substitute creates an object which prints incorrectly (PR#9427)

Bill.Venables at csiro.au wrote:

Thanks Peter.  I see the dilemma.  It is serious in my view, though,
even if I can't see an elegant way round it.

I guess the only possibilities are

1. Only keep the source in printing or, much more seriously, dumping, if
the source attribute parses to an object structually identical to the
function itself (even I can see this is going to be impractical)

  

Yes, dumping is the serious issue here. One should also consider that 
inconsistencies can be created by other means than substitute(), e.g. by 
brute force.

2. Make the default keep.source option FALSE rather than TRUE and warn
people that switching it on can be unsafe in language manipulation.
This would be practical, I suggest, if comments were kept as part of the
function itself, as well as in the source attribute, but if comments are
only kept in the source attribute (as appears to be the case now) I
concede this becomes impractical.

  

The whole source attribute construction was created because there was no 
way (that we could find, anyway) to make comment and whitespace handling 
sane without either changing the language or making the parser insanely 
complicated. Old R hacks will remember how comments intended for the end 
of a for loop could move to the top when the function was listed.

The basic issue is that S's syntactic sugar coat was modeled on C, but C 
is a compiled language, so its parsers just skip comments and 
whitespace. Trying to retain comments leads to complications.... Even in 
a simple function call f(a=b), there are 7 places to stick in comments, 
so at the very least you need to deal with pre-comments, post-comments, 
and sometimes in-the-middle-comments. And what is worse, there are 
ambiguities, which make automatic parser generators unhappy: Does a 
comment between two expressions belong with the former or the latter? Etc...

3. Modify substitute() so that it strips source attributes (or anything
else apparently visible that it will not manipulate) from objects.
Sorry folks, too dangerous.  (I concede this appears to be a bit of an
overkill, too.)

  

Actually, this is the easier fix to my mind. Where do you see the danger?

Perhaps the compromise has to be to warn people that keep.source=TRUE
can be dangerous in this way, both in the help informaton for options()
and for substitute().

?

Bill Venables.

-----Original Message-----
From: Peter Dalgaard [mailto:p.dalgaard at biostat.ku.dk] 
Sent: Friday, 22 December 2006 9:47 PM
To: Venables, Bill (CMIS, Cleveland)
Cc: r-devel at stat.math.ethz.ch; R-bugs at biostat.ku.dk
Subject: Re: [Rd] substitute creates an object which prints incorrectly
(PR#9427)

Bill.Venables at csiro.au wrote:
  

The function "substitute" seems to fail to make a genuine
substitution, although the printed verision seems fine.  Here is an
example.

  
    

m <- substitute(Y <- function(x) FUN(x+1), 
    
      

+ list(Y = as.name("y"), FUN = as.name("sin")))
  
    

m
    
      

y <- function(x) sin(x + 1)
  
    

eval(m)
y
    
      

function(x) FUN(x+1)

However the story doesn't end there.  The substitution appears to have
been made, even though the printed version, this time, suggests
otherwise.

  
    

y(pi)
    
      

[1] -0.841471
  
    

sin(pi+1)
    
      

[1] -0.841471
  

  
    

Yes, this is (fairly) well known. It has to do with the retention of 
function source.

The thing to notice is that it is only the printing of y that is really 
confused. If you do

dput(y)
attr(y, "source")
attr(y, "source") <- NULL
y

then you should see the point. The tricky bit is that the "source" 
attribute exists in an intermediate form inside m. Notice that m 
contains, not the function itself, but a call to the function `function`

which creates the function when eval'ed. This call contains the function

source as its 4th element (look at m[[3]][[4]] in your example), and you

might try setting it to NULL and see how things will clear up.

The issue with substitute is that it cannot sensibly substitute into 
character vectors, so it just leaves  the source as is, which gives the 
symptoms you see. It could, however, and probably should, recognize 
calls to `function` and NULL out their 4th element. It cannot be done 
completely failsafe though (`function` could result from a computation, 
or even be part of the substitution), so one has to decide that the 
extreme cases are too extreme worry about them.
 
-pd

  

Bill Venables 
CMIS, CSIRO Laboratories, 
PO Box 120, Cleveland, Qld. 4163 
AUSTRALIA 
Office Phone (email preferred): +61 7 3826 7251 
Fax (if absolutely necessary):    +61 7 3826 7304 
Mobile (rarely used):                +61 4 1963 4642 
Home Phone:                          +61 7 3286 7700 
mailto:Bill.Venables at csiro.au 
http://www.cmis.csiro.au/bill.venables/ 


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