[Rd] substitute(x$y)) corrupts 'y' component (PR#731)

[Thomas Lumley]

On Wed, 8 Nov 2000 cberry@tajo.ucsd.edu wrote:

> viz.
> 
> 
> > (function(x,y=a) substitute(x$y))(x)
> x$a
> > (function(x,y=a) substitute(x$y))(list(y=1:3))
> list(y = 1:3)$a
> > (function(x,y) eval(substitute(substitute(y))))(x=list(y=1:3),y=x$y)
> list(y = 1:3)$x$y
> 
> 
> The behavior I expect and want is like that in Splus 3.4:
> 
> > (function(x,y=a) substitute(x$y))(x)
> x$y
> > (function(x,y=a) substitute(x$y))(list(y=1:3))
> list(y = 1:3)$y
> > (function(x,y) eval(substitute(substitute(y))))(x=list(y=1:3),y=x$y)
> list(y = 1:3)$y


I don't think this is a bug - substitute is performing as intended and as
documented -- but it is an incompatibility.  There are two issues here,
one easy and one difficult.

In your first two examples the problem is that you want to substitute only
for x (though in the second example I don't see why you expect this to
happen).  If you want to substitute only for x you can do
	substitute(x$y,list(x=x))


In the third example you want to perform substitution on the value of a
variable.  This is a useful thing to want to do, and I don't know how to
do it.

eg
     z<-quote(x$y)
     x<-list(y=1:3)
and you want to produce
     list(y=1:3)$y

Suggestions?

	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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