Partial Function Application with %<%, %-<%.

curry

curry is yet another attempt at providing a native currying/partial application mechanism in R. Other examples of implementations of this can be found in purrr and functional (and probably others). curry sets itself apart in the manner it is used and in the functions it creates. curry is operator based and a partially applied function retains named arguments for easier autocomplete etc. curry provides three mechanisms for partial application: `%<%` (`curry()`), `%-<%` (`tail_curry()`), and `%><%` (partial()) - see the examples for the differences

Example

Currying is the reduction of the arity of a function by fixing the first argument, returning a new function lacking this.

# Equivalent to curry(`+`, 5)
add_5 <- `+` %<% 5
add_5(10)
#> [1] 15

# ellipsis are retained when currying
bind_5 <- cbind %<% 5
bind_5(1:10)
#>       [,1] [,2]
#>  [1,]    5    1
#>  [2,]    5    2
#>  [3,]    5    3
#>  [4,]    5    4
#>  [5,]    5    5
#>  [6,]    5    6
#>  [7,]    5    7
#>  [8,]    5    8
#>  [9,]    5    9
#> [10,]    5   10

Tail currying is just like currying except it reduces the arity of the function from the other end by fixing the last argument.

# Equivalent to tail_curry(`/`, 5)
divide_by_5 <- `/` %-<% 5
divide_by_5(10)
#> [1] 2

no_factors <- data.frame %-<% FALSE
df <- no_factors(x = letters[1:5])
class(df$x)
#> [1] "character"

When the argument you wish to fix is not in either end of the argument list it is necessary to use a more generalised approach. Using %><% (or partial()) it is possible to fix any (and multiple) arguments in a function using a list of values to fix.

dummy_lengths <- vapply %><% list(FUN = length, FUN.VALUE = integer(1))
test_list <- list(a = 1:5, b = 1:10)
dummy_lengths(test_list)
#>  a  b 
#>  5 10

Other efforts in this has the drawback of returning a new function with just an ellipsis, making argument checks and autocomplete impossible. With curry the returned functions retains named arguments (minus the fixed ones).

args(no_factors)
#> function (..., row.names = NULL, check.rows = FALSE, check.names = TRUE, 
#>     fix.empty.names = TRUE) 
#> NULL
args(dummy_lengths)
#> function (X, ..., USE.NAMES = TRUE) 
#> NULL

Installation

curry is still a work in progress but can be installed through devtools:

if (!require(devtools)) {
    install.packages(devtools)
}
devtools::install_github('thomasp85/curry')