[R] Lexical scoping is not what I expect

[John Fox]

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.?

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
> use...{{dropped:8}}
> >
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.