[Rd] Bug in handling of promises?

[Duncan Murdoch]

A followup:  doing an assignment of the value works even after the
return:

> f
function( H ) {
    cat('Evaluate H to get \n')
    print(H)
    return(environment())
}
> env <- f( function(x) x^2 )
Evaluate H to get 
function(x) x^2
> env$H
<promise: 0117C2D0>
> env$H(1)
Error: attempt to apply non-function
> H <- env$H
> H(1)
[1] 1

So the oddity is that I can evaluate H within f() without any problem,
but outside of f() I need extra work before I can do it.

Duncan Murdoch


On Tue, 08 Mar 2005 18:00:10 +0000, Duncan Murdoch
<murdoch at stats.uwo.ca> wrote :

>On Tue, 8 Mar 2005 17:44:41 +0000 (GMT), Prof Brian Ripley
><ripley at stats.ox.ac.uk> wrote :
>
>>The following note in ?force may help
>>
>>Note:
>>
>>      'force' does not force the evaluation of promises.
>>
>>It is there because people have been confused before.
>
>Yes, but it also says that it "forces the evaluation of a function
>argument", which is what I was trying to do.
>
>But mainly what may be a bug is the fact that H was available in env
>but its value was not, even though it had already been evaluated in
>that environment.  
>
>My examples were unnecessarily complicated last time, because they
>were too much like the original.  Here are simpler versions:
>
>> f <- function( H ) {
>+     cat('Evaluate H to get ', H, '\n')
>+     return(environment())
>+ }
>> 
>> env <- f( 1 )
>Evaluate H to get  1 
>> env$H
><promise: 0118BF1C>
>> 
>> g <- function( H ) {
>+     force(H)
>+     return(environment())
>+ }
>> 
>> env <- g( 2 )
>> env$H
><promise: 0118A148>
>> 
>> 
>> h <- function( H ) {
>+     H <- H
>+     return(environment())
>+ }
>> 
>> env <- h( 3 )
>> env$H
>[1] 3
>
>Duncan Murdoch
>>
>>
>>On Tue, 8 Mar 2005, Duncan Murdoch wrote:
>>
>>> I'm working on a function that does adaptive sampling, and I thought
>>> it would be handy to return the function's environment as part of the
>>> result so that I could re-use local variables in a subsequent run.  My
>>> first try didn't work, and it came down to code like this:
>>>
>>>> f <- function( H, prevEnv = NULL) {
>>> +     if (!is.null(prevEnv)) H <- prevEnv$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.
>
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