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

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]]

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!

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


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

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
> 
> 
>