Re: local "module" pattern without module [was Re: [ANN] SLAXML - pure Lua, robust-ish, SAX-like streaming XML processor]

On Feb 18, 2013, at 09:59 AM, Justin Cormack <justin@specialbusservice.com> wrote:

all requires should be assigning to a local variable.

[...]

local scxml = {}

scxml.state = require "lib/state"

scxml.event = require "lib/event"

return scxml

This works for the simplest cases I supplied where each file adds exactly one completely independent table to the master. However, I have three more cases that aren't quite as clean:

* Multiple Datatypes per File

* Multiple Files Augmenting the Same Table

* One File Referencing Tables from Another

1) Multiple Datatypes per File

One of the file declares a few (currently-global) new datatypes that are used by various methods throughout. Would you recommend:

# lxsc.lua

local LXSC = {}

...require others...

for name,t in pairs(require('lib/datatypes') do LXSC[name]=t end

return LXSC

# lib/datatypes.lua

local Queue = {}; ...

local OrderedSet = {}; ...

local List = {}; ...

return {Queue=Queue, OrderedSet=OrderedSet, List=List}

This is easily worked around by making one file per 'object', but that feels like an ugly burden. Further, while this makes these classes available on the master LXSC table, there is no way for the sub-tables to access this master. Which brings me more explicitly to problem #2:

2) Multiple Files Augmenting the Same Table

Some of the files do not add a new table, but augment another existing table. For example:

# lxsc.lua

LXSC = {}

require 'lib/scxml'

require 'lib/runtime'

# lib/scxml.lua

LXSC.SCXML = {}

# lib/runtime.lua

function LXSC.SCXML:go() ... end -- modify a table defined elsewhere

If there was only one case like this, I could 'invert' the hierarchy like so:

# lxsc.lua

local LXSC = {}

LXSC.SCXML = require 'lib/runtime'

return LXSC

# lib/runtime.lua

local SCXML = require 'lib/scxml'

function SCXML:go() ... end -- modify a table defined elsewhere

return SCXML

# lib/scxml.lua

local SCXML = {}

return SCXML

...but that technique does not work when multiple files augment the same datatype. Is there a pattern that allows multiple files to work on the same common table without polluting the global namespace?

3) One File Referencing Tables from Another

Various files and methods need to talk to other high-level objects in the system. For example:

# lib/parse.lua

function LXSC:parse(...) -- augments the master table

local name = getSomeNameString()

local object = LXSC[name]() -- dynamically picks methods from the master table

end

# lib/transition.lua

function Transition:addTarget(...)

self.targets = LXSC.List() -- accesses a datatype from the namespace

end

How would I cause the master table to be exposed to each child file that needs to access it?

Is setfenv() the proper way to go about all this? Is there something better?

On Mon, Feb 18, 2013 at 2:43 PM, Gavin Kistner <phrogz@me.com> wrote:

On Feb 17, 2013, at 10:15 PM, Miles Bader <miles@gnu.org> wrote:

> Gavin Kistner <phrogz@me.com> writes:
>> * Adds only a single `SLAXML` key to the environment; there is no spam
>> of utility functions polluting the global namespace.
>
> It should not add _any_ keys to the global environment.
>
>> ## Usage
>> require 'slaxml'
>
> local slaxml = require 'slaxml'

Thank you for the suggestion. I've updated the library to use this pattern.

This works fine for this project where I only have two files. How would others suggest enforcing the same pattern for a different project that has many files all augmenting the same table?

For example, in LXSC[1] I currently have 10+ files like so:

lxsc.lua
lib/event.lua
lib/scxml.lua
lib/state.lua
lib/...etc...

and I use this to build the common object like so:

# lxsc.lua
LXSC = { VERSION="0.3.1" }
require 'lib/state'
require 'lib/scxml'
require 'lib/event'
...etc.

# lib/event.lua
LXSC.Event = { ... }

# lib/scxml.lua
LXSC.SCXML = { ... }

# lib/state.lua
LXSC.State = { ... }

How might I modify this small multiple of files to work together in a way that doesn't spam the global namespace?

I can't do something like:

# lxsc.lua
local LXSC = { VERSION="0.3.1" }
require 'lib/state'
return LXSC

# lib/state.lua
local LXSC = require 'lxsc'
LXSC.State = { ... }

…because that causes a loop in the loader. Should I instead do this?

LXSC = { VERSION="0.3.1" } # Spam the global for now
require 'lib/state'
require 'lib/scxml'
require 'lib/event'
local LXSC = LXSC
_G.LXSC = nil # remove the global spam
return LXSC

Your experienced advice is requested :)


[1] https://github.com/Phrogz/LXSC