[Rd] Defining a `show` function breaks the print-ing of S4 object -- bug or expected?

[Duncan Murdoch]

On 30/06/2015 7:04 PM, Paul Gilbert wrote:
> 
> 
> On 06/30/2015 11:33 AM, Duncan Murdoch wrote:
>> On 30/06/2015 5:27 PM, Lorenz, David wrote:
>>> There is something I'm really missing here. The function show is a
>>> standardGeneric function, so the correct way to write it as method like
>>> this:
>>
>> That describes methods::show.  The problem is that the default print
>> mechanism isn't calling methods::show() (or base::print() as Luke says),
>> it's calling show() or print() in the global environment, so the user's
>> function overrides the generic, and you get the error.
> 
> These are two different problems aren't they? I can see that you might 
> want to ensure that base::print() calls methods::show(), but forcing the 
> default print to go to base::print(), rather than whatever print() is 
> first on the search path, would seem like a real change of philosophy. 
> What about all the other base functions that can be overridden by 
> something in the global environment?

I'd guess it's a minority of R users who know that print() or show() is
being called when you just evaluate an expression.  Most would think R
just shows you the value of the expression.  That's why they'd be
surprised when their local function suddenly stops the display of
variables from working.

On the other hand, if someone defined a print or show *method* in the
global environment, I think that one should override one defined in a
package namespace.  It does now, and I wouldn't change that.  The
difference is that I'd expect someone defining a method to know what
they're doing, but just defining a function doesn't imply that.

Duncan Murdoch

> 
> Paul
>>
>> Luke, are you going to look at this, or should I?
>>
>> Duncan Murdoch
>>
>>>
>>> setMethod("show",  "Person", function(object) {
>>>
>>> for an object of class "Person" for example.
>>
>>
>>> Dave
>>>
>>> On Tue, Jun 30, 2015 at 10:11 AM, <luke-tierney at uiowa.edu> wrote:
>>>
>>>> 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
>>>>
>>>>
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