Programmable Non-Standard Evaluation

Non-Standard Evaluation (NSE hereafter) occurs when R expressions are captured and evaluated in a manner different than if they had been executed without intervention. subset is a canonical example, which we use here with the built-in iris data set:

subset(iris, Sepal.Width > 4.1)

Sepal.Width does not exist in the global environment, yet this works because subset captures the expression and evaluates it within iris.

A limitation of NSE is that it is difficult to use programmatically:

exp.a <- quote(Sepal.Width > 4.1)
subset(iris, exp.a)

oshka::expand facilitates programmable NSE, as with this simplified version of subset:

subset2 <- function(x, subset) {
  sub.exp <- expand(substitute(subset), x, parent.frame())
  sub.val <- eval(sub.exp, x, parent.frame())
  x[! & sub.val, ]
subset2(iris, exp.a)

expand is recursive:

exp.b <- quote(Species == 'virginica')
exp.c <- quote(Sepal.Width > 3.6)
exp.d <- quote(exp.b & exp.c)

subset2(iris, exp.d)

We abide by R semantics so that programmable NSE functions are almost identical to normal NSE functions, with programmability as a bonus.

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oshka documentation built on May 1, 2019, 9:19 p.m.