From murdoch at stats.uwo.ca  Tue Mar  8 18:15:58 2005
From: murdoch at stats.uwo.ca (Duncan Murdoch)
Date: Tue Mar  8 18:16:01 2005
Subject: [Rd] Bug in handling of promises?
Message-ID: <bemr21dafvf5sp687mk0jpob2rual9ilan@4ax.com>

I'm working on a function that does adaptive sampling, and I thought
it would be handy to return the function's environment as part of the
result so that I could re-use local variables in a subsequent run.  My
first try didn't work, and it came down to code like this:

> f <- function( H, prevEnv = NULL) {
+     if (!is.null(prevEnv)) H <- prevEnv$H
+     cat('Evaluate H to get ', H(1), '\n')  
+     return(environment(NULL))
+ }

I thought that evaluating H would force it, so that H would be
available in the environment returned by the function.  But this is
not so:

> env <- f( function(x) x^2 )
Evaluate H to get  1 
> env$H
<promise: 012094D8>
> env$H(1)
Error: attempt to apply non-function

So I tried to explicitly force it:

> g <- function( H, prevEnv = NULL) {
+     if (!is.null(prevEnv)) H <- prevEnv$H
+     force(H)   
+     return(environment(NULL))
+ }

but this still doesn't work:

> env <- g( function(x) x^2 )
> env$H
<promise: 01206FC0>
> env$H(1)
Error: attempt to apply non-function

It seems that I need to do an assignment to convert H from a promise
to an evaluated object:

> h <- function( H, prevEnv = NULL) {
+     if (!is.null(prevEnv)) H <- prevEnv$H
+     H <- H     
+     return(environment(NULL))
+ }
> env <- h( function(x) x^2 )
> env$H
function(x) x^2
> env$H(1)
[1] 1

Is this a bug, or just the way things are?

I get the same results in both R-patched and R-devel.

Duncan Murdoch


[Rd] Bug in handling of promises?
Duncan Murdoch