Re: RFE: table ctor syntax enhancement for many-to-one mappings

[Lorenzo Donati <lorenzodonatibz@...>]

Thanks for the reply

On 18/08/2011 23.40, HyperHacker wrote:
> On Thu, Aug 18, 2011 at 14:46, Lorenzo Donati
> <lorenzodonatibz@interfree.it>  wrote:
> > Hi all!

[...]

> > In other words I propose to enhance the table constructor syntax to allow
> > not just one expression inside "[]", but an expression list, so that:
> >
> > [exp1,exp2,exp3,...] = val
> >
> > would be expanded as
> >
> > [exp1] = val, [exp2] = val, [exp3] = val, ...
[...]

> >
>
> This seems not too far from the typical multiple assignments:
> a, b, c = 1, 2, 3
> I often wondered why that can't be done in tables. I'm guessing the
> answer is ambiguity: does a, b, c = 1, 2 mean [1]=a, ['b']=1, ['c']=2
> or ['a']=1, ['b']=2, [1]=c or ['a']=1, ['b']=2, ['c']=nil? This might
> be resolvable with some kind of syntax extension such as (a,b,c) =
> (1,2,3) and just requiring equal number of values on left/right sides,
> since you can't store nil in a table anyway (so a,b,c = 1,2 doesn't
> really make sense).

My proposal is far less ambitious. It would only add new semantics, not 
modify the existing one: it will only affect the ctor in the cases where 
the square brackets are used, and would allow only one value on the RHS:

so

{ a, b, c = 1, 2 }

will have the usual meaning, whereas:
(<=> means "equivalent" for short)


{ ["a", "b", "c"] = 1, 2 }  <=>
{ ["a"] = 1, ["b"] = 1, ["c"] = 1, 2 } <=>
{ a = 1, b = 1, c = 1, 2 }

and

{ [a, b, c] = 1, 2 }  <=>
{ [a] = 1, [b] = 1, [c] = 1, 2 }



[...]

> Something similar to your idea could be done with a 'rep' function: a,
> b, c = rep(3, true) --returns true, true, true. That would let you do
> similar to my snippet above without needing the local aliases, which
> might look a little nicer.

nope, it won't work with current implementation:

-- example rep
function rep( n, what )
  if n ==  0 then return end
  return what, rep( n - 1, what )
end

t = { a, b, c = rep(3, true) } --> { c = true }
-------------------------------

since:

{ a, b, c = rep(3, true) } <=>
{ a = nil, b = nil, c = true } <=> { c = true }

since in this case the return values of rep are adjusted to 1 - the 
manual says [1]:

"If the *last field* in the list has the form *exp* and the expression 
is a function call or a vararg expression, then all values returned by 
this expression enter the list consecutively (see §2.5.8). "


[1] http://www.lua.org/manual/5.1/manual.html#2.5.7


-- Lorenzo