.
int inc(int x) {
return x + 1;
}
(int, ()) ~inc(int x) {
return (x + 1, ());
}
And then call it like that:
x~inc();
int y = inc(x);
int z = x.inc();
The first call will modify x; the second and third won't.
In summary, when a function with the name foo is called as a non-modifying or modifying method (i.e. with .foo or ~foo syntax), the FunC compiler uses the definition of .foo or ~foo correspondingly if such a definition is presented, and if not, it uses the definition of foo.
Operators
Note that currently all of the unary and binary operators are integer operators. Logical operators are represented as bitwise integer operators (cf. absence of boolean type).
Unary operators
There are two unary operators:
~is bitwise not (priority 75)-is integer negation (priority 20)
They should be separated from the argument:
- xis ok.-xis not ok (it's a single identifier)
Binary operators
With priority 30 (left-associative):
*is integer multiplication/is integer division (floor)~/is integer division (round)^/is integer division (ceil)%is integer reduction by modulo (floor)~%is integer reduction by modulo (round)^%is integer reduction by modulo (ceil)/%returns the quotient and the remainder&is bitwise AND
With priority 20 (left-associative):
+is integer addition-is integer subtraction|is bitwise OR^is bitwise XOR
With priority 17 (left-associative):
<<is bitwise left shift>>is bitwise right shift~>>is bitwise right shift (round)^>>is bitwise right shift (ceil)
With priority 15 (left-associative):
==is integer equality check!=is integer inequality check<is integer comparison<=is integer comparison>is integer comparison>=is integer comparison<=>is integer comparison (returns -1, 0 or 1)
They also should be separated from the argument:
x + yis okx+yis not ok (it's a single identifier)
Conditional operator
It has the usual syntax.
<condition> ? <consequence> : <alternative>
For example:
x > 0 ? x * fac(x - 1) : 1;
It has priority 13.
Assignments
Priority 10.
Simple assignment = and counterparts of the binary operations: +=, -=, *=, /=, ~/=, ^/=, %=, ~%=, ^%=, <<=, >>=, ~>>=, ^>>=, &=, |=, ^=.
Loops
FunC supports repeat, while, and do { ... } until loops. The for loop is not supported.
Repeat loop
The syntax is a repeat keyword followed by an expression of type int. Repeats the code for the specified number of times. Examples:
int x = 1;
repeat(10) {
x *= 2;
}
;; x = 1024
int x = 1, y = 10;
repeat(y + 6) {
x *= 2;
}
;; x = 65536
int x = 1;
repeat(-1) {
x *= 2;
}
;; x = 1
If the number of times is less than -2^31 or greater than 2^31 - 1, range check exception is thrown.
While loop
Has the usual syntax. Example:
int x = 2;
while (x < 100) {
x = x * x;
}
;; x = 256
Note that the truth value of condition x < 100 is of type int (cf. absence of boolean type).
Until loop
Has the following syntax:
int x = 0;
do {
x += 3;
} until (x % 17 == 0);
;; x = 51
If statements
Examples:
;; usual if
if (flag) {
do_something();
}
;; equivalent to if (~ flag)
ifnot (flag) {
do_something();
}
;; usual if-else
if (flag) {
do_something();
}
else {
do_alternative();
}
;; Some specific features
if (flag1) {
do_something1();
} else {
do_alternative4();
}
The curly brackets are necessary. That code wouldn't be compiled:
if (flag1)
do_something();
Try-Catch statements
Available in func since v0.4.0
Executes the code in try block. If it fails, completely rolls back changes made in try block and executes catch block instead; catch receives two arguments: the exception parameter of any type (x) and the error code (n, integer).
Unlike many other languages in the FunC try-catch statement, the changes made in the try block, in particular the modification of local and global variables, all registers' changes (i.e. c4 storage register, c5 action/message register, c7 context register and others) are discarded if there is an error in the try block and consequently all contract storage updates and message sending will be reverted. It is important to note that some TVM state parameters such as codepage and gas counters will not be rolled back. This means, in particular, that all gas spent in the try block will be taken into account, and the effects of OPs that change the gas limit (accept_message and set_gas_limit) will be preserved.
Note that exception parameter can be of any type (possibly different in case of different exceptions) and thus funC can not predict it on compile time. That means that developer need to "help" compiler by casting exception parameter to some type (see Example 2 below):
Examples:
try {
do_something();
} catch (x, n) {
handle_exception();
}
forall X -> int cast_to_int(X x) asm "NOP";
...
try {
throw_arg(-1, 100);
} catch (x, n) {
x.cast_to_int();
;; x = -1, n = 100
return x + 1;
}
int x = 0;
try {
x += 1;
throw(100);
} catch (_, _) {
}
;; x = 0 (not 1)
Block statements
Block statements are also allowed. They open a new nested scope:
int x = 1;
builder b = begin_cell();
{
builder x = begin_cell().store_uint(0, 8);
b = x;
}
x += 1;