[R] list of closures

[Gabor Grothendieck]

On Thu, Aug 26, 2010 at 12:04 AM, Stephen T. <obsessively at hotmail.com> wrote:
>
> Hi, I wanted to create a list of closures. When I use Map(), mapply(), lapply(), etc., to create this list, it appears that the wrong arguments are being passed to the main function. For example:
> Main function:
>> adder <- function(x) function(y) x + y
> Creating list of closures with Map():
>> plus <- Map(adder,c(one=1,two=2))> plus$one(1)[1] 3> plus$two(1)[1] 3
> Examining what value was bound to "x":
>> Map(function(fn) get("x",environment(fn)),plus)$one[1] 2$two[1] 2
>
> This is what I had expected:
>> plus <- list(one=adder(1),two=adder(2))> plus$one(1)[1] 2> plus$two(1)[1] 3
>> Map(function(fn) get("x",environment(fn)),plus)$one[1] 1$two[1] 2
>
> Anyone know what's going on? Thanks much!

R uses lazy evaluation of function arguments.  Try forcing x:

> adder <- function(x) { force(x); function(y) x + y }
> plus <- Map(adder,c(one=1,two=2))
> plus$one(1)
[1] 2
> plus$two(1)
[1] 3