Re: Abolishing the numeric-for

[Fabio Mascarenhas <mascarenhas@...>]

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