[Rd] Substitute unaware when promise objects are evaluated

Duncan Murdoch murdoch.duncan at gmail.com
Thu May 16 00:03:57 CEST 2013

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

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