Re: modules, require, magic

[Philipp Janda <siffiejoe@...>]

On 19.10.2011 21:36, Petite Abeille wrote:
> On Oct 19, 2011, at 9:08 PM, Philipp Janda wrote:
>
> > What happens if the name passed to require and the name passed to module differ?
>
> Then hopefully you know what you are doing... or you have a typo...
>
> > Wouldn't require just return true
>
> In case of name mismatch, require raises an exception...
>
> E.g.:
>
> local foo = require( 'baz' )
>
>   module 'baz' not found

require raises an exception if "require-name" and "file-name" don't 
match, but it cannot handle a mismatch between "require-name" and 
"module-name", because it doesn't know "module-name".

E.g.:
$ mkdir extern
$ cat > extern/mod.lua
local print = print
_ENV = module( "mod" )

print( (...), _NAME )

function echo( ... )
  print( ... )
end
^D

$ cat > main.lua
function module( aName, ... )
  local aModule = package.loaded[ aName ]
  if type( aModule ) ~= 'table' then
    aModule = {}
    aModule._M = aModule
    aModule._NAME = aName
    aModule._PACKAGE = aName:sub( 1, aName:len() - ( 
aName:reverse():find( '.', 1, true ) or 0 ) )
    package.loaded[ aName ] = aModule
    for anIndex = 1, select( '#', ... ) do
      select( anIndex, ... )( aModule )
    end
  end
  return aModule
end

local mod = require( "extern.mod" )
mod.echo( "hello world" )
^D

$ lua5.2 main.lua
extern.mod	mod
lua5.2: main.lua:17: attempt to index local 'mod' (a boolean value)
stack traceback:
	main.lua:17: in main chunk
	[C]: in ?


> > and the actual module ends up somewhere hidden in package.loaded?
>
> Modules are named. They will end up under their assigned name.

Yes, they are named by require and put in package.loaded under 
"require-name" and they are named again by module and if "module-name" 
is different put in package.loaded again under "module-name".


> > It seems to me that the name parameter to module might be problematic if you don't set globals.
>
> How so? This is not meant to prevent people from doing silly mistakes. Just to name their modules.

No, it enables people to make silly mistakes (s.a). It's a bad way to 
just name a module, because it forces users to edit the name if they 
want to move your module to a subdirectory -- why not use a comment instead.

> > Maybe you shouldn't unconditionally replace all non-table values from package.loaded in case some other module placed a function or a string there (which could happen if "require-name" and "module-name" differed).
>
> Ah, yes... there is a sort of marker put in package.loaded by require to sort out circular dependencies during loading... I don't recall the details, but module circa 5.1 has all this sorted out... e.g.:
>
> http://www.lua.org/source/5.1/loadlib.c.html#ll_module
> http://www.lua.org/source/5.1/loadlib.c.html#ll_require

Not what I meant, but good point as well. What I meant is that *I* put 
functions in package.loaded, and I would hate it if some module 
overwrites my modules by doing module( "samename" ).

> > The proposed module-function doesn't mess with globals anymore, but it does mess with package.loaded
>
> Well... "mess" might not be the right term... it simply sets it... just as module 5.1 does... module&  require work both in terms of package.loaded in tandem...

You are right, module 5.1 would also overwrite non-table modules. I 
didn't know that. What it should do is: create new table if 
package.loaded[ x ] is nil, reuse the table if package.loaded[ x ] is a 
table and raise an error otherwise.

> > and therefore should make sure that it does not disrupt other legitimate uses of package.loaded.
>
> Sure.

Good :-)


Philipp