Berwin A Turlach wrote:
>
>> foo = function(arg) arg$foo = foo
>>
>> e = new.env()
>> foo(e)
>> e$foo
>>
>> are you sure this is pass by value?
>>
>
> But that is what environments are for, aren't they?
might be.
> And it is
> documented behaviour.
sure!
> Read section 2.1.10 ("Environments") in the R
> Language Definition,
haven't objected to that. i object to your 'r uses pass by value',
which is only partially correct.
> in particular the last paragraph:
>
> Unlike most other R objects, environments are not copied when
> passed to functions or used in assignments. Thus, if you assign the
> same environment to several symbols and change one, the others will
> change too. In particular, assigning attributes to an environment can
> lead to surprises.
>
> [..]
>
>> and actually, in the example we discuss, 'names<-' does *not* return
>> an updated *tmp*, so there's even less to entertain.
>>
>
> How do you know? Are you sure? Have you by now studied what goes on
> under the hood?
>
yes, a bit. but in this example, it's enough to look into *tmp* to see
that it hasn't got the names added, and since x does have names, names<-
must have returned a copy of *tmp* rather than *tmp* changed:
x = 1
tmp = x
x = 'names<-'(tmp, 'foo')
names(tmp)
# NULL
you suggested that "One reads the manual, (...) one reflects and
investigates, ..." -- had you done it, you wouldn't have asked the question.
>
>> for fun and more guesswork, the example could have been:
>>
>> x = x
>> x = 'names<-'(x, value=c('a', 'b'))
>>
>
> But it is manifestly not written that way in the manual; and for good
> reasons since 'names<-' might have side effects which invokes in the
> last line undefined behaviour. Just as in the equivalent C snippet
> that I mentioned.
>
i just can't get it why the manual does not manifestly explain what
'names<-' does, and leaves you doing the guesswork you suggest.
vQ
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