[R] Lexical scoping is not what I expect

[Prof Brian Ripley]

On 28/06/2013 15:54, John Fox wrote:
> Dear Duncan and Steve,
>
> Since Steve's example raises it, I've never understood why it's legal to
> change the built-in global "constants" in R, including T and F. That just
> seems to me to set a trap for users. Why not treat these as reserved
> symbols, like TRUE, Inf, etc.?

Because people wanted to use them as names of things.  Maybe a T (not t) 
statistic.

And BTW, you can change the value of T for yourself, but you will not 
change it for any package code, including R itself, since the base 
namespace is ahead of the workspace in namespace scoping.  Because of 
that, it is years since I have seen anyone actually trip themselves up 
over this.

>
> Best,
>   John
>
>> -----Original Message-----
>> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
>> project.org] On Behalf Of Duncan Murdoch
>> Sent: Friday, June 28, 2013 9:40 AM
>> To: S Ellison
>> Cc: r-help at r-project.org
>> Subject: Re: [R] Lexical scoping is not what I expect
>>
>> On 28/06/2013 9:28 AM, S Ellison wrote:
>>>
>>>>> I too find R's lexical scoping rules straightforward.
>>>>> However, I'd say that if your code relies on lexical
>>>>> scoping to find something, you should probably rewrite your code.
>>>>
>>>> Except of course that almost every function relies on lexical
>>>> scoping to some extent!
>>>
>>> This could get messy, because a) that's true and b) it actually leads
>> to some genuine risks when 'globals' get redefined or masked*.
>>>
>>> How about I amend the assertion to "if your code relies on lexical
>> scoping to find a variable you defined, you should probably rewrite
>> your code."
>>> and leave it at that, subject to some common sense about whether you
>> know what you're doing?
>>
>> That still isn't right, because users should feel free to define
>> functions and call them from their other functions.
>>
>> I think who defined it isn't the issue, the issue is whether it might
>> change unexpectedly.  The user owns globalenv().  The package author
>> owns the package namespace.  So packages should almost never read or
>> write things directly from/to globalenv() (the user might change them),
>> but they can create their own private environments and write there.
>>
>> Where it gets a little less clear is when the user writes a function.
>> I
>> would say functions should never write directly to globalenv(), but
>> it's
>> perfectly fine to reference constants there (like other functions
>> written by the user).  Referencing things there that change is the
>> risky
>> thing.
>>
>> Duncan Murdoch
>>
>>
>>>
>>> Steve E
>>>
>>>
>>> *Example
>>>> sin.deg  <- function(deg) sin(deg * pi/180)
>>>> sin.deg(45)
>>> [1] 0.7071068
>>> 	#looks about right
>>>
>>>> pi <- 3.2       #Indiana General Assembly bill #247, 1897.
>>>> sin.deg(45)
>>> [1] 0.7173561
>>> 	#oops ...
>>>
>>>
>>>
>>>
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>
> ______________________________________________
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> https://stat.ethz.ch/mailman/listinfo/r-help
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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