Htydjtk

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?

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.

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.

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.)

After digging a little bit through the source code, here are a few observations:

1. The actual function creation is done by mkCLOSXP(). This is what gets called by function() {}, by as.function.default() and by .Primitive("function") (a.k.a. `function`)




2. as.function.default() gets routed to do_asfunction(), which also calls CheckFormals(). However, it directly constructs these formals a few lines above that.




3. As you pointed out, the other place where CheckFormals() gets called is inside do_function(). However, I don't think do_function() gets called by anything other than .Primitive("function"), so this is the only situation where CheckFormals() is called on the user's input.




4. CheckFormals() does actually correctly validate a pairlist object.

You can check the last point yourself by running parts of the CheckFormals() function using inline::cfunction

inline::cfunction( c(x="ANY"),

  'Rprintf("is list?: %d\nTag1 OK?: %d\nTag2 OK?: %d\nTag3 NULL?: %d\n",

     isList(x), TYPEOF(TAG(x)) == SYMSXP, TYPEOF(TAG(CDR(x))) == SYMSXP,

     CDR(CDR(x)) == R_NilValue); return R_NilValue;' )( formals(mean) )



# is list?: 1

# Tag1 OK?: 1

# Tag2 OK?: 1

# Tag3 NULL?: 1

So, somewhere between you passing formals(means) to .Primitive("function") and it getting forwarded to CheckFormals() by do_function(), the argument loses its validity. (I don't know the R source well enough to tell you how that happens.) However, since do_function() is only called by .Primitive("function"), you don't encounter this situation with any other examples.