luau/rfcs/syntax-goto-statement.md

# Feature name

Goto statement

## Summary

goto statement for jumps and making scripting a bit easier.

## Motivation

### Problems

#### Breaking in nested loops
Let's say for example:
```lua
while true do
    for i=1, 100 do
        if (i == 10) then
            break -- This will only break the current for loop
        end
    end
end
```

In Luau there isn't a easy/simple way to break nested loops(without a variable)<br>

Current solution we have:
```lua
local RunLoop = true
while RunLoop do
    for i=1, 100 do
        if (i == 10) then
            RunLoop = false
            break
        end
    end
end
```
The problems that this solution has are:
Messy: In a bigger scale this will get messy and confusing.<br>
Uneeded checks: Why to do those checks when we could just jump out directly?




### Possible solution

A goto statement would make Luau more easy to script and cleaner, if used properly.<br>
```lua
while true do
    for i=1, 100 do
        if (i == 10) then
            goto LoopEnd         
        end
    end
end
LoopEnd: -- Could follow Lua syntax(:LABEL:)
```


## Design



`<ConstantString>:` Sets a label<br>
Notes:<br>
A label can't be used in another scope
```lua
function A()
    FunctionStart:
        
    do
        if Y == Y then
            goto FunctionStart -- However, this would work
        end
    end

    if X == X then
        goto FunctionStart -- This would act the same as a while loop.
    end
end

function B()
    goto FunctionStart -- Undefined label compile error
end
```

`goto <ConstantString>` Jumps to a set label


`goto <Number>` Jumps to X line
Notes:<br>
If the number is a constant number, the compiler should optmize it<br>
Otherwise if the number is a variable, any registers needed should be saved before the jump<br>
If the X line is in another scope a error should be thrown<br>


## Drawbacks

Cross thread jumping could be a problem(but it would be very interesting if implemented and used correctly).<br>
Lambdas/Functions that are called could bypass the scope boundary without runtime checks.<br>


## Alternatives

A break for nested loops
```lua
while true do
    for i=1, 100 do
        if (i == 10) then
            :break -- The ':' means "break depth"
        end
    end
end
```