In Lua, is `repeat ... until condition` equivalent to `while true ... if condition break`? - Stack Overflow

In Lua, are these two snippets exactly equivalent?

repeat                  
    ...
until <condition>


while true do
    ...
    if <condition> then break end
end

At first glance they seem to be equivalent - and I believe they are even in corner cases, for example:

• Lua evaluates until <condition> in the scope of the loop body. (Unlike in C, where a similar pair of do/while and while loops would not be equivalent because C evaluates the condition in the outer scope.)

• Lua doesn't have continue, which would jump to until <condition> in the first loop and while true in the second.

However, are there any other corner cases where they behave differently? Or are they exactly equivalent?

In Lua, are these two snippets exactly equivalent?

repeat                  
    ...
until <condition>


while true do
    ...
    if <condition> then break end
end

At first glance they seem to be equivalent - and I believe they are even in corner cases, for example:

• Lua evaluates until <condition> in the scope of the loop body. (Unlike in C, where a similar pair of do/while and while loops would not be equivalent because C evaluates the condition in the outer scope.)

• Lua doesn't have continue, which would jump to until <condition> in the first loop and while true in the second.

However, are there any other corner cases where they behave differently? Or are they exactly equivalent?

• while-loop
• lua
• do-while

Share Improve this question Follow edited Mar 25 at 19:04 user200783 asked Mar 25 at 10:43 user200783user200783 14.4k1515 gold badges7979 silver badges142142 bronze badges 3

• 2 If you are concerned about continue then should you be concerned about goto (lua-users./wiki/GotoStatement ) as well? – Richard Chambers Commented Mar 25 at 19:14
• 1 @RichardChambers - I think any gotos and labels inside the ... would behave the same in both cases – user200783 Commented Mar 25 at 19:19
• 1 Not really an "answer", but an exploratory method might be to just plug different permutations into a bytecode visualizer (like luac.nl). The instructions are definitely not 1-1 identical. – Optimum Commented Mar 26 at 17:07

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1 Answer 1

Sorted by: Reset to default 0

The bytecode is almost identical in both cases (using Lua 5.4.7):

% cat 1
local a,b
repeat                  
    a()
until b()

% luac -l 1
main <1:0,0> (9 instructions at 0x7f842d406230)
0+ params, 3 slots, 1 upvalue, 2 locals, 0 constants, 0 functions
    1   [1] VARARGPREP  0
    2   [1] LOADNIL     0 1 ; 2 out
    3   [3] MOVE        2 0
    4   [3] CALL        2 1 1   ; 0 in 0 out
    5   [4] MOVE        2 1
    6   [4] CALL        2 1 2   ; 0 in 1 out
    7   [4] TEST        2 0
    8   [4] JMP         -6  ; to 3
    9   [4] RETURN      2 1 1   ; 0 out

% cat 2
local a,b
while true do
    a()
    if b() then break end
end

% luac -l 2
main <2:0,0> (10 instructions at 0x7fa46dc06230)
0+ params, 3 slots, 1 upvalue, 2 locals, 0 constants, 0 functions
    1   [1] VARARGPREP  0
    2   [1] LOADNIL     0 1 ; 2 out
    3   [3] MOVE        2 0
    4   [3] CALL        2 1 1   ; 0 in 0 out
    5   [4] MOVE        2 1
    6   [4] CALL        2 1 2   ; 0 in 1 out
    7   [4] TEST        2 1
    8   [4] JMP         1   ; to 10
    9   [4] JMP         -7  ; to 3
    10  [5] RETURN      2 1 1   ; 0 out