README.md
In oshka: Recursive Quoted Language Expansion

oshka - Recursive Quoted Language Expansion

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.Length Sepal.Width Petal.Length Petal.Width Species
## 16          5.7         4.4          1.5         0.4  setosa
## 34          5.5         4.2          1.4         0.2  setosa

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)
## Error in subset.data.frame(iris, exp.a): 'subset' must be logical

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[!is.na(sub.val) & sub.val, ]
}
subset2(iris, exp.a)
##    Sepal.Length Sepal.Width Petal.Length Petal.Width Species
## 16          5.7         4.4          1.5         0.4  setosa
## 34          5.5         4.2          1.4         0.2  setosa

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)
##     Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
## 118          7.7         3.8          6.7         2.2 virginica
## 132          7.9         3.8          6.4         2.0 virginica

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

Intro vignette for a more in depth introduction to oshka, including a brief comparison to rlang.
NSE Functions with oshka in which we recreate simplified versions of dplyr and data.table that implement programmable NSE with oshka::expand.

This package is proof-of-concept. If it elicits enough interest we will re-write the internals in C and add helper functions for common use patterns.

install.packages('oshka')
# or development version
devtools::instal_github('brodieg/oshka@development')

Feedback is welcome, particularly if you are aware of some NSE pitfalls we may be ignoring.

R Core for developing and maintaining such a wonderful language.
CRAN maintainers, for patiently shepherding packages onto CRAN and maintaining the repository, and Uwe Ligges in particular for maintaining Winbuilder.
Lionel Henry for his early feedback on this project.
Jim Hester because covr rocks.
Dirk Eddelbuettel and Carl Boettiger for the rocker project, and Gábor Csárdi and the R-consortium for Rhub, without which testing bugs on R-devel and other platforms would be a nightmare.
Hadley Wickham for devtools and roxygen2.
Yihui Xie for knitr and J.J. Allaire etal for rmarkdown, and by extension John MacFarlane for pandoc.