Re: Syntax sugar for default arguments

[Dirk Laurie <dirk.laurie@...>]

2013/4/21 Steve Litt <slitt@troubleshooters.com>:

> By the way, are you going to let the other shoe drop and do the same
> thing in the subroutine *call*?
>
> function f(x=3, y=4)
>    whatever()
> end
>
> f(y=7)   -- no need to articulate x

This is immeasurably harder. The other one is mere syntax sugar, a trivial
modification to the source string before passing it along.

This requires code to examine run-time debugging information. At compile
time it is not known which function object is currently associated with the
name f. Therefore it is not known which parameter of f was called y when
the function was defined. One will have to generate code that repeatedly
calls debug.getlocal and works through all the local variables one by one.

All this, for a simple function call.

What will be easier is default values for entries in keyword-style
table arguments:

function f{x=3,y=4}
  whatever()
end

This will translate to the equivalent of:

function f(param)
  for k,v in pairs(default[f]) do f[k]=f[k] or v end
  whatever()
end
default[f]={x=3,y=4}



default_parameters[f]