[R] Recall() and sapply()

[Gabor Grothendieck]

I believe that the function that Recall executes is the
function in which Recall, itself, is evaluated -- not the
function in which Recall appears.  In normal cases these are
the same but if you pass Recall to another function then
they are not the same.  Here Recall is being passed to
sapply (which in turn is likely passing it to other
functions).  Because of lazy evaluation Recall does not get
evaluated until it is found within sapply (or a function
called by it or called by one called by it, etc.) and at
that point its recalling the wrong function.  AFAICS one
cannot pass Recall to another function.

You could rewrite the expression that uses sapply to use 
iteration instead or you could do it as shown below.  
In this example, the use of f2 within supply refers to 
the inner f2 which does not change even if the name of 
the outer f2 does.

  f2 <- function(n) {
     f2 <- function(n) if(length(n)>1) sapply(n,f2) else matrix(n,n,n)
     f2(n)
   }
   f3 <- f2
   f2(1:3)
   f3(1:3)  # gives same result



On Wed, 30 Mar 2005 09:28:08 +0100, Robin Hankin
<r.hankin at soc.soton.ac.uk> wrote:
> Hi.
> 
> I'm having difficulty following the advice given in help(Recall).
> Consider the two
> following toy functions:
> 
> f1 <- function(n){
>   if(length(n)>1){return(sapply(n,f1))}
>   matrix(n,n,n)
> }
> 
> f2 <- function(n){
>   if(length(n)>1){return(sapply(n,Recall))}
>   matrix(n,n,n)
> }
> 
> f1() works as desired (that is, f(1:3), say, gives me a three element
> list whose i-th element
> is an i-by-i matrix whose elements are all i).
> 
> But f2() doesn't.
> 
> How do I modify either function to use Recall()?  What exactly is
> Recall() calling here?
> 
> --
> Robin Hankin
> Uncertainty Analyst
> Southampton Oceanography Centre
> European Way, Southampton SO14 3ZH, UK
>  tel  023-8059-7743
> 
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