Monday, November 30, 2015

Common config that can be utilized by multiple languages

Often, I would want to separate out some config/initial data out of a program, and keep it in a way that is accessible to different parts of the system. If all parts of the system are in the same language, then the config can be very simple - just the function invocation with values:

For e.g. in shell:

   # Format:
   # Employee name role department
   Employee "Bheem"  "Fighter"  "Security"
   Employee "Chutki" "Hiring"   "HR"

Then, the invoker would just have to define the function Employee and source this file!

So far, so good if all parts were in shell. But, what if I want to use this config in some C code, and in Lua scripts etc. ?

(Don't think file-read/parse/split/tokenize .. No no! yuck!)

Can we define (somehow), a common format which is usable by different environments (languages), with minimal or no effort!? Observe the calling conventions:

C function call looks like:

   Employee (arg1, arg2 /* ... */)

Shell (is the simplest):

     Employee "arg1" "arg2" # ...


   Employee (arg1, arg2) -- ...

But, there is a special notation in Lua, when the argument is a string or a hash table - we can get rid of the parentheses! like:

   Employee "arg1"

OK, but what if we have more than 1 argument ? Now, we need to get a bit deeper into function-chaining:

    function Employee (name)
        return function (role)
            return function (department)
               add_to_db(name, role, department)

Without going into much details about anonymous functions, Lexical scoping, upvalues etc - the function returns a function, which returns yet another function - that means, when you invoke

    Employee "Bheem"  "Fighter"  "Security"

The first function reads the first value, and returns its inner function, which reads the second value and that in turn returns another function which finally does some work (and can access all the 3 values - this is what we want)! Cool, ain't it? :-)

Wait, we solved the problem for shell and Lua, what about C code ?

In case of C code, we can always use the well-documented Lua C APIs to read Lua config! :)
(after all, Lua evolved from data-description language)

If you think, all this is too much work - think again, know the DRY principle.