[Rd] Bug in handling of promises?

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Tue 08 Mar 2005 - 17:15:58 GMT


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



R-devel@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel Received on Wed Mar 09 04:27:37 2005

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