Re: documentation: metatable is a reference (refman is unclear)

[Ross Berteig <ross@...>]

On 5/7/2017 3:02 AM, nobody wrote:
> (unless I fail at searching), the reference manual does not explicitly
> state that metatables are "stored" by reference.  (I.e. `setmetatable`
> doesn't copy the thing passed as metatable, but puts a reference to it
> in a suitable location.) 

The reference manual is very careful to be specific. If the table were 
copied, it would say that because that is different from how tables are 
used. It doesn't need to say that the table is not copied into internal 
storage because that is the normal case in any other use of tables. (The 
manual doesn't say that arithmetic never returns a string either. It 
doesn't, but saying so would raise the question of when it might.)

One could argue that this habit does make the reference manual harder to 
read for a language learner. But that is not the role of a reference 
manual. See Programming in Lua for one example (and likely the best 
example) of a language textbook that a learner should start with.

This lack of copying also follows from the way in which tables are 
treated as values. Since

    a = {}
    b = a
    b.one = 1
    print(a.one)     -- prints "1"

is the way tables work in all other contexts, why would this be 
particularly different:

    mt = {}
    c = setmetatable({}, mt)
    mt.one = 1
    print(getmetatable(c).one) -- prints "1"

The fact that tables are "stored by reference" is just an implementation 
detail that makes the value semantics of tables practical to implement. 
This is similar to strings being internalized, which is also just an 
implementation detail that make their value semantics practical to 
implement.

--
Ross Berteig                               Ross@CheshireEng.com
Cheshire Engineering Corp.           http://www.CheshireEng.com/
+1 626 303 1602