Skip to content

How to assign NULL value to pairlist element while keeping it a pairlist?

9 messages · Michael Lawrence, Martin Maechler, Luke Tierney +1 more

Original
Henrik Bengtsson
# 
Hi, I seem to not be able to assign NULL to an element of a pairlist
without causing it to be coerced to a plain list.  For example:
[1] "pairlist"
[1] "list"

This actually true for all [()<- assignments regardless of list value, e.g.
[1] "list"

I also tried assigning a pairlist(), but still the same problem:
[1] "list"

The only workaround I'm aware of is to:

x <- as.pairlist(x)

at the end.  Any other suggestions?

Thanks,

Henrik
Michael Lawrence
# 
The coercion is probably the most viable workaround for now, as it's
consistent with what happens internally for calls. All pairlists/calls
are converted to list for subassignment, but only calls are converted
back. My guess is that the intent was for users to move from using a
pairlist to the "new" (almost 20 years ago) list. In my opinion,
consistency trumps "convenience" in this case. If others agree, I'll
change it to also coerce back to pairlist.

Michael

On Wed, Oct 12, 2016 at 9:20 AM, Henrik Bengtsson
<henrik.bengtsson at gmail.com> wrote:
Michael Lawrence
# 
Hi Henrik,

It would help to understand your use case for pairlists.

Thanks,
Michael
On Wed, Oct 12, 2016 at 9:40 AM, Michael Lawrence <michafla at gene.com> wrote:
Henrik Bengtsson
# 
Michael, thanks for this info.

I've stumbled upon this in a case where I walk an R expression (the
AST) and (optionally) modifies it (part of the globals package).  In R
expressions, a function definition uses a pairlist to represent the
arguments.  For example,
List of 4
 $ : symbol function
 $ :Dotted pair list of 1
  ..$ x: num 1
 $ : symbol x
 $ :Class 'srcref'  atomic [1:8] 1 15 1 29 15 29 1 1
.. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile'
<environment: 0x13918b8>

Here the 2nd element is a pairlist:
Dotted pair list of 1
 $ x: num 1
[1] "pairlist"

Now say that I want to update the default value of argument 'x', which
is currently 1, to NULL.  Then I do:
At this step, I end up with an expression 'expr' where the arguments
are no longer represented by a pairlist:
List of 1
 $ x: NULL
[1] "list"

More importantly, at this point 'expr' no longer holds a valid R expression:
Error: badly formed function expression

The solution is to make sure we have a pairlist:
function(x = NULL) x


I agree it would be nice to fix this for consistency, but if you bump
into major issues, at least I can live with having to use an explicit
as.pairlist().

Thanks

Henrik

On Wed, Oct 12, 2016 at 10:53 AM, Michael Lawrence
<lawrence.michael at gene.com> wrote:
Michael Lawrence
# 
Thanks, this was what I expected. There is a desire to eliminate the
usage of pairlist from user code, which suggests the alternative of
allowing for function arguments to be stored in lists. That's a much
deeper change though.

On Wed, Oct 12, 2016 at 12:31 PM, Henrik Bengtsson
<henrik.bengtsson at gmail.com> wrote:
2 days later
Martin Maechler
# 
> Thanks, this was what I expected. There is a desire to
    > eliminate the usage of pairlist from user code, which
    > suggests the alternative of allowing for function
    > arguments to be stored in lists. That's a much deeper
    > change though.

and I hope we would not go there just for the purpose of
eliminating pairlists from user code, would we ?

As nobody else has mentioned it, I'd really  like to mention the
two (actually 3) functions important for dealing with function
argument lists much more transparently than the
as.list(<function>) things below:

  formals(<f>)
  formals(<f>) <- <arglist>      #  and
  alist()

for creating / modifying function argument lists (which are
pairlists, but the user does not need to know really).
Or did you imply, Henrik, that would you want is not achievable
with these?

Martin

    > On Wed, Oct 12, 2016 at 12:31 PM, Henrik Bengtsson
> <henrik.bengtsson at gmail.com> wrote:
>> Michael, thanks for this info.
    >> 
    >> I've stumbled upon this in a case where I walk an R expression (the
    >> AST) and (optionally) modifies it (part of the globals package).  In R
    >> expressions, a function definition uses a pairlist to represent the
    >> arguments.  For example,
    >> 
    >>> expr <- quote(function(x = 1) x)
    >>> str(as.list(expr))
    >> List of 4
    >> $ : symbol function
    >> $ :Dotted pair list of 1
    >> ..$ x: num 1
    >> $ : symbol x
    >> $ :Class 'srcref'  atomic [1:8] 1 15 1 29 15 29 1 1
    >> .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile'
    >> <environment: 0x13918b8>
    >> 
    >> Here the 2nd element is a pairlist:
    >> 
    >>> str(expr[[2]])
    >> Dotted pair list of 1
    >> $ x: num 1
    >>> typeof(expr[[2]])
    >> [1] "pairlist"
    >> 
    >> Now say that I want to update the default value of argument 'x', which
    >> is currently 1, to NULL.  Then I do:
    >> 
    >>> expr[[2]][1] <- list(x = NULL)
    >> 
    >> At this step, I end up with an expression 'expr' where the arguments
    >> are no longer represented by a pairlist:
    >> 
    >>> str(expr[[2]])
    >> List of 1
    >> $ x: NULL
    >>> typeof(expr[[2]])
    >> [1] "list"
    >> 
    >> More importantly, at this point 'expr' no longer holds a valid R expression:
    >> 
    >>> expr
    >> Error: badly formed function expression
    >> 
    >> The solution is to make sure we have a pairlist:
    >> 
    >>> expr[[2]] <- as.pairlist(expr[[2]])
    >>> expr
    >> function(x = NULL) x
    >> 
    >> 
    >> I agree it would be nice to fix this for consistency, but if you bump
    >> into major issues, at least I can live with having to use an explicit
    >> as.pairlist().
    >> 
    >> Thanks
    >> 
    >> Henrik
    >> 
    >> On Wed, Oct 12, 2016 at 10:53 AM, Michael Lawrence
>> <lawrence.michael at gene.com> wrote:
>>> Hi Henrik,
    >>> 
    >>> It would help to understand your use case for pairlists.
    >>> 
    >>> Thanks,
    >>> Michael
    >>>
>>> On Wed, Oct 12, 2016 at 9:40 AM, Michael Lawrence <michafla at gene.com> wrote:
>>>> The coercion is probably the most viable workaround for now, as it's
    >>>> consistent with what happens internally for calls. All pairlists/calls
    >>>> are converted to list for subassignment, but only calls are converted
    >>>> back. My guess is that the intent was for users to move from using a
    >>>> pairlist to the "new" (almost 20 years ago) list. In my opinion,
    >>>> consistency trumps "convenience" in this case. If others agree, I'll
    >>>> change it to also coerce back to pairlist.
    >>>> 
    >>>> Michael
    >>>> 
    >>>> On Wed, Oct 12, 2016 at 9:20 AM, Henrik Bengtsson
>>>> <henrik.bengtsson at gmail.com> wrote:
>>>>> Hi, I seem to not be able to assign NULL to an element of a pairlist
    >>>>> without causing it to be coerced to a plain list.  For example:
    >>>>>
>>>>> [1] "pairlist"
    >>>>>
>>>>> [1] "list"
    >>>>> 
    >>>>> This actually true for all [()<- assignments regardless of list value, e.g.
    >>>>>
>>>>> [1] "list"
    >>>>> 
    >>>>> I also tried assigning a pairlist(), but still the same problem:
    >>>>>
>>>>> [1] "list"
    >>>>> 
    >>>>> The only workaround I'm aware of is to:
    >>>>> 
    >>>>> x <- as.pairlist(x)
    >>>>> 
    >>>>> at the end.  Any other suggestions?
    >>>>> 
    >>>>> Thanks,
    >>>>> 
    >>>>> Henrik
    >>>>> 
    >>>>> ______________________________________________
    >>>>> R-devel at r-project.org mailing list
    >>>>> https://stat.ethz.ch/mailman/listinfo/r-devel

    > ______________________________________________
    > R-devel at r-project.org mailing list
    > https://stat.ethz.ch/mailman/listinfo/r-devel
4 days later
Henrik Bengtsson
# 
On Sat, Oct 15, 2016 at 2:00 AM, Martin Maechler
<maechler at stat.math.ethz.ch> wrote:
Martin, thanks for bringing these options up.  Just to make sure no
one misreads these comments - I'm not operating on as.list(expr) - it
was just used to show the content of the expression.    In my case,
I'm operating / modifying expressions and not functions per se, e.g.

  expr <- quote(function(x = 1) x)

It's not clear to me how I would go about to modify this to be the
equivalent of:

  expr <- quote(function(x = NULL) x)

without having to:

  f <- eval(expr)
  formals(f) <- list(x=NULL)
  expr <- bquote(.(f))

/Henrik
Luke Tierney
#
On Wed, 19 Oct 2016, Henrik Bengtsson wrote:

            

      
      
      
    

        
You don't need to go through the eval\bquote if you use

expr <- quote(function(x = 1) x)
alist <- expr[[2]]
alist[1] <- list(NULL)
expr[[2]] <- as.pairlist(alist)

Best,

luke

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Henrik Bengtsson
    

    

      
      #
    

  


  

      
Thanks Luke.  Yes, this is what I wrote as a workaround in my original
post (and my first follow up):

expr[[2]][1] <- list(x = NULL)
expr[[2]] <- as.pairlist(expr[[2]])

but your

alist <- expr[[2]]
alist[1] <- list(NULL)
expr[[2]] <- as.pairlist(alist)

makes it a bit more clear what the issue is; a pairlist 'x' (here
expr[[2]] and expr) gets coerced to a plain list if one try to do a
`[<-()` assignment.  It would be nice / useful / less surprising /
more consistent(?) if it would remain a pairlist also in those cases.

/Henrik

        
On Wed, Oct 19, 2016 at 11:47 AM,  <luke-tierney at uiowa.edu> wrote: