Lexical scoping is not what I expect
On 28/06/2013 10:54 AM, 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.?
If we weren't allowed to change T and F, would we be allowed to change other "constants", like functions mean or max? If we couldn't change anything defined in the base package, would we be allowed to change things defined in other packages like stats or utils or graphics? I think it's simply a matter of setting the line somewhere, and R has chosen to set it in the most permissive place that's reasonable. It assumes that its users know what they are doing. Why not allow changes to TRUE or Inf? TRUE is a constant, whereas T is a variable containing TRUE. Inf is also a constant, corresponding to a length one vector containing that value. Those are treated by the parser like 2 or "hello". It would be really bad if someone could change the meaning of 2 (though I hear some old Fortran compilers allowed that), but is it really so bad to allow someone to define their own plot function, or temperature variable named T? Duncan Murdoch
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 ... ******************************************************************* This email and any attachments are confidential. Any
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