On 28/03/17 04:21, Barry Rowlingson wrote:
On Mon, Mar 27, 2017 at 1:17 AM, Rolf Turner <r.tur...@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
--
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276
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