如何通过match.call编程？

Question

I'm trying to program over a function inside a package, but I'm stuck with the function internally using match.call() to parse one of its arguments.

A super-simplified example of the function with the usual utilization could look like this:

f1 = function(x, y=0, z=0, a=0, b=0){ #lots of arguments not needed for the example
  mc = match.call()
  return(mc$x)
  #Returning for testing purpose.
  #Normally, the function later uses calls as character:
  r1 = as.character(mc$x[1])
  r2 = as.character(mc$x[2])
  #...
}
x1 = f1(x = foo(bar))
x1
# foo(bar)
class(x1)
# [1] "call"

In my case, I need to get the value of x from a variable ( value in the following code). Expected utilisation of f1 is as following :

value = "foo(bar)" #this line could also be anything else
f1(x=some_magic_function(value))
# Expected result = foo(bar)
# Unwanted result = some_magic_function(value)

Unfortunately, match.call() always return the very input value. I'm quite out of my league here so I only tried few functions.

Is there any way I could trick match.call() so it could accept external variable ?

Failed attempts so far:

#I tried to create the exact same call using rlang::sym()
#This may not be the best way...
value = call("foo", rlang::sym("bar"))
value
# foo(bar)
class(value)
# [1] "call"
x1==value
# [1] TRUE

f1(x=value)
# value
f1(x=eval(value))
# eval(value)
f1(x=substitute(value))
# substitute(value)

Answer 1

There's nothing you can include as a parameter to f1 to make this work. Instead, you would dynamically need to build your call to f1 . With base R you might do this with do.call .

do.call("f1", list(parse(text=value)[[1]]))

or with rlang

eval_tidy(quo(f1(!!parse_expr(value))))