[R-pkg-devel] Using function with same name as argument as default argument

Hi Andrew and Ivan,

Thanks for the suggestion. Both are good suggestions; better than what I
have.


Andrews solution:

  > foo <- function (x, schema = schema(x))
  > {
  >      if (missing(schema)) {
  >          rm(schema)
  >          schema <- schema(x)
  >      }
  > }

Ivans:

  > function(x, schema = mypackagename::schema(x))

Both would work in this case. Ivans is a bit cleaner/simpler; Andrews
appears simpler from a user perspective. Not sure yet which one I will
use; I am currently leaning towards Ivans solution.

Thanks!
Jan





On 08-08-2022 15:42, Andrew Simmons wrote:

Hello,


This isn't something that can be fixed in the parser. If an argument
isn't provided, its default value is evaluated inside the function, so
it gives you a loop where schema = schema(x), but then what's schema,
it's schema(x), thus the recursion error. you could do something like this:

foo <- function (x, schema = schema(x))
{
  ? ? if (missing(schema)) {
  ? ? ? ? rm(schema)
  ? ? ? ? schema <- schema(x)
  ? ? }
}

which, while kinda gross looking, means that schema = schema(x) can be
evaluated without causing a recursion error.

On Mon, Aug 8, 2022, 09:11 Jan van der Laan <rhelp at eoos.dds.nl
<mailto:rhelp at eoos.dds.nl>> 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