[Rd] A trap for young players with the lapply() function.

Rolf Turner r.turner at auckland.ac.nz
Thu Mar 30 04:53:39 CEST 2017

On 29/03/17 20:32, Enrico Schumann wrote:
> (inline)
> On Tue, 28 Mar 2017, Rolf Turner writes:
>> On 28/03/17 04:21, Barry Rowlingson wrote:
>>> On Mon, Mar 27, 2017 at 1:17 AM, Rolf Turner <r.turner at auckland.ac.nz> wrote:
>>>> Is there any way to trap/detect the use of an optional argument called
>>>> "X" and thereby issue a more perspicuous error message?
>>>> This would be helpful to those users who, like myself, are bears of very
>>>> little brain.
>>>> Failing that (it does look impossible)
>>> You can get the names of named arguments:
>>>  > z = function(x,X){cos(x*X)}
>>>  > names(formals(z))
>>>  [1] "x" "X"
>> That doesn't seem to help.  I tried putting a browser inside lapply()
>> and looked at formals(FUN).  This gave NULL, because FUN has already
>> been taken to be the list argument "x" to which lapply() is being
>> applied.
> You can get the call, without the arguments being
> matched, with `sys.call`. In your call of lapply,
> saying `sys.call()` before anything else would give
> you
>     lapply(y, function(x, X) {
>         cos(x * X)
>     }, X = 2 * pi)
> which would allow you to get at the argument names of
> your function.
>     if ("X" %in% names(sys.call()[[3L]][[2L]]))
>        warnings("found 'X'")
> But more would be needed: you need to figure out which
> argument you actually meant to be FUN. (In the above,
> I simply assumed it was the second passed argument.)
> That is, you would need to figure out which passed
> argument is a function, which is not safe either,
> since ... may also contain functions.

This idea appears to work for the particular example that I used, but it 
is not clear to me how to make it work in general.  E.g. if we define

   foo <- function(x,X){X*x}

and then do


we find that sys.call[[3]] is of length 1 and consists only of the 
*name* "foo".  One can then inspect


and find "X" therein, on the basis of which to trigger a warning.

However I don't know how to approach distinguish the two cases, or how 
to discern if there may be other cases lurking in the bushes.

So the problem still looks insoluble to me.



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