[Rd] as.function()

[Duncan Murdoch]

Robin Hankin wrote:
> Hi
>
> [this after some considerable thought as to R-help vs R-devel]
>
>
>
> I want to write a (S3) method for as.function();
> toy example follows.
>
> Given a matrix "a", I need to evaluate trace(ax) as a function of
> (matrix) "x".
>
> Here's a trace function:
>
> tr <-  function (a)  {
>      i <- seq_len(nrow(a))
>      return(sum(a[cbind(i, i)]))
> }
>
>
> How do I accomplish the following:
>
>
> 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)
+ }

(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