[Rd] as.list method for by Objects

Hervé Pagès hpages at fredhutch.org
Tue Jan 30 23:24:17 CET 2018

I agree that it makes sense to expect as.list() to perform
a "strict coercion" i.e. to return an object of class "list",
*even* on a list derivative. That's what as( , "list") does
by default:

   # on a data.frame object
   as(data.frame(), "list")  # object of class "list"
                             # (but strangely it drops the names)

   # on a by object
   x <- by(warpbreaks[, 1:2], warpbreaks[,"tension"], summary)
   as(x, "list")  # object of class "list"

More generally speaking as() is expected to perform "strict
coercion" by default, unless called with 'strict=FALSE'.

That's also what as.list() does on a data.frame:

   as.list(data.frame())  # object of class "list"

FWIW as.numeric() also performs "strict coercion" on an integer

   as.numeric(1:3)  # object of class "numeric"

So an as.list.env method that does the same as as(x, "list")
would bring a small touch of consistency in an otherwise
quite inconsistent world of coercion methods(*).


(*) as(data.frame(), "list", strict=FALSE) doesn't do what you'd
     expect (just one of many examples)

On 01/29/2018 05:00 PM, Dario Strbenac wrote:
> Good day,
> I'd like to suggest the addition of an as.list method for a by object that actually returns a list of class "list". This would make it safer to do type-checking, because is.list also returns TRUE for a data.frame variable and using class(result) == "list" is an alternative that only returns TRUE for lists. It's also confusing initially that
>> class(x)
> [1] "by"
>> is.list(x)
> [1] TRUE
> since there's no explicit class definition for "by" and no mention if it has any superclasses.
> --------------------------------------
> Dario Strbenac
> University of Sydney
> Camperdown NSW 2050
> Australia
> ______________________________________________
> R-devel at r-project.org mailing list
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Hervé Pagès

Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N, M1-B514
P.O. Box 19024
Seattle, WA 98109-1024

E-mail: hpages at fredhutch.org
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