Re: Abolishing the numeric-for
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- Subject: Re: Abolishing the numeric-for
- From: Fabio Mascarenhas <mascarenhas@...>
- Date: 2010年5月25日 15:20:39 -0300
On Tue, May 25, 2010 at 2:58 PM, Roberto Ierusalimschy
<roberto@inf.puc-rio.br> wrote:
>> Yes, that is a problem with this proposal, it adds the overhead of an
>> extra type check to every numeric for (at least it is a one-time cost,
>> not per-iteration).
>
> I cannot see how to avoid a type check at each iteration, as each
> iteration needs completely different behavior in each case.
Sorry if the description was not clear, what I meant is that a statement like:
for var_1, ···, var_n in explist do
<block>
end
would be equivalent to the code:
do
local e1, e2, e3 = explist
if type(e1) == "number" then -- numeric for
local var, limit, step = e1, tonumber(e2), tonumber(e3) or 1
if not limit then error() end
while (step > 0 and var <= limit) or (step <= 0 and var >= limit) do
local v = var
<block>
var = var + step
end
else -- __iter or regular generic for
local f, s, var = e1, e2, e3
if type(f) ~= "function" then -- __iter metamethod
f, s, var = metatable(f).__iter(f)
end
while true do -- generic for
local var_1, ···, var_n = f(s, var)
var = var_1
if var == nil then break end
<block>
end
end
end
end
The values of the explist when you enter the loop decide what "kind"
of loop the for is, you can't switch from generic to numeric in the
middle of the loop.
> -- Roberto
>
--
Fabio Mascarenhas