How to parse and build an URL query string with multiple value parameters in R

Question

Is there a straightforward way to parse and build URL query string with multiple value parameters in R ?

I would expect something like

myqueryString <- parse_url("http://www.mysite.com/?a=1&a=2&b=val")$query
myqueryString
  $a
  [1] 1 2
  $b
  [1] "val"

and

urlElements <- list(scheme="http",path="www.mysite.com/",query=list(a=c(1,2),b="val"))
setattr(urlElements,"class","url")
build_url(urlElements)
  [1] "http://www.mysite.com/?a=1&a=2&b=val"

However httr gives

parse_url("http://www.mysite.com/?a=1&a=2&b=val")$query
 $a
 [1] "1"

 $a
 [1] "2"

 $b
 [1] "val"

and

builtURL <- build_url(urlElements)
builtURL
   [1] "http:///www.mysite.com/?a=c%281%2C%202%29&b=val"

This latest URL can be reprocessed

parse_url(builtURL)$query
 $a
 [1] "c(1, 2)"

 $b
 [1] "val"

I understand that I can use parse() + eval() to get a back but it looks fairly unsafe to eval code that can be freely dumped to an URL. Any suggestions?

Answer 1

See if these work to convert between those two argument list formats:

mergeUrlArgs <- function(x) sapply(unique(names(x)), function(z) unlist(x[names(x) == z], use.names=FALSE), simplify=FALSE)

expandUrlArgs <- function(x) structure(do.call(c, lapply(x, function(z) as.list(z))), names=rep(names(x), sapply(x, length)))