In crowding/nse: Tools for Lazy and Non-Standard Evaluation

"Non-standard evaluation" is the name the R community gives to techniques that subvert the normal order of evaluation in the R language.

Things that are standard evaluation: * Calling functions * Returing a value from a function

Things that are non-standard evaluation:

The most common case of non-standard evaluation occurs e.g. in plotting functions, where. If this is your use case, just use arg_expr.

What is Standard Evaluation?

Consider the following R code

plot_with_mean <- function(x, ...) {

}

recenter_median <- function(x, ...) {
  tmp <- mean(x)
}

recenter_mode <- function(x, ...) {
  tmp <- mode(x)
  x=x, ...
}

list <- function(x, ...) {

}

What

Trivial non-standard evaluation

Information about the present call

Information about the arguments to the call

How was the argument written?

What environment did the argument come from?

What are all the arguments to the function?

as.args(environment())

Information about the call itself

What is the function being evaluated

What was this function called, in the caller?

Note that functions do not have a "name" in R as such. Functions may be bound to a name in a particular environment, and they

How was this call written, in the caller's code?

call_expr(capture_call())

This is OK for generating plot labels and debugging labels, but if you are implementing NSE you should almost certainly use [capture_call] instead.

What environment was the present function called from?

Note: this is NOT necessarily the same as what environments the arguments are from. If your function has an argument, 9 times out of ten it's [the environment of the arg] that you want to know.

caller()
call_env(capture_call())
env(capture_call()[[1]])

Capture a version of this call and all its arguments, unevaluated

capture_call()

Information about other calls

Recall that every time a function is called, an R environment object is created.

Construct, modify or invoke arguments.

Bind a lazy value

set_arg(x, quo(x+1))
set_arg(x, quo(x+1))
set_arg(x, quo_(quote(x+1), environment())

Bind a lazy value in another environment

Make a lazy variable from some expression

Assign a lazy variable to some name

Create an argument list from expression objects

Create an argument list from promises

Create a list of arguments

Create a call from a function and arguments

call(function, as.args(x+1))

crowding/nse documentation built on Jan. 5, 2024, 12:14 a.m.