Re: Bizarre behavior with debug.setupvalue() in Lua 5.3

[Paul K <paul@...>]

Hi Sean,

> print(debug.setupvalue(meta.print,1,meta))
> name,env = debug.getupvalue(meta.print,1) -- line 16
> When I run it with Lua 5.3, I get:
> _ENV
> lua-53: /tmp/t.lua:16: attempt to index a nil value (global 'debug')

I get the same error in Lua 5.3 and Lua 5.2. Aren't you setting the
first upvalue for meta.print (which is _ENV) to "meta", which replaces
_ENV for your current code with "meta", which makes "debug" not
available?

If you make "meta.print" an empty function, you won't get the error
(since it won't have _ENV as its first upvalue). You need to localize
debug.getupvalue if you want to access it after resetting _ENV.

Paul.

On Fri, Mar 27, 2015 at 2:05 PM, Sean Conner <sean@conman.org> wrote:
>
> The following script fails:
>
>         meta =
>         {
>           io       = io,
>           tostring = tostring,
>           select   = select,
>           print    = function(...)
>             io.stdout:write("HERE I AM, JH ... ")
>             for i = 1 , select('#',...) do
>               io.stdout:write(tostring(select(i,...)),"\t")
>             end
>             io.stdout:write("\n")
>           end
>         }
>
>         print(debug.setupvalue(meta.print,1,meta))
>         name,env = debug.getupvalue(meta.print,1) -- line 16
>
> When I run it with Lua 5.3, I get:
>
> _ENV
> lua-53: /tmp/t.lua:16: attempt to index a nil value (global 'debug')
> stack traceback:
>         /tmp/t.lua:16: in main chunk
>         [C]: in ?
>
>   My intent is to have meta.print() use meta as its environment.  I can work
> around it with:
>
>         PRINT = [[
>                 io.stdout:write("HERE I AM, JH ... ")
>                 for i = 1 , select('#',...) do
>                   io.stdout:write(tostring(select(i,...)),"\t")
>                 end
>                 io.stdout:write("\n")
>         ]]
>
>         meta =
>         {
>           io       = io,
>           tostring = tostring,
>           select   = select,
>         }
>
>         meta.print = load(PRINT,"code","t",meta)
>         name,env = debug.getupvalue(meta.print,1)
>         print(name,env,meta)
>         meta.print("three","two",1,true)
>
> and when run:
>
> _ENV    table: 0x8159068        table: 0x8159068
> HERE I AM, JH ... three two     1       true
>
>   I'm curious---what am I doing wrong in the first case?
>
>   -spc (And for the curious---this is a testcase for some code I'm playing
>         around with ... )
>