Re: continuing continue - was Re: [patch] continue statement
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- Subject: Re: continuing continue - was Re: [patch] continue statement
- From: Rici Lake <lua@...>
- Date: 2005年9月23日 23:17:30 -0500
On 23-Sep-05, at 11:27 AM, jrs@tbrve.net wrote:
Ripke <paul-lua@malete.org> wrote:
> ...
too sad you are losing the semantics of break -- bad deal.
likewise with
i=0
while (function () -- clearly you'd rather create function only once
print(i) -- but it would be less readable
if i<3 then return true end -- continue
-- break
end)() do
i=i+1
end
now you have break and continue plus a "continue block",
but sacrificy return.
I've actually never really noticed the lack of continue in Lua, since
you can almost always achieve the effect simply by inverting the if
statement:
for x in iterator() do
if condition(x) then continue end
...
end
-->
for x in iterator() do if not condition(x) then
...
end end
Or:
for x in iterator() do
if condition1(x) then continue end
if condition2(x) then continue end
...
end
-->
for x in iterator() do
if not condition1(x)
and not condition2(x) then
...
end
end
Or:
for x in iterator() do
if condition1(x) then continue end
local y = f(x)
if condition2(y) then continue end
...
end
-->
for x in iterator() do
if not condition1(x) then
local y = f(x)
if not condition2(y) then
...
end
end
end
Once in a blue moon, this is a bit awkward:
for x in iterator() do
if condition1(x) then
if condition2(x) then
continue
else
break
end
end
...
end
This sort of thing can be dealt with by using a tailcall, although that
means expanding the for statement:
local function loop(nexter, obj, state)
local x = nexter(obj, state)
if condition1(x) then
if condition2(x) then
return loop(nexter, obj, x) -- continue
else
return -- break
end
end
...
return loop(nexter, obj, x) -- explicit continue
end
loop(iterator())
If you really really need to be able to 'return' as well, you can wrap
the whole thing in a coroutine. I can't say I've ever encountered this
situation, and there are probably better ways to handle it. However:
function foo()
...
for x in iterator() do
if condition1(x) then
if condition2(x) then
continue
else
break
end
end
return x
end
-- fall through in certain cases :)
print "Failed to find anything"
end
-->
function foo()
local try = coroutine.wrap(function()
...
local function loop(nexter, obj, state)
local x = nexter(obj, state)
if condition1(x) then
if condition2(x) then
-- continue
return loop(nexter, obj, x)
else
-- break
return
end
end
-- return
coroutine.yield(x)
end
loop(iterator())
-- "fall through"
print "Failed to find anything"
end)
return try()
end
I'd really love to see a non-contrived use case for a loop like that. :)
R.