[R-pkg-devel] Using function with same name as argument as default argument
John Fox
j|ox @end|ng |rom mcm@@ter@c@
Mon Aug 8 18:57:26 CEST 2022
Dear Jan,
Here's yet another solution that takes advantage of partial matching:
> foo <- function(x, schema. = schema(x)) {
+ if (is.null(schema.)) stop("schema missing")
+ x
+ }
> schema <- function(x) attr(x, "schema")
> y <- 1
> z <- 2
> attr(y, "schema") <- "bar"
> foo(y)
[1] 1
attr(,"schema")
[1] "bar"
> foo(z)
Error in foo(z) : schema missing
> foo(z, schema="baz")
[1] 2
I hope this helps,
John
On 2022-08-08 9:10 a.m., Jan van der Laan wrote:
> Not sure if this belongs on r-help or r-package-devel; decided for the
> latter as the question is mainly relevant when writing code to be used
> by others.
>
> The issue I run into is that I want to write a function similar to:
>
>
> foo <- function(x, schema = schema(x)) {
> if (is.null(schema)) stop("schema missing")
> # ...
> }
>
> with 'schema()' something like (strongly simplified):
>
> schema <- function(x) attr(x, "schema")
>
>
> However using both the argument schema and calling the schema() function
> in one of the default arguments is not allowed ans results in the
> following somewhat cryptic error message:
>
> Error in foo(1:3) :
> promise already under evaluation: recursive default argument
> reference or earlier problems?
>
> I am looking for a clean solution to this. I can rename the argument or
> function, but calling it something other than schema feels impractical
> as both refer to the same thing (the schema of x). The best solution I
> have come up with until now is to define a second function to be used in
> default function arguments:
>
> schema_ <- schema
>
> foo <- function(x, schema = schema_(x)) {
> if (is.null(schema)) stop("schema missing")
> # ...
> }
>
> I guess another solution would be:
>
> foo <- function(x, schema) {
> if (missing(schema)) schema <- schema(x)
> }
>
> But then it is not clear for the user from the interface that the
> 'schema' argument can be omitted.
>
> I am hoping some of you have other suggestions.
>
> And, is this something that could be 'fixed' in the R-parser?
>
>
> Thanks,
> Jan
>
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--
John Fox, Professor Emeritus
McMaster University
Hamilton, Ontario, Canada
web: https://socialsciences.mcmaster.ca/jfox/
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