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Function generation; change default values ​​of other functions (partial)

I need a function generator that takes another function and any arguments to that function and sets new default values. I thought @hadley pryr::partialis a magic feature. It does exactly what I want, but you cannot change this new default value. Therefore, here I can change sepin my new function paste, but not as a new default value collapse = "_BAR_". How can I do it partialthis way (i.e. by default - collapse = "_BAR_"but enable it if necessary collapse = NULL)? If this is not possible using partial, there is a way to rewrite the code for partialto do this: https://github.com/hadley/pryr/blob/master/R/partial.r

library(pryr)
.paste <- pryr::partial(paste, collapse = "_FOO_")

.paste(1:5)
.paste(1:5, LETTERS[1:5], sep="_BAR_")
.paste(1:5, collapse=NULL)

> .paste(1:5)
[1] "1_FOO_2_FOO_3_FOO_4_FOO_5"

> .paste(1:5, LETTERS[1:5], sep="_BAR_")
[1] "1_BAR_A_FOO_2_BAR_B_FOO_3_BAR_C_FOO_4_BAR_D_FOO_5_BAR_E"

> .paste(1:5, collapse=NULL)
Error in paste(collapse = "_FOO_", ...) : 
  formal argument "collapse" matched by multiple actual arguments
+4
3

partial , , .

.paste <- paste
formals(.paste)$collapse <- "_FOO_"


args(.paste)
# function (..., sep = " ", collapse = "_FOO_") 
# NULL


.paste(1:5)
# [1] "1_FOO_2_FOO_3_FOO_4_FOO_5"
.paste(1:5, LETTERS[1:5], sep="_BAR_")
# [1] "1_BAR_A_FOO_2_BAR_B_FOO_3_BAR_C_FOO_4_BAR_D_FOO_5_BAR_E"
.paste(1:5, collapse=NULL)
# [1] "1" "2" "3" "4" "5"
+6

, @MrFlick :

hijack <- function(FUN, ...){

    .FUN <- FUN

    args <- list(...)
    invisible(lapply(seq_along(args), function(i) {
        formals(.FUN)[[names(args)[i]]] <<- args[[i]]
    }))
    .FUN
}

#

.paste <- hijack(paste, collapse = "_FOO_")

.paste(1:5)
.paste(1:5, LETTERS[1:5], sep="_BAR_")
.paste(1:5, collapse=NULL)


> .paste(1:5)
[1] "1_FOO_2_FOO_3_FOO_4_FOO_5"

> .paste(1:5, LETTERS[1:5], sep="_BAR_")
[1] "1_BAR_A_FOO_2_BAR_B_FOO_3_BAR_C_FOO_4_BAR_D_FOO_5_BAR_E"

> .paste(1:5, collapse=NULL)
[1] "1" "2" "3" "4" "5"
+4

.paste <- function(..., collapse = "_FOO_"){paste(..., collapse = collapse)}


> .paste <- function(..., collapse = "_FOO_"){paste(..., collapse = collapse)}
> .paste(1:5)
[1] "1_FOO_2_FOO_3_FOO_4_FOO_5"
> .paste(1:5, collapse = NULL)
[1] "1" "2" "3" "4" "5"
+2

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