Function generation; change defaults of other functions (partial)

Question

I have the need for a function generator that takes another function and any arguments of that function and sets new defaults. I thought @hadley's pryr::partial was that magic function. It does exactly what I want except you can't then change that new default. So here I can change sep in my new paste function but not the new default of collapse = "_BAR_". How can I make partial perform this way (i.e., default to collapse = "_BAR_" but enable setting it to collapse = NULL if desired)? If this is not possible with partial is there a way to rewrite the code for partial to 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

Perhaps, also, `Defaults::setDefaults` could be helpful? – alexis_laz Aug 18 '14 at 10:10 — alexis_laz, Aug 18 '14 at 10:10

score 7 · Accepted Answer · answered Aug 18 '14 at 02:11

7

partial is good for fixing certain parameter values, but if you want to change defaults, you might consider a different strategy. This would work

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

This changes the parameters to the function

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

Then you can do

.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"

answered Aug 18 '14 at 02:11

MrFlick

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How about removing the default altogether? – Aaron McDaid Sep 04 '15 at 15:45
(to answer my own comment) To remove a default, it appears to be sufficient to do `formals(.paste)$collapse <- alist(dummy=)$dummy`. Now `.paste` has no default for `collapse`. – Aaron McDaid Sep 04 '15 at 16:16

score 5 · Answer 2 · answered Aug 18 '14 at 14:58

This is a canned function taking @MrFlick's great response and putting it into a function for future searchers:

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

    .FUN <- FUN

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

# Now Try It

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

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

Yielding

> .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"

score 2 · Answer 3 · answered Aug 19 '14 at 02:26

2

You could just write a simple wrapper

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

which gives

> .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"

answered Aug 19 '14 at 02:26

Dason

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True but the idea was to make it have the same function arguments. – Tyler Rinker Aug 19 '14 at 02:54
And this doesn't how? You can still pass in `sep` if you want. – Dason Aug 19 '14 at 02:57

Function generation; change defaults of other functions (partial)

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