Add native map traversing functionality

[Gusarich]

This PR introduces the for loop - currently just the map traversing version of it, but I'm also thinking about implementing general purpose for loops in future.

Syntax (may be changed, it's up for discussions):

for (k, v in m) {
    // k: key, v: value, m: map
}

What have been already done:

• grammar
• ast definitions
• statement resolving

What have to be done:

• actual implementation of map traversing loop
• tests

[Gusarich]

I've ran into a little problem when was trying to implement it. Here's an example of how to traverse a map in FunC:

(int key, slice val, int flag) = d.udict_get_min?(256);
while (flag) {
    ;; do something with pair key->val
    
    (key, val, flag) = d.udict_get_next?(256, key);
}

This loop uses 3 variables: key, value and flag that indicates whether the loop should be terminated or not (actually it's set to false when there are no more keys in map).
We can easily define key and value variables by using same names in the FunC code (id(keyName) etc), but I'm not sure about the best solution regarding the flag variable. It must not be overlapped with any other existing variable names including other flag variables in case with nested loops.

Well, we can just ban nested map traversing loops... but I don't think it's a good idea.

@anton-trunov what do you think?

[Gusarich]

«just generate random name like for_flag_RANDOM_HASH ?»

@xsr I've thought about this, but it adds some kind of indeterminacy to compiler which isn't good.

[nonam3e]

I've ran into a little problem when was trying to implement it. Here's an example of how to traverse a map in FunC:

(int key, slice val, int flag) = d.udict_get_min?(256);
while (flag) {
    ;; do something with pair key->val
    
    (key, val, flag) = d.udict_get_next?(256, key);
}

This loop uses 3 variables: key, value and flag that indicates whether the loop should be terminated or not (actually it's set to false when there are no more keys in map).
We can easily define key and value variables by using same names in the FunC code (id(keyName) etc), but I'm not sure about the best solution regarding the flag variable. It must not be overlapped with any other existing variable names including other flag variables in case with nested loops.

Well, we can just ban nested map traversing loops... but I don't think it's a good idea.

@anton-trunov what do you think?

Maybe can it be solved by checking key instead of flag (key.is_null?)?

[Gusarich]

Maybe can it be solved by checking key instead of flag (key.is_null?)?

@nonam3e Well, it’ll work, but will consume more gas (18 for each iteration).

[novusnota]

Found an interesting read on designing a for-each loop syntax: https://harelang.org/blog/2024-04-01-introducing-for-each-loops-in-hare. This won't help much with this issue, but it can be somewhat inspirational nonetheless, despite coming from a systems programming language :)

[anton-trunov]

We can easily define key and value variables by using same names in the FunC code (id(keyName) etc), but I'm not sure about the best solution regarding the flag variable. It must not be overlapped with any other existing variable names including other flag variables in case with nested loops.

@Gusarich The usual approach here is to define a fresh identifier generator. Something like this:

// fresh.ts

let counter = 0;    // a mutable variable

function freshIdentifier(prefix: string): string {
    const fresh = `__tact_freshId${prefix}${counter}`;
    counter += 1;
    return fresh;
}

and then you can use it to generate fresh flag variables with freshIdentifier("flag"). Using a prefix is not necessary, it should make debugging a bit easier.

[anton-trunov]

HUGE!