From Waclaw.Marcin.Kusnierczyk at idi.ntnu.no  Fri Feb  6 11:44:40 2009
From: Waclaw.Marcin.Kusnierczyk at idi.ntnu.no (Wacek Kusnierczyk)
Date: Fri, 06 Feb 2009 11:44:40 +0100
Subject: [Rd] Bug in the parser (?) (PR#13504)
In-Reply-To: <498AF44D.6080805@biostat.ku.dk>
References: <20090205131004.AE42C283415C@mail.pubhealth.ku.dk>
	<498AF44D.6080805@biostat.ku.dk>
Message-ID: <498C1498.6050305@idi.ntnu.no>

Peter Dalgaard wrote:
> johnc.deva at yahoo.com wrote:
>   
>> Full_Name: John C. Deva
>> Version: 2.8.1
>> OS: Fedora Linux 8, 64 bit
>> Submission from: (NULL) (193.200.150.189)
>>
>>
>> I notice that it is possible to redefine 'if' as a function of an arbitrary
>> number of arguments.  Such redefined 'if' can then be used as any other user
>> function, except for that the parser still demands exactly three arguments to be
>> given to if.  Furthermore, even if 'if' is defined with three arguments, its
>> application must still be made with the original syntax, and not the usual
>> syntax of function application:
>>
>>     
>>> `if` <- function(a,b,c) 
>>>       
>> + {
>> + assign(deparse(substitute(a)), b+c, envir=parent.frame()
>> + }
>>
>>     
>>> if (x) 1 else 2
>>> x
>>>       
>> [1] 3
>>
>>     
>>> if(x, 1, 2)
>>>       
>> Error: unexpected ',' in "if(x,"
>>
>> The later expression above should be the only valid when 'if' is redefined as
>> above, but it is not the case.
>>     
>
>
> A bug report with a ? in the title is in general ill-advised. If you are
> not sure that something is a bug, then ask a question instead.
>
> In this case, no, it is not a bug. Syntax is syntax and "if(x,1,2)" is a
> syntax error no matter how if() is defined. Parsing is controlled by the
> fact that "if" is a keyword, the function definition is not used at all
> at that stage. `if`(x,1,2) works perfectly well, though, at least until
> you try deparsing it:
>
>   
>> quote(`if`(x,1,2))
>>     
> if (x) 1 else 2
>
>
> Redefining "if" is a really Bad Idea, excepting things like code
> analysis tools (which I believe are the main reason it is not explicitly
> forbidden).
>
>   

i'm not quite sure i'm getting this last point (after that redefining
'if' is a bad idea, which it is, mostly).

on further exploration, i found that r can be used to produce fancy
patterns:

'function' = function(...) { print(list(...)); function(...) list(...) }
# a function that prints its arguments and calls itself with the
arguments and a list of the arguments

# maximize the terminal, minimize the font size, and execute:
function(whatever) NULL

enjoy.

vQ



[Rd] Bug in the parser (?) (PR#13504)
Wacek Kusnierczyk