Re: [Rd] Bug in handling of promises?

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Tue 08 Mar 2005 - 18:10:26 GMT

Duncan Murdoch <murdoch@stats.uwo.ca> writes:

[snip]
> 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.

Hmm, I don't think it is a bug as such that a promise in a function environment stays a promise. Otherwise the semantics of substitute() get messed up. However, it is a bit peculiar that the evaluator doesn't extract the value when such an object is encountered in an expression. Apparently (conjecture!) this is only done during variable lookup, but not on return values from functions:

> env <- g( 2 )
> env$H+2

Error in env$H + 2 : non-numeric argument to binary operator
> with(env,H+2)

[1] 4
> x <- env$H
> x+2

[1] 4

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907

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Received on Wed Mar 09 05:29:06 2005

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