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.bengts...@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.mich...@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 <micha...@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.bengts...@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@r-project.org mailing list >>>> https://stat.ethz.ch/mailman/listinfo/r-devel ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
