I have been dealing a lot with Lua in the past few months, and I really like most of the features but I'm still missing something among those:
- Why is there no
continue? - What workarounds are there for it?
164
votes
11 Answers
91
votes
72
votes
The way that the language manages lexical scope creates issues with including both goto and continue. For example,
local a=0
repeat
if f() then
a=1 --change outer a
end
local a=f() -- inner a
until a==0 -- test inner a
The declaration of local a inside the loop body masks the outer variable named a, and the scope of that local extends across the condition of the until statement so the condition is testing the innermost a.
If continue existed, it would have to be restricted semantically to be only valid after all of the variables used in the condition have come into scope. This is a difficult condition to document to the user and enforce in the compiler. Various proposals around this issue have been discussed, including the simple answer of disallowing continue with the repeat ... until style of loop. So far, none have had a sufficiently compelling use case to get them included in the language.
The work around is generally to invert the condition that would cause a continue to be executed, and collect the rest of the loop body under that condition. So, the following loop
-- not valid Lua 5.1 (or 5.2)
for k,v in pairs(t) do
if isstring(k) then continue end
-- do something to t[k] when k is not a string
end
could be written
-- valid Lua 5.1 (or 5.2)
for k,v in pairs(t) do
if not isstring(k) then
-- do something to t[k] when k is not a string
end
end
It is clear enough, and usually not a burden unless you have a series of elaborate culls that control the loop operation.
57
votes
You can wrap loop body in additional repeat until true and then use do break end inside for effect of continue. Naturally, you'll need to set up additional flags if you also intend to really break out of loop as well.
This will loop 5 times, printing 1, 2, and 3 each time.
for idx = 1, 5 do
repeat
print(1)
print(2)
print(3)
do break end -- goes to next iteration of for
print(4)
print(5)
until true
end
This construction even translates to literal one opcode JMP in Lua bytecode!
$ luac -l continue.lua
main <continue.lua:0,0> (22 instructions, 88 bytes at 0x23c9530)
0+ params, 6 slots, 0 upvalues, 4 locals, 6 constants, 0 functions
1 [1] LOADK 0 -1 ; 1
2 [1] LOADK 1 -2 ; 3
3 [1] LOADK 2 -1 ; 1
4 [1] FORPREP 0 16 ; to 21
5 [3] GETGLOBAL 4 -3 ; print
6 [3] LOADK 5 -1 ; 1
7 [3] CALL 4 2 1
8 [4] GETGLOBAL 4 -3 ; print
9 [4] LOADK 5 -4 ; 2
10 [4] CALL 4 2 1
11 [5] GETGLOBAL 4 -3 ; print
12 [5] LOADK 5 -2 ; 3
13 [5] CALL 4 2 1
14 [6] JMP 6 ; to 21 -- Here it is! If you remove do break end from code, result will only differ by this single line.
15 [7] GETGLOBAL 4 -3 ; print
16 [7] LOADK 5 -5 ; 4
17 [7] CALL 4 2 1
18 [8] GETGLOBAL 4 -3 ; print
19 [8] LOADK 5 -6 ; 5
20 [8] CALL 4 2 1
21 [1] FORLOOP 0 -17 ; to 5
22 [10] RETURN 0 1
18
votes
Straight from the designer of Lua himself:
Our main concern with "continue" is that there are several other control structures that (in our view) are more or less as important as "continue" and may even replace it. (E.g., break with labels [as in Java] or even a more generic goto.) "continue" does not seem more special than other control-structure mechanisms, except that it is present in more languages. (Perl actually has two "continue" statements, "next" and "redo". Both are useful.)
17
votes
The first part is answered in the FAQ as slain pointed out.
As for a workaround, you can wrap the body of the loop in a function and return early from that, e.g.
-- Print the odd numbers from 1 to 99
for a = 1, 99 do
(function()
if a % 2 == 0 then
return
end
print(a)
end)()
end
Or if you want both break and continue functionality, have the local function perform the test, e.g.
local a = 1
while (function()
if a > 99 then
return false; -- break
end
if a % 2 == 0 then
return true; -- continue
end
print(a)
return true; -- continue
end)() do
a = a + 1
end
12
votes
I've never used Lua before, but I Googled it and came up with this:
Check question 1.26.
This is a common complaint. The Lua authors felt that continue was only one of a number of possible new control flow mechanisms (the fact that it cannot work with the scope rules of repeat/until was a secondary factor.)
In Lua 5.2, there is a goto statement which can be easily used to do the same job.
9
votes
Lua is lightweight scripting language which want to smaller as possible. For example, many unary operation such as pre/post increment is not available
Instead of continue, you can use goto like
arr = {1,2,3,45,6,7,8}
for key,val in ipairs(arr) do
if val > 6 then
goto skip_to_next
end
# perform some calculation
::skip_to_next::
end
7
votes
6
votes
We encountered this scenario many times and we simply use a flag to simulate continue. We try to avoid the use of goto statements as well.
Example: The code intends to print the statements from i=1 to i=10 except i=3. In addition it also prints "loop start", loop end", "if start", and "if end" to simulate other nested statements that exist in your code.
size = 10
for i=1, size do
print("loop start")
if whatever then
print("if start")
if (i == 3) then
print("i is 3")
--continue
end
print(j)
print("if end")
end
print("loop end")
end
is achieved by enclosing all remaining statements until the end scope of the loop with a test flag.
size = 10
for i=1, size do
print("loop start")
local continue = false; -- initialize flag at the start of the loop
if whatever then
print("if start")
if (i == 3) then
print("i is 3")
continue = true
end
if continue==false then -- test flag
print(j)
print("if end")
end
end
if (continue==false) then -- test flag
print("loop end")
end
end
I'm not saying that this is the best approach but it works perfectly to us.
4
votes
-9
votes
Why is there no continue?
Because it's unnecessary¹. There's very few situations where a dev would need it.
A) When you have a very simple loop, say a 1- or 2-liner, then you can just turn the loop condition around and it's still plenty readable.
B) When you're writing simple procedural code (aka. how we wrote code in the last century), you should also be applying structured programming (aka. how we wrote better code in the last century)
C) If you're writing object-oriented code, your loop body should consist of no more than one or two method calls unless it can be expressed in a one- or two-liner (in which case, see A)
D) If you're writing functional code, just return a plain tail-call for the next iteration.
The only case when you'd want to use a continue keyword is if you want to code Lua like it's python, which it just isn't.²
What workarounds are there for it?
Unless A) applies, in which case there's no need for any workarounds, you should be doing Structured, Object-Oriented or Functional programming. Those are the paradigms that Lua was built for, so you'd be fighting against the language if you go out of your way to avoid their patterns.³
Some clarification:
¹ Lua is a very minimalistic language. It tries to have as few features as it can get away with, and a continue statement isn't an essential feature in that sense.
I think this philosophy of minimalism is captured well by Roberto Ierusalimschy in this 2019 interview:
add that and that and that, put that out, and in the end we understand the final conclusion will not satisfy most people and we will not put all the options everybody wants, so we don’t put anything. In the end, strict mode is a reasonable compromise.
² There seems to be a large number of programmers coming to Lua from other languages because whatever program they're trying to script for happens to use it, and many of them want don't seem to want to write anything other than their language of choice, which leads to many questions like "Why doesn't Lua have X feature?"
Matz described a similar situation with Ruby in a recent interview:
The most popular question is: "I’m from the language X community; can’t you introduce a feature from the language X to Ruby?", or something like that. And my usual answer to these requests is… "no, I wouldn’t do that", because we have different language design and different language development policies.
³ There's a few ways to hack your way around this; some users have suggested using goto, which is a good enough aproximation in most cases, but gets very ugly very quickly and breaks completely with nested loops. Using gotos also puts you in danger of having a copy of SICP thrown at you whenever you show your code to anybody else.
gotostatement which can be used to implement continue. See the answers below. – lhf