oshka - Recursive Quoted Language Expansion

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.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.

Documentation

• 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.

Installation

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.

Acknowledgements

• 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.

About

Brodie Gaslam is a hobbyist programmer based on the US East Coast.

The name of this package is derived from “matryoshka”, the Russian nesting
dolls.