From luke-tierney at uiowa.edu  Wed Oct 19 20:47:45 2016
From: luke-tierney at uiowa.edu (luke-tierney at uiowa.edu)
Date: Wed, 19 Oct 2016 13:47:45 -0500
Subject: [Rd] How to assign NULL value to pairlist element while keeping
 it a pairlist?
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Message-ID: <alpine.DEB.2.20.1610191346290.2688@luke-Latitude>

On Wed, 19 Oct 2016, Henrik Bengtsson wrote:

> On Sat, Oct 15, 2016 at 2:00 AM, Martin Maechler
> <maechler at stat.math.ethz.ch> wrote:
>>>>>>> Michael Lawrence <lawrence.michael at gene.com>
>>>>>>>     on Wed, 12 Oct 2016 15:21:13 -0700 writes:
>>
>>    > 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, 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))

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
>
>>
>> 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:
>>    >>>>>
>>>>>>> x <- pairlist(1, 2)
>>>>>>> class(x)
>>    >>>>> [1] "pairlist"
>>    >>>>>
>>>>>>> x[1] <- list(NULL)
>>>>>>> class(x)
>>    >>>>> [1] "list"
>>    >>>>>
>>    >>>>> This actually true for all [()<- assignments regardless of list value, e.g.
>>    >>>>>
>>>>>>> x <- pairlist(1, 2)
>>>>>>> x[1] <- list(0)
>>    >>>>> [1] "list"
>>    >>>>>
>>    >>>>> I also tried assigning a pairlist(), but still the same problem:
>>    >>>>>
>>>>>>> x <- pairlist(1, 2)
>>>>>>> x[1] <- pairlist(0)
>>    >>>>> [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
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
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[Rd] How to assign NULL value to pairlist element while keeping it a pairlist?
luke-tierney at uiowa.edu