Using non-standard evaluation to put values into a function or list

Question

If I define a list as follows: list(param = "value1"), then the result will be:

> list(param = "value1") #$param #[1] "value1"

I'm trying to define a new operator %!=% such that I can define a list as follows: list(param %!=% "value1") and I want the result to be:

> list(param %!=% "value1") #$param.ne #[1] "value1"

As background, I'm writing an R wrapper for a RESTful API that is pulling data from a database. When you make a request, you can tell the API to return results that match a parameter value param1 = value1 -- this will be included directly in the query string of the GET request.

However, the API also allows you to pull results that do NOT match the parameter value: param != value1. Putting != into the query string of a request presents a problem. The API is designed to use param.ne = value1 in place of param != value1.

Currently I'm trying to do this using some elements from non-standard evaluation:

`%!=%` = function(query, value) { query = deparse(substitute(query)) query = paste0("`", query, ".ne", "`", "=", "\"", value, "\"") parse(text = query) }

This function will figure out the type of query (param, param2, param3, etc) using non-standard evaluation, convert the query to a string, add a .ne to the end and send it back out as an expression.

However, if I try to do list(eval(param1 %!=% "value1")) then the result will be:

> list(eval(address %!=% "value1")) #[[1]] #[1] "value1"

Is there a simple way to achieve the above?

EDIT: In light of posts below, an additional question:
Is there anyway to adapt the above to work with a function that has a default parameter? For example:

newlist <- function(param1 = "default1", param2 = "default2", ...)

and I wanted to do something like

newlist(param2 = "non-default", param3 %!=% "stuff")

with the result being a list with param1 = "default1", param2 = "non-default" and param3.ne = "stuff"?

MrFlick · Accepted Answer · 2015-11-23 03:58:16Z

You're better off tacking this at the list() level call rather than trying to interfere with parameter name replacements. For example, consider this function

newlist <- function(...) { dots <- substitute(...()) op <- sapply(sapply(dots, '[[', 1), deparse) car <- sapply(sapply(dots, '[[', 2), deparse) cdr <- sapply(sapply(dots, "[[", 3), eval.parent) stopifnot(all(op %in% c("==","!="))) setNames(as.list(cdr), ifelse(op=="!=", paste0(car, ".ne"), car)) } newlist(a == "b", c != "d") # $a # [1] "b" # $c.ne # [1] "d"

Here we look at the expressions you are passing to the function and make sure they are == or != and can build the appropriate list.

But if you really wanted to use your function, you could do

`%!=%` = function(query, value) { field = paste0(deparse(substitute(query)), ".ne") setNames(list(value), field) } a %!=% 5 # $a.ne # [1] 5

Note that this will return a list directly. You can combine them with c

c(a %!=% 5, b %!=% "fred")

could you explain how this, ...(), little bit of magic forms a list please
I wish it were easier to search for symbols on this site. Here's a question just about that syntax: stackoverflow.com/questions/12523548/confused-by
Is there anyway to adapt the above to work with a function that has a default parameter? For example, newlist <- function(param1 = "default1", param2 = "default2", ...) and I wanted to do something like newlist(param2 = "non-default", param3 %!=% "stuff") and the result is a list with param1 = "default1", param2 = "non-default" and param3.ne = "stuff"?
@hadley Do you have a reference or can you suggest an alternative?

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