Well, two comments, in two non-compatible directions.

1.  I have to say that I find the idea of using "+" to paste character 
strings together aesthetically ugly.

IMO, one thing that makes functional object-based languages attractive 
is that the generic function retains a consistent _function_, that is, 
purpose and meaning, of which the methods are implementations.

It escapes me totally why I should think of pasting strings as addition 
in the mathematical or intuitive sense (as Brian points out re 
commutativity, it fails a number of axiomatic properties).  And if so, 
what about "-", "*",  "/" and so on?  The mind boggles.

Its excuse presumably is to save typing, but I would favor using some 
%thing% operator at the cost of a couple of extra key strokes.

2.  Having said that,  it's a reasonable hope that efficiency of 
dispatch will not be a serious problem.  There are a bunch of fixes, for 
semantic correctness and efficiency, nearly ready to commit (the 
Bioconductor folks have been doing some valuable testing).  These should 
help, and more important perhaps it's fairly easy to see how dispatch in 
this form can be tuned for performance if necessary.

John

Bill Dunlap wrote:
>>>     >> There have been propositions to make "+" work in S (and
>>>     >> R) like in some other languages, namely for character
>>>     >> (vectors),
>>>     >>
>>>     >> a + b := paste(a,b, sep="")
>>> ...
>>> yes.  I think however if we keep speed and clarity and catching
>>> user errors all in mind, it would be enough - and better - to
>>> only dispatch to paste(.,.) when both arguments are character
>>> (vectors), i.e., the above case needed
>>>  "a" + as.character(1:7) or "a" + paste(1:7) or "a" + format(1:7)
>>> which after all is really more clearer, even more for cases of
>>>  "1" + 2  which I'd rather want keeping to give errors.
>>>
>>> If  Char + Num  should work like above, then also
>>>     Num + Char  should (since after all,  "+" should be commutative
>>> 			apart from floating point precision issues).
>>>
>>> and so the internal C code gets a bit more complicated and slightly
>>> slower..  something we had in mind we should strongly avoid...
>>>       
>> I doubt that it would be measurably slower, but I agree that requiring
>> both args to be Char could be done in fewer operations than just
>> requiring one.
>>
>> However, I think the consistency argument is stronger.  We have a rule
>> that operations on mixed types promote the more restrictive type to the
>> less restrictive one, and I don't think we should handle this case
>> differently.
>>
>> So I'd say we should allow all of Char + Num, Num + Char, and Char +
>> Char, or, if this costs too much at evaluation time, we shouldn't allow
>> any of them.
>>     
>
> Currently doing arithmetic on mixed class data.frames
> produces useful warnings and errors.  E.g.,
>
>   > z <- data.frame(Factor=factor(c("Lo","Med","High")),
>                   Char=letters[1:3],
>                   Num1=exp(0:2),
>                   Num2=(1:3)*pi,
>                   stringsAsFactors=FALSE)
>   > z+1
>   Error in FUN(left, right) : non-numeric argument to binary operator
>   In addition: Warning message:
>   + not meaningful for factors in: Ops.factor(left, right)
>   > z[,-2] + 1
>     Factor     Num1      Num2
>   1     NA 2.000000  4.141593
>   2     NA 3.718282  7.283185
>   3     NA 8.389056 10.424778
>   Warning message:
>   + not meaningful for factors in: Ops.factor(left, right)
>
> If we made + do paste(sep="") for character+number then
> we would lose the messages and let garbage flow further
> down the pipe.
>
> Should factor data be treated as character data in this
> case (e.g., pasting to the levels)?  That would be weird,
> but many users confound character and factor data when
> they are buried in data.frames.
>
> ----------------------------------------------------------------------------
> Bill Dunlap
> Insightful Corporation
> bill at insightful dot com
> 360-428-8146
>
>  "All statements in this message represent the opinions of the author and do
>  not necessarily reflect Insightful Corporation policy or position."
>
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>   

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