[R] mapply & assign to generate functions

[Brian Diggs]

On 3/12/2012 10:47 AM, J Toll wrote:
> Hi,
>
> I have a problem that I'm finding a bit tricky.  I'm trying to use
> mapply and assign to generate curried functions.  For example, if I
> have the function divide
>
> divide<- function(x, y) {
>    x / y
> }
>
> And I want the end result to be functionally equivalent to:
> half<- function(x) divide(x, 2)
> third<- function(x) divide(x, 3)
> quarter<- function(x) divide(x, 4)
>
> But I want to do it using mapply and assign:
> mapply(assign,
>         c("half", "third", "quarter"),
>         lapply(2:4, function(i) {function(x) divide(x, i)}),
>         pos = 1)

The problem is with lazy argument evaluation/promises. i will be the 
last value of i (in this case 4) for all instances.  You can change that.

mapply(assign,
        c("half", "third", "quarter"),
        lapply(2:4, function(i) {force(i); function(x) divide(x, i)}),
        pos = 1)

Note the inclusion of the force function.  This gives expected results.

 > half(12)
[1] 6
 > third(12)
[1] 4
 > quarter(12)
[1] 3


> The problem is in the third line.  I end up with 3 functions that are
> all functionally equivalent to quarter().  Any suggestions on how to
> get this to work properly.
>
> Thanks.
>
>
> James
>


-- 
Brian S. Diggs, PhD
Senior Research Associate, Department of Surgery
Oregon Health & Science University