wrapr, is an R package that supplies powerful tools for writing and debugging R code.

Introduction

Primary wrapr services include:

  • let() (let block)
  • %.>% (dot arrow pipe)
  • := (named map builder)
  • λ() (anonymous function builder)
  • DebugFnW() (function debug wrappers)

let()

let() allows execution of arbitrary code with substituted variable names (note this is subtly different than binding values for names as with base::substitute() or base::with()).

The function is simple and powerful. It treats strings as variable names and re-writes expressions as if you had used the denoted variables. For example the following block of code is equivalent to having written “a + a”.

library("wrapr")

a <- 7

let(
  c(VAR = 'a'),
  
  VAR + VAR
)
 #  [1] 14

This is useful in re-adapting non-standard evaluation interfaces (NSE interfaces) so one can script or program over them.

We are trying to make let() self teaching and self documenting (to the extent that makes sense). For example try the arguments “eval=FALSE” prevent execution and see what would have been executed, or debug=TRUE to have the replaced code printed in addition to being executed:

let(
  c(VAR = 'a'),
  eval = FALSE,
  {
    VAR + VAR
  }
)
 #  {
 #      a + a
 #  }

let(
  c(VAR = 'a'),
  debugPrint = TRUE,
  {
    VAR + VAR
  }
)
 #  $VAR
 #  [1] "a"
 #  
 #  {
 #      a + a
 #  }
 #  [1] 14

Please see vignette('let', package='wrapr') for more examples. Some formal documentation can be found here.

For working with dplyr 0.7.* we strongly suggest wrapr::let() (or even an alternate approach called seplyr).

%.>% (dot pipe or dot arrow)

%.>% dot arrow pipe is a pipe with intended semantics:

a %.>% b” is to be treated approximately as if the user had written “{ . <- a; b };” with “%.>%” being treated as left-associative.

Other R pipes include magrittr and pipeR.

The following two expressions should be equivalent:

cos(exp(sin(4)))
 #  [1] 0.8919465

4 %.>% sin(.) %.>% exp(.) %.>% cos(.)
 #  [1] 0.8919465

The notation is quite powerful as it treats pipe stages as expression parameterized over the variable “.”. This means you do not need to introduce functions to express stages. The following is a valid dot-pipe:

1:4 %.>% .^2 
 #  [1]  1  4  9 16

The notation is also very regular as we show below.

1:4 %.>% sin
 #  [1]  0.8414710  0.9092974  0.1411200 -0.7568025
1:4 %.>% sin(.)
 #  [1]  0.8414710  0.9092974  0.1411200 -0.7568025
1:4 %.>% base::sin
 #  [1]  0.8414710  0.9092974  0.1411200 -0.7568025
1:4 %.>% base::sin(.)
 #  [1]  0.8414710  0.9092974  0.1411200 -0.7568025

1:4 %.>% function(x) { x + 1 }
 #  [1] 2 3 4 5
1:4 %.>% (function(x) { x + 1 })
 #  [1] 2 3 4 5

1:4 %.>% { .^2 } 
 #  [1]  1  4  9 16
1:4 %.>% ( .^2 )
 #  [1]  1  4  9 16

Regularity can be a big advantage in teaching and comprehension. Please see “In Praise of Syntactic Sugar” for more details. Some formal documentation can be found here.

  • Obvious oblivious right-hand sides are rejected. Pipelines are meant to move values through a sequence of transforms, and not just for side-effects. Example: 5 %.>% 6 deliberately stops as 6 is a right-hand side that obviously does not use its incoming value. This check is only applied to values, not functions on the right-hand side.
  • Trying to pipe into a an “zero argument function evaluation expression” such as sin() is prohibited as it looks too much like the user declaring sin() takes no arguments. One must pipe into either a function, function name, or an non-trivial expression (such as sin(.)). A useful error message is returned to the user: wrapr::pipe does not allow direct piping into a no-argument function call expression (such as "sin()" please use sin(.)).
  • Some reserved words can not be piped into. One example is 5 %.>% return(.) is prohibited as the obvious pipe implementation would not actually escape from user functions as users may intend.
  • Obvious de-references (such as $, ::, @, and a few more) on the right-hand side are treated performed (example: 5 %.>% base::sin(.)).
  • Outer parenthesis on the right-hand side are removed (example: 5 %.>% (sin(.))).
  • Anonymous functions constructions are evaluated so the function can be applied (example: 5 %.>% function(x) {x+1} returns 6, just as 5 %.>% (function(x) {x+1})(.) does).
  • Checks and transforms are not performed on items inside braces (example: 5 %.>% { function(x) {x+1} } returns function(x) {x+1}, not 6).

:= (named map builder)

:= is the “named map builder”. It allows code such as the following:

'a' := 'x'
 #    a 
 #  "x"

The important property of named map builder is it accepts values on the left-hand side allowing the following:

name <- 'variableNameFromElsewhere'
name := 'newBinding'
 #  variableNameFromElsewhere 
 #               "newBinding"

A nice property is := commutes (in the sense of algebra or category theory) with R’s concatenation function c(). That is the following two statements are equivalent:

c('a', 'b') := c('x', 'y')
 #    a   b 
 #  "x" "y"

c('a' := 'x', 'b' := 'y')
 #    a   b 
 #  "x" "y"

The named map builder is designed to synergize with seplyr.

λ() (anonymous function builder)

λ() is a concise abstract function creator or “lambda abstraction”. It is a placeholder that allows the use of the -character for very concise function abstraction.

Example:

# Make sure lambda function builder is in our enironment.
wrapr::defineLambda()

# square numbers 1 through 4
sapply(1:4, λ(x, x^2))
 #  [1]  1  4  9 16

DebugFnW()

DebugFnW() wraps a function for debugging. If the function throws an exception the execution context (function arguments, function name, and more) is captured and stored for the user. The function call can then be reconstituted, inspected and even re-run with a step-debugger. Please see our free debugging video series and vignette('DebugFnW', package='wrapr') for examples.

Installing

Install with either:

install.packages("wrapr")

or

# install.packages("devtools")
devtools::install_github("WinVector/wrapr")

More Information

More details on wrapr capabilities can be found in the following two technical articles:

Note

Note: wrapr is meant only for “tame names”, that is: variables and column names that are also valid simple (without quotes) R variables names.