weasel

’Round and ’round the R workspace, code-monkey chased the {weasel}

Code-monkey stopped to extract the first val…

pop() goes the {weasel}

Installation

You can install the development version of weasel like so:

# install.packages("remotes")
remotes::install_github("jonocarroll/weasel")

Motivation

Inspired by other (less functional/immutable) languages where one might be familiar with being able to pop() a value from the top of the stack, this package provides similar functionality.

If we define a vector a containing some values

a <- c(3, 1, 4, 1, 5, 9)

and we wish to extract the first value, we can certainly do so with

a[1]
#> [1] 3

but, due to the nature of R, the vector a is unchanged

a
#> [1] 3 1 4 1 5 9

Instead, we could remove the first value of a with

a[-1]
#> [1] 1 4 1 5 9

but again, a remains unchanged - in order to modify a we must redefine it as e.g.

a <- a[-1]
a
#> [1] 1 4 1 5 9

This differs from other languages were we might want to extract that first value for use, and leave the rest of the vector in place.

Usage

{weasel} offers that functionality

a <- c(3, 1, 4, 1, 5, 9)
a
#> [1] 3 1 4 1 5 9
first_val <- pop(a)
a
#> [1] 1 4 1 5 9
first_val
#> [1] 3

Note that this did not require explicitly¹ redefining a; the return value of pop() is the popped value.

This is also a Generic, so it is defined for some classes (vector, list, data.frame) and can be extended.

With a list:

a <- list(x = c(2, 3), y = c("foo", "bar"), z = c(3.1, 4.2, 6.9))
a
#> $x
#> [1] 2 3
#> 
#> $y
#> [1] "foo" "bar"
#> 
#> $z
#> [1] 3.1 4.2 6.9
x <- pop(a)
a
#> $y
#> [1] "foo" "bar"
#> 
#> $z
#> [1] 3.1 4.2 6.9
x
#> [1] 2 3

With a data.frame:

a <- data.frame(x = c(2, 3, 4), y = c("foo", "bar", "baz"), z = c(3.1, 4.2, 6.9))
a
#>   x   y   z
#> 1 2 foo 3.1
#> 2 3 bar 4.2
#> 3 4 baz 6.9
x <- pop(a)
a
#>   x   y   z
#> 2 3 bar 4.2
#> 3 4 baz 6.9
x
#>   x   y   z
#> 1 2 foo 3.1

{weasel} also offers push() functionality

a <- c(1, 4, 1, 5, 9)
a
#> [1] 1 4 1 5 9
push(a, 3)
a
#> [1] 3 1 4 1 5 9

which is also Generic

a <- list(y = c("foo", "bar"), z = c(3.1, 4.2, 6.9))
a
#> $y
#> [1] "foo" "bar"
#> 
#> $z
#> [1] 3.1 4.2 6.9
push(a, list(new_vals = c(99, 77)))
a
#> $new_vals
#> [1] 99 77
#> 
#> $y
#> [1] "foo" "bar"
#> 
#> $z
#> [1] 3.1 4.2 6.9

a <- data.frame(y = c("foo", "bar", "baz"), z = c(3.1, 4.2, 6.9))
a
#>     y   z
#> 1 foo 3.1
#> 2 bar 4.2
#> 3 baz 6.9
push(a, data.frame(y = 99, z = 77))
a
#>     y    z
#> 1  99 77.0
#> 2 foo  3.1
#> 3 bar  4.2
#> 4 baz  6.9

Warnings

Please note that this is intended only for informal use and education. This functionality is not idiomatic R and is more of a footgun than should be tolerated.

Explanation

Internally, the evaluation of each ‘method’ is a macro defined using gtools::defmacro() which enables substitution of input arguments and redefinition in the parent namespace.

This blogpost covers the full details. This article in the R News Newsletter covers the construction and motivation of defmacro().

Footnotes

1. It certainly is redefined internally, but the use of defmacro() means it does not need to be explicit ↩