On 14 Jan 2008, at 11:50, Duncan Murdoch wrote:
> Robin Hankin wrote:
>> Hi
>>
[snip]
>> a <- crossprod(matrix(rnorm(12),ncol=3))
>> class(a) <- "foo"
>>
>> f <- as.function(a) # need help to write as.function.foo()
>> x <- diag(3)
>>
>> f(x) #should give tr(ax)
>>
>> a <- 4
>> f(x) # should still give tr(ax) even though "a" has been
>> reassigned.
>>
>>
> Brian's answer was what you want. A less general version is this:
>
> > as.function.foo <- function(x, ...) {
> + function(b) tr(x %*% b)
> + }
>
Wow. Got it! Looks like I'll have to read the R Language Definition
again.
Thanks everyone.
> (I switched the names of the args, because the first arg to
> as.function.foo should match the name of the first arg to
> as.function).
>
> I was a little surprised that this worked even if a was changed
> without ever evaluating f, because I thought lazy evaluation would
> mess up that case. But of course the value of x is forced when R
> evaluates it to find out the class for dispatch to the method.
>
> Duncan Murdoch
--
Robin Hankin
Uncertainty Analyst and Neutral Theorist,
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
tel 023-8059-7743
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