Declare All Function Parameters in R Without Calling the Function

Question

I want to a function where I throw any given other function with multiple parameters that use both single and double quotes.

For example:

1. just_params("some_function(param1="this",param2='that')")

or

2. just_params(some_function(param1="this",param2='that'))

which results in the function not being called but two new string variables param1 and param2 being declared in the environment. Either options works but I think #1 would be easier.

Throwing this all in a string directly is tough because of a mix of double and single quotes. I am hoping to use this while debugging other people's code and am assuming that they are using double and single quotes willy-nilly.

Yes, I know I can just copy and delete the commas but sometimes I am debugging function calls with 20 parameters (also cringe, I know) but that just how it is and I don't want to go in and copy and paste and delete commas each time.

Answer 1

Here's one way to do it:

just_params <- function(call) {
  # Get the expression that was in the call without evaluating it.
  
  call <- substitute(call)
  
  # Expand all arguments to their full names.  This needs the
  # called function to exist and be visible to our caller.
  
  call <- match.call(get(call[[1]], mode = "function", 
                         envir = parent.frame()), 
                     call)
  
  # Change the name of the function to "list" to return the arguments
  
  call[[1]] <- `list`
  
  # Evaluate it in the parent frame
  
  eval(call, envir = parent.frame())
}

some_function <- function(param1, param2, longname) {}

just_params(some_function(param1 = "this", param2 = 'that', long = 1 + 1))
#> $param1
#> [1] "this"
#> 
#> $param2
#> [1] "that"
#> 
#> $longname
#> [1] 2

^{Created on 2023-09-03 with reprex v2.0.2}

Notes:

• If the call abbreviated a name you'll see the full name (the example used long instead of longname)
• If the call used an expression, it will be evaluated (the example used 1 + 1 but returned 2.
• The type of quotes is not retained.