R devel - May 2011 - Using substitute to access the expression related to a promise

Hi all,

The help for delayedAssign suggests that you can use substitute to
access the expression associated with a promise, and the help for
substitute says: "If it is a promise object, i.e., a formal argument
to a function or explicitly created using ?delayedAssign()?, the
expression slot of the promise replaces the symbol.

But this doesn't seem to work:
> a <- 1
> b <- 2
> delayedAssign("x", {message("assigning..."); a + b})
> substitute(x)
x
> x
[1] 3

Is this a bug in substitute?
> sessionInfo()
R version 2.13.0 (2011-04-13)
Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)
...

Hadley

-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/

On 02/05/2011 9:53 AM, Hadley Wickham wrote:
> Hi all,
>
> The help for delayedAssign suggests that you can use substitute to
> access the expression associated with a promise, and the help for
> substitute says: "If it is a promise object, i.e., a formal argument
> to a function or explicitly created using ?delayedAssign()?, the
> expression slot of the promise replaces the symbol.
>
> But this doesn't seem to work:
>
> >  a<- 1
> >  b<- 2
> >  delayedAssign("x", {message("assigning..."); a +
b})
> >  substitute(x)
> x
> >  x
> [1] 3
>
> Is this a bug in substitute?
>
I think it is a design flaw rather than a bug:  the global environment 
is handled specially.  If you put those lines into a function you'll see 
different behaviour.  I think if you really carefully read the 
documentation you'll find it says this.

I suggested regularizing this several years ago, but there were worries 
that it would break some common usage.

Duncan Murdoch
> >  sessionInfo()
> R version 2.13.0 (2011-04-13)
> Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)
> ...
>
> Hadley
>

On Mon, May 2, 2011 at 9:53 AM, Hadley Wickham <hadley at rice.edu>
wrote:
> Hi all,
>
> The help for delayedAssign suggests that you can use substitute to
> access the expression associated with a promise, and the help for
> substitute says: "If it is a promise object, i.e., a formal argument
> to a function or explicitly created using ?delayedAssign()?, the
> expression slot of the promise replaces the symbol.
>
> But this doesn't seem to work:
>
>> a <- 1
>> b <- 2
>> delayedAssign("x", {message("assigning..."); a +
b})
>> substitute(x)
> x
>> x
> [1] 3
>
> Is this a bug in substitute?
>
>> sessionInfo()
> R version 2.13.0 (2011-04-13)
> Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)
It works differently if the assign environment is the global
environment or not.  This is actually mentioned on the help page for
substitute though its precise meaning may not be completely clear from
the wording.

Try this:
> e <- new.env()
> delayedAssign("x", {message("assigning..."); a + b},,
e)
> substitute(x, e)
{
    message("assigning...")
    a + b
}



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