[R] Functions returning functions

[Romain Francois]

Romain Francois wrote:
> Paulo Grahl wrote:
>> Dear All:
>>
>> I have a question regarding the behavior of functions.
>> Say I define a function that returns another function :
>> A <- function(parameters) {
>>      # calculations w/ parameters returning 'y'
>>      tmpf <- function(x) { # function of 'y' }
>>      return(tmpf)
>> }
>>
>> The value of the parameters are stored in an environment local to the
>> function. Then I call
>> x<- something
>> B<-A(x)
>>
>> When R executes this last statement,  does it perform all the
>> calculations inside function A again 
> no
... but there are ways around it

f <- function( x = 3 ){
    # y will be get a value the first time
    # the function created below is called
    delayedAssign( "y", {
        cat( "\nevaluating `y` right now\n" )
        x + 5
    } )
   
    # z will get a new value each time the function
    # created below is called
    makeActiveBinding( "z", function( data ){
        if( missing(data) ){
            cat( "\nevaluating `z` right now\n" )
            rnorm(x)
        }
    }, environment() )
       function( ){
          list( y, z )
       }
}

 > h <- f()
 > h()

evaluating `y` right now

evaluating `z` right now
[[1]]
[1] 8

[[2]]
[1]  1.0991189 -1.1033016 -0.5410733

 > h()

evaluating `z` right now
[[1]]
[1] 8

[[2]]
[1]  0.7102276 -0.6371956 -0.7703179

 > environment(h)[["z"]]

evaluating `z` right now
[1] -0.6713595  2.2006114  0.9335674
 > environment(h)[["z"]]

evaluating `z` right now
[1]  0.1243523  0.6178176 -0.9043380

> see this:
>
> > f <- function( x = 3){
> +    function( ){}
> + }
> > g <- f()
> > ls.str( environment(g) )
> x :  num 3
> > h <- f(6)
> > ls.str( environment(h) )
> x :  num 6
> > ls.str( environment(g) )
> x :  num 3
>
> See also colorRampPalette which uses this feature:
>
> > colorRampPalette
> function (colors, ...)
> {
>    ramp <- colorRamp(colors, ...)
>    function(n) {
>        x <- ramp(seq.int(0, 1, length.out = n))
>        rgb(x[, 1], x[, 2], x[, 3], maxColorValue = 255)
>    }
> }
> <environment: namespace:grDevices>
> > pal <- colorRampPalette( c("blue", "white", "red") )
> > pal
> function (n)
> {
>    x <- ramp(seq.int(0, 1, length.out = n))
>    rgb(x[, 1], x[, 2], x[, 3], maxColorValue = 255)
> }
> <environment: 0x99aba4c>
> > ls.str( environment( pal ) )
> colors :  chr [1:3] "blue" "white" "red"
> ramp : function (x)
>> (i.e., all the calculations that
>> yield 'y')
>>  or the value of 'y' is already stored in the function's local 
>> environment ?
>>
>> Many thanks.
>> Paulo Gustavo Grahl, CFA
>>   
>


-- 
Romain Francois
Independent R Consultant
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr