Chaining multiple replacement functions in R - r

Question

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 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:
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

Answer 1

This is what I came up with. It makes the recursive function return the position of the 'name' and still be able to chain the calls together. I am not sure if this is a great way to do it... but it seems to be working... This is based off the fact that app[[c(3,3,1,)]] is a valid way to index in R.
rmatch.pos <- function(object, name, seq=NA, level=NULL){
##return the vector of integers corresponding to the first match
##of 'name' to a label in object or NULL if no match is found
###object: a list, likely deeply nested
##name: the name of the label to look for
##seq: starting point to search for 'name' in 'object' i.e. c(2,3,3)
##level: don't touch this; it keeps track of how deep the recursive execution is
##can be chained together to reduce ambiguity or result:
##obj <- list(a=1, b=list(c=2, d=list(e=1, attr1="really?", f=list(attr1 = "found me!"))))
##obj[[rmatch.pos(obj, "attr1", rmatch.pos(obj, "f"))]]
if(is.null(seq)){
#short circuit if NULL gets passed
#when chaining, this forces the whole 'chain'
#to NULL when any 'link' is NULL
return(NULL)
}
if(is.null(level)){
level <- length(na.omit(seq))
}
if(any(is.na(seq))){
temp <- object
}else{
temp <- object[[seq]]
}
level <- level + 1
pos <- match(name, names(temp))
if(!is.na(pos)){
seq[level] <- pos
return(seq)
}
for(el in seq_along(temp)){
if(class(temp[[el]]) == "list"){
seq[level] <- el
out <- Recall(object, name, seq, level)
if(!is.null(out)){
return(out)
}
}
}
}
###Examples
rmatch.pos(app, "ThirdKey")
rmatch.pos(app, "attr2")
###chaining example
rmatch.pos(app, "attr2", rmatch.pos(app, "FirstKey"))
rmatch.pos(app, "attr2", rmatch.pos(app, "SecondKey"))
rmatch.pos(app, "attr1", rmatch.pos(app, "ERROR"))
rmatch.pos(app, "ERROR", rmatch.pos(app, "attr1"))