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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