R devel - May 2013 - Substitute unaware when promise objects are evaluated

R-devel,
I used the 'substitute' function to create labels for objects inside an
environment, without actually evaluating the objects, as the objects might be
promises.

However, I was surprised to see that 'substitute' returns the expression
slot of the original promise even after the promise has been forcibly evaluated.
(Doesn't the promise go away after evaluation?) This behavior probably falls
under the "...no guarantee that the resulting expression makes any
sense" clause of the ?substitute documentation, but in case there's
something actually wrong here, I thought I'd send an example.

Here's an example showing how the evaluated expression returned by
substitute does not match the actual variable value:
> env <- new.env()
> z <- 0
> delayedAssign("var", z+2, assign.env=env)
> substitute(var, env=env)
z + 2
> force(env$var)
[1] 2
> z <- 10
> substitute(var, env=env)
z + 2
> eval(substitute(var, env=env))
[1] 12
> force(env$var)
[1] 2

Is there any obvious way to code around this behavior, e.g. can I explicitly
check if an object in an environment is an unevaluated promise?

Thanks, 
Robert
> R.version
               _
platform       x86_64-pc-linux-gnu
arch           x86_64
os             linux-gnu
system         x86_64, linux-gnu
status
major          3
minor          0.0
year           2013
month          04
day            03
svn rev        62481
language       R
version.string R version 3.0.0 (2013-04-03)
nickname       Masked Marvel

Robert McGehee, CFA
Geode Capital Management, LLC
One Post Office Square, 28th Floor | Boston, MA | 02109
Direct: (617)392-8396

On 13-05-15 11:54 AM, McGehee, Robert wrote:
> R-devel,
> I used the 'substitute' function to create labels for objects
inside an environment, without actually evaluating the objects, as the objects
might be promises.
>
> However, I was surprised to see that 'substitute' returns the
expression slot of the original promise even after the promise has been forcibly
evaluated. (Doesn't the promise go away after evaluation?) This behavior
probably falls under the "...no guarantee that the resulting expression
makes any sense" clause of the ?substitute documentation, but in case
there's something actually wrong here, I thought I'd send an example.
I think you misunderstand promises.

A promise has two (or three, depending how you count) parts:  an 
expression with an associated environment, and a value.  The value isn't 
filled in until the expression is evaluated, but the expression doesn't 
go away then.  You can still see it until you change the variable that 
holds the promise.

> Here's an example showing how the evaluated expression returned by
substitute does not match the actual variable value:
>
>> env <- new.env()
>> z <- 0
>> delayedAssign("var", z+2, assign.env=env)
>> substitute(var, env=env)
> z + 2
The documentation for substitute may not be clear on this, but for a 
promise, the env argument will be ignored.  It was the eval.env argument 
to delayedAssign that set the promise's environment.
>> force(env$var)
> [1] 2
>> z <- 10
>> substitute(var, env=env)
> z + 2
>> eval(substitute(var, env=env))
> [1] 12
>> force(env$var)
> [1] 2
>
> Is there any obvious way to code around this behavior, e.g. can I
explicitly check if an object in an environment is an unevaluated promise?
Not at R level. In C code you could, but you probably shouldn't.  Think 
of promises as values where you can look up the expression that gave the 
value, and sometimes delay the calculation until you need it.

Duncan Murdoch