On Sat, Oct 15, 2016 at 2:00 AM, Martin Maechler <maech...@stat.math.ethz.ch> wrote: >>>>>> Michael Lawrence <lawrence.mich...@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)) /Henrik > > Martin > > > 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 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
