How do I call the `function` function?

Question

I am trying to call the function `function` to define a function in R code.

As we all know™️, `function`is a .Primitive that’s used internally by R to define functions when the user uses the conventional syntax, i.e.

mean1 = function (x, ...) base::mean(x, ...) 

But there’s nothing preventing me from calling that primitive directly. Or so I thought. I can call other primitives directly (and even redefine them; for instance, in a moment of madness I overrode R’s builtin `for`). So this is in principle possible.

Yet I cannot get it to work for `function`. Here’s what I tried:

# Works mean2 = as.function(c(formals(mean), quote(mean(x, ...))))  # Works mean3 = eval(call('function', formals(mean), quote(mean(x, ...))))  # Error: invalid formal argument list for "function" mean4 = `function`(formals(mean), quote(mean(x, ...))) 

The fact that mean3 in particular works indicates to me that mean4 should work. But it doesn’t. Why?

I checked the definition of the `function` primitive in the R source. do_function is defined in eval.c. And I see that it calls CheckFormals, which ensures that each argument is a symbol, and this fails. But why does it check this, and what does that mean?

And most importantly: Is there a way of calling the `function` primitive directly?

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Just to clarify: There are trivial workarounds (this question lists two, and there’s at least a third). But I’d like to understand how this (does not) works.

Answer 1

This is because function is a special primitive:

typeof(`function`) #> [1] "special" 

The arguments are not evaluated, so you have actually passed quote(formals(mean)) instead of the value of formals(mean). I don't think there's a way of calling function directly without evaluation tricks, except with an empty formals list which is just NULL.

Answer 2

For completeness’ sake, Lionel’s answer hints at a way of calling `function` after all. Unfortunately it’s rather restricted, since we cannot pass any argument definition except for NULL:

mean5 = `function`(NULL, mean(x, ...)) formals(mean5) = formals(mean) 

(Note the lack of quoting around the body!)

This is of course utterly unpractical (and formals<- internally calls as.function anyway.)