Re: 'with' statement

[Daurnimator <quae@...>]

On 14 Mar 2016 04:25, "Dirk Laurie" <dirk.laurie@gmail.com> wrote:
>
> Thanks to everyone who replied. The present protocol is:
>
> --- with(tbl[,context])
> -- Creates a new table with write access to `tbl` and read access to
> --   `tbl` and `context`, which defaults to `_ENV`. If current _ENV
> --   is local or an upvalue, it must be explicitly given as `context`.
> --   If really no context is required, specify an explicit `nil`.
> -- Designed for the idiom
> --    do local _ENV=with(tbl)
> --      ...
> --    end
> -- which mimics the `with` keyword in Pascal, Visual Basic etc.
>
> The code is:
>
> local
> function with(tbl,...)
>   local context = ...
>   if select('#',...)==0 then context = _ENV end
>   if context then
>     return setmetatable({},{__newindex=tbl,
>       __index=function(_,key) return tbl[key] or context[key] end})
>   else
>     return tbl
>   end
> end

The optional context can't default to _ENV, as environments are lexical, and will be taken from the _ENV of where `with` is declared.