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

[Rolf Turner]

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.

>> might it not be a good idea to
>> add a warning to the help for lapply(), to the effect that if FUN has an
>> optional argument named "X" then passing this argument via "..." will
>> cause this argument to be taken as the first argument to lapply() and
>> thereby induce an error?
>
> Another idea might be to use purrr:map instead, which is quite happy
> with X in your function:
>
>  >  xxx <- purrr::map(y,function(x,X){cos(x*X)},X=2*pi)
>  > xxx
> [[1]]
> [1] 0.08419541
>
> [[2]]
> [1] 0.6346404
>
> [[3]]
> [1] 0.9800506
>
> [[4]]
> [1] 0.8686734
>
> [[5]]
> [1] -0.9220073
>
> But don't feed `.x` to your purrrring cats, or fails silently:
>
>  >  xxx <- purrr::map(y,function(x,.x){cos(x*.x)},.x=2*pi)
>  > xxx
> [[1]]
> NULL
>
> But who would have a function with `.x` as an argument?

Indeed.  It struck me that a possible workaround would be to change the 
name of the first argument of lapply() from "X" to ".X".  No-one would 
have a function with an argument names ".X" --- at least I wouldn't, so 
this would solve the problem for me.

It seems to me that this change could be made without breaking anything.
But perhaps I am engaging in my usual PollyAnna-ish optimism! :-)

cheers,

Rolf

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