[Rd] copying promise

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Wed, 19 Sep 2007 20:05:26 -0400

  1. Is there some way to copy a promise so that the copy has the same expression in its promise as the original. In the following we y is a promise that we want to copy to z. We want z to be a promise based on the expression x since y is a promise based on the expression x. Thus the answer to the code below is desired to be z=2 but its 1, 1 and y in the next three examples so they are not the answer. See examples at end.
  2. Is there some way to determine if a variable holds a promise without evaluating it?

This code relates to question 1.

# example 1

x <- 0
delayedAssign("y", x)
x <- 1
# this forces y which is not what we want
z <- y
x <- 2
z # 1

# example 2
# this connects to z to x via y which is not what we want
# since if y is forced then z takes its value from that, not x
x <- 0
delayedAssign("y", x)
delayedAssign("z", y)
x <- 1
y
x <- 2
z # 1

# example 3
# this attempts to assign the expression underlying promise y to z
# which seems closest in spirit to what we want
# but it does not work as intended
x <- 0
delayedAssign("y", x)
delayedAssign("z", substitute(y))
x <- 1
y
x <- 2
z # y

> R.version.string # Vista
[1] "R version 2.6.0 alpha (2007-09-06 r42791)"



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