[R] Chaining multiple replacement functions in R

[Daniel Gabrieli]

I cross posted this on Stack Overflow:
http://stackoverflow.com/questions/27694466/chaining-multiple-replacement-functions-in-r


I am using R to work with a large JS object (using the library rjsonio). As
such, I have a lot of nested lists, which are getting somewhat cumbersome
to work with. I have a simplified example below. I am trying to work with
this object by creating some form of ‘getter’ and ‘setter’ functions. After
looking around, I have found a pretty nice ‘getter’ function that recurses
through the object and returns the value for the first matching label. This
is especially great because it lends itself to chaining functions together.
However, I can not figure out a way to get the same effect for a ‘setter’
function. Any thoughts on how to create a ‘setter’ function that can be
chained together in a similar fashion?

#example, simplified, object
app = list(
  1,
  2,
  d=list(a=123,
         b=456,
         list(
           FirstKey=list(attr1='good stuff', attr2=12345),
           SecondKey=list(attr1='also good stuff', attr2=4321)
           )
         )
  )


#Return a function that returns the value
#associated with first label that matches 'name'
getByName <- function(name){
  rmatch <- function(x) {
    pos <- match(name, names(x))
    if (!is.na(pos))
      return(x[[pos]])
    for (el in x) {
      if (class(el) == "list") {
        out <- Recall(el)
        if (!is.null(out)) return(out)
      }
    }
  }
  rmatch
}

getFirstKey <- getByName("FirstKey")
getAttr1 <- getByName("attr1")
getAttr2 <- getByName("attr2")

#I like that I can chain these functions together
getAttr1(getFirstKey(app))
getAttr2(getFirstKey(app))

# I would like to be able to do something like this
# But this won't work
###    getAttr1(getFirstKey(app)) <- 9876

# This does work,,, but I loose the ability to chain functions together
# Closure around a replacement function
setterKeyAttr <- function(keyName, attr){
  function(x, value){
    x$d[[3]][[keyName]][[attr]] <- value
    x
  }
}

`setFirstKeyAttr2<-` <- setterKeyAttr("FirstKey", "attr2")
setFirstKeyAttr2(app) <- 22222
#check the answer is correct
getAttr2(getFirstKey(app))



references:

http://stackoverflow.com/questions/23124096/r-decorator-to-change-both-input-and-output

http://r.789695.n4.nabble.com/How-to-get-a-specific-named-element-in-a-nested-list-td3037430.html

http://adv-r.had.co.nz/Functions.html

        [[alternative HTML version deleted]]

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