R devel - Jun 2015 - Defining a `show` function breaks the print-ing of S4 object -- bug or expected?

On 30/06/2015 1:57 PM, Hadley Wickham wrote:
> A slightly simpler formulation of the problem is:
> 
> show <- function(...) stop("My show!")
> methods::setClass("Person", slots = list(name =
"character"))
> methods::new("Person", name = "Tom")
> #> Error in (function (...)  : My show!
Just to be clear:  the complaint is that the auto-called show() is not
methods::show?  I.e. after

x <- methods::new("Person", name = "Tom")

you would expect

show(x)

to give the error, but not

x

??

Duncan Murdoch
> 
> Hadley
> 
> On Tue, Jun 30, 2015 at 9:02 AM, Dean Attali <daattali at gmail.com>
wrote:
>> Hi r-devel
>>
>> If you define a function named `show` or attach a package with an
exported
>> `show` function, then printing/vieweing S4 objects breaks. This is
probably
>> because the `print` function calls `show`, which is now masked.
>>
>> Example:
>>
>> show <- function() {}
>>> setClass("Person", slots = list(name =
"character"))
>>> tom <- new("Person", name = "Tom")
>>> tom # error
>>> methods::show(tom) # works
>>
>>
>> The error was a surprise to me because  I was under the assumption that
>> `show` would be namespaced and therefore should not break.
>> I'm wondering if this is intended behaviour, or if this is a
problem.  My
>> intuition, and Hadley agreed on Twitter, is that defining a `show`
method
>> should not have such a grave effect on printing S4 objects.
>>
>> Thanks
>>
>> ---
>> http://deanattali.com
>>
>>         [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
> 
> 
>

On Tue, Jun 30, 2015 at 2:20 PM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:
> On 30/06/2015 1:57 PM, Hadley Wickham wrote:
>> A slightly simpler formulation of the problem is:
>>
>> show <- function(...) stop("My show!")
>> methods::setClass("Person", slots = list(name =
"character"))
>> methods::new("Person", name = "Tom")
>> #> Error in (function (...)  : My show!
>
> Just to be clear:  the complaint is that the auto-called show() is not
> methods::show?  I.e. after
>
> x <- methods::new("Person", name = "Tom")
>
> you would expect
>
> show(x)
>
> to give the error, but not
>
> x
>
> ??
Correct - I'd expect print() to always call methods::show(), not
whatever show() is first on the search path.

Hadley

-- 
http://had.co.nz/

Same thing happens with S3 if you redefine print(). I thought that
code was actually calculating the function to call rather than the
symbol to use, but apparently not. Shouldn't be too hard to fix.

luke

On Tue, 30 Jun 2015, Hadley Wickham wrote:
> On Tue, Jun 30, 2015 at 2:20 PM, Duncan Murdoch
> <murdoch.duncan at gmail.com> wrote:
>> On 30/06/2015 1:57 PM, Hadley Wickham wrote:
>>> A slightly simpler formulation of the problem is:
>>>
>>> show <- function(...) stop("My show!")
>>> methods::setClass("Person", slots = list(name =
"character"))
>>> methods::new("Person", name = "Tom")
>>> #> Error in (function (...)  : My show!
>>
>> Just to be clear:  the complaint is that the auto-called show() is not
>> methods::show?  I.e. after
>>
>> x <- methods::new("Person", name = "Tom")
>>
>> you would expect
>>
>> show(x)
>>
>> to give the error, but not
>>
>> x
>>
>> ??
>
> Correct - I'd expect print() to always call methods::show(), not
> whatever show() is first on the search path.
>
> Hadley
>
>
-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu