Message-ID: <25095.33965.301976.460342@stat.math.ethz.ch>
Date: 2022-02-12T09:58:05Z
From: Martin Maechler
Subject: str2lang throws error when the string is empty
In-Reply-To: <3D923E32-CF64-4AC5-BE66-61048A30A55B@gmail.com>
>>>>> Dipterix Wang
>>>>> on Fri, 11 Feb 2022 06:55:44 -0500 writes:
> Hi,
> str2lang("") raises an error in current version.
on purpose.
> Would it
> be good if it returns a missing value expression?
Well, others may be able to better explain why "the empty name"
aka your "missing value expression" or just "the missing" is a
"dangerous" object and ideally it would not be available at all
on the R level. OTOH, it is available e.g. via alist(), maybe
slightly "less ugly" as alist(.=)$.
but I don't think there are really good use cases.
(see below)
> One use-case would be to build an expression that subsets an
> array:
> # Expected: x[index1, ]
> as.call(list(quote(`[`), quote(x), quote(index1), str2lang("")))
> Right now I'm using the following, which is ugly
> as.call(list(quote(`[`), quote(x), quote(index1), alist(x=)[[1]]))
Well, in such cases, much less ugly than both your version is to
use substitute(),
here e.g.,
> substitute(x[I,], list(I = quote(index1)))
x[index1, ]
>
> Thanks, - Dipterix
You are welcome,
Martin
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