Would my DSL for Lua work...? (this seems too simply to be true)

Question

I really love Lua as a programming language BUT, it bugs me unbelievably to have to constantly type "local" for all my local variables.

It just makes my code look more cluttered.

So I am wondering, can I create a Domain Specific Language (DSL) on top of Lua to simply have the following variable naming convention.

1. If a variable name is in ALL CAPITAL LETTERS, then it's a global variable
2. Else, the variable is a local variable

Question: Would this work - Yes or no?

In other words:

-- In Lua 5.2
isGlobalinLua  = "is global in default Lua"
GLOBALVAR      = "is global var in default Lua"
local localvar = "is local var in default Lua"

-- In my DSL Lua language
isLocalinDSLLua = "is local in DSL Lua"  -- translates to: local isLocalinDSLLua = ...
GLOBALVAR       = "is global DSL Lua"
localvar        = "is local var in DSL Lua"  -- translates to: local localvar = ...

So now, the following code in default Lua:

myglobal = 10
local a = 1
if a > 1 then
    local b = 2
    print b
else
    local c = 3
    print c + myglobal
end

With my DSL Lua:

MYGLOBAL = 10
a = 1
if a > 1 then
    b = 2
    print b
else
    c = 3
    print c + MYGLOBAL
end

UPDATE:

What about local functions?

How would the following code work?

myfunc = function (...)   -- local myfunc = function (...)

I'm not certain I'd want to make every global function in all caps.

Maybe I just ignore functions and require the 'local' identifier ... thoughts?

Octonary

Interesting question. +1 I am interested in seeing if this is correct or what the correct way of doing this would be. – Durware

Of course you "can" do this; write a preprocessor that reads your DSL and outputs Lua source. This is not a trivial task, nor is it obvious to me how worthwhile it would be. I personally see little point in DSLs that work at the same level of abstraction as the implementation language; they do not save time nor prevent errors. The "domain" in a "domain-specific" language usually refers to the end-user domain; such a game player, accountant, businessman, et al. – Tackett

So, after a year, what came out of this idea? – Goeger

@lhf, I didn't create this but it's given me inspiration. moonscript.org – Octonary

Answer 1

Moonscript already has all variables local by default and you only need to use export keyword to declare a global. It's a very nice next-generation, coffeescript-like language and it compiles to Lua. I use it wherever I used to use Lua.

foo = 'bar' -- local
square (x) -> x*x -- local

-- globals
export square
export MY_GLOBAL = 12

export class Foo
    new: (bar) => @bar = bar -- self.bar = bar
    get_bar: => @bar

Pitching

Unfortunately, I'm not a coffeescript fan. Do you know how I could pull out the portion of Moonscript that I want to use without using the rest? – Octonary

Answer 2

Since the change you want is relatively simple, you basically have two options. You can try out Metalua:

Metalua is a language and a compiler which provide ...

• A complete macro system, similar in power to what's offfered by Lisp dialects or Template Haskell; manipulated programs can be seen as source code, as abstract syntax trees, or as an arbitrary mix thereof, whichever suits your task better.
• A dynamically extensible parser, which lets you support your macros with a syntax that blends nicely with the rest of the language.

Or you can use token filters:

The token filter works by giving you the opportunity to inspect and alter the stream of tokens coming from the lexer before they go to the parser. You get to see only tokens and you are only allowed to generate tokens -- you're not allowed to see the text coming to the lexer nor to generate text to go into the lexer.

Both approaches have their pros and cons. Metalua allows you to do high-level language modifications in Lua, but has a steeper learning curve. Token filters allow you to do simple modifications to the token stream, but have a limited power (see the 2005 talk).

I'm not sure if token filters are enough for your case, because inserting local before every lowercase-starting identifier in assignment will only work for simple cases. What about the following code?

a = 1
a = a * a

Do you want to convert it to a single local, or you want two?

local a = 1     vs.    local a = 1
a * a                  local a = a * a  -- valid in Lua, creates a new variable

Mosque

Answer 3

Yes, you can do this, but the result is not called a "domain-specific language". It is called a "preprocessor".

I, too, was once incredibly bugged by a problem with Lua—it lacks a switch or case statement. I actually wound up modifying the implementation to support this feature. But eventually I decided I didn't care to be programming in a dialect that only I used.

If you have C programming skills, the Lua implementation is very clean, and the easiest way for you to achieve your goal is probably just to modify the implementation.

Grattan