what's performance difference between local xxx = require('xxx') and require('xxx') ?

[xfwang...@gmail.com]

Hello

Why does the document  https://github.com/openresty/lua-nginx-module#lua-variable-scope  say:

Care must be taken when importing modules and this form should be used:

local xxx = require('xxx')

instead of the old deprecated form:

require('xxx')

Will the latter create a global variable eventually?

Thanks

[Kng]

I dont know it theres any perfomance difference.

The difference is instead in how you do modules.

New way: Populate a table with functions and return the table.

Old way: Call "module("mymodule", package.seeall)" which makes a global table and adds all subsequent globals to that table, It's confusing

You can read up on it here

But yeah, The latter will create a global variable.

[Aapo Talvensaari]

On Fri, Dec 9, 2016 at 8:06 AM, Kng <kngr...@gmail.com> wrote:
> Den torsdag 8 december 2016 kl. 09:45:48 UTC+1 skrev xfwang...@gmail.com:
>> Why does the document  https://github.com/openresty/lua-nginx-module#lua-variable-scope  say:
>> Care must be taken when importing modules and this form should be used:
>> local xxx = require('xxx')
>>
>> instead of the old deprecated form:
>> require('xxx')
>>
>> Will the latter create a global variable eventually?

>

> I dont know it theres any perfomance difference.
>
> The difference is instead in how you do modules.
> New way: Populate a table with functions and return the table.
> Old way: Call "module("mymodule", package.seeall)" which makes a global table and adds all subsequent globals to that table, It's confusing
> You can read up on it here
>
> But yeah, The latter will create a global variable.

Polluting global namespace is bad. And local variables do have slight performance advantage. It is matter of index lookup vs. hash lookup. Index lookup is obviously a bit faster.

[xfwang...@gmail.com]

OK, thanks, I understand local var has better performance than global var.

OK. Let me explain my question more details

If I only need call require("xxx") to initialize but I don't need get its return result, should I declare a local var to store the require result if I don't need access it latter?

[Itamar Gilad]

To avoid polluting the global namespace, I think it would be best to store it to a temporary variable that you discard ( local ignored = require("xxx") )

-Itamar

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Best regards,

Itamar Gilad

CTO & co-founder, Metapacket

[Javier Guerra Giraldez]

On 12 December 2016 at 10:41, Itamar Gilad <ita...@metapacket.com> wrote:

To avoid polluting the global namespace, I think it would be best to store it to a temporary variable that you discard ( local ignored = require("xxx") )

the one polluting the global namespace or not is the module itself, not how it's called.

IOW: the module can mess with the environment, either directly or by using the (deprecated) 'module()' function.  if it does so, assigning to a local variable won't change that.

if it doesn't store anything on the global, then the only effect would be to return something, so you have to get that somewhere, typically on a local variable.  if you don't, then it will be (almost) as if you hadn't require()'d it.

But since the original question stated that the return value wasn't needed, then it seems that the purpose of the module is to modify some part of the environment, not about the returned value; so doing the assignment or not wouldn't make any difference.

[*] "almost" because the require() function stores whatever the module returns in the 'packages.loaded' table.

--

Javier

[xfwang...@gmail.com]

ok, it makes sense, thanks.