I'm experimenting with simple manipulation of R code and trying to generate an object equivalent to
substitute(function(x) x)
I am aware I can do something around these lines
as.call(list(as.symbol("function"), as.pairlist(alist(x=)), as.symbol("x")))
but I am looking for a way to achieve as.pairlist(alist(x=)) without resorting alist or, if that's not possible, allowing me to generate equivalent expression without hard-coding the thing or parsing strings.
I was tinkering with things like as.call(list(as.symbol("="), as.symbol("x"))), but that's seems to be a dead end for now.
A programmatic alternative to as.pairlist(alist(...)) is to initialize a pairlist of positive length then assign names (well, "tags") and values:
zzz <- vector("pairlist", 1L)
names(zzz) <- "x"
zzz[[1L]] <- substitute()
identical(zzz, as.pairlist(alist(x = )))
## [1] TRUE
More generally:
pl <- function(n, tags, values) {
r <- vector("pairlist", n)
if (n >= 1L) {
names(r) <- tags
for (i in seq_len(n)) {
v <- values[[i]]
if (missing(v) || !is.null(v))
r[[i]] <- values[[i]]
}
}
r
}
identical(pl(3L, c("x", "y", "z"), list(substitute(), 0, NULL)),
as.pairlist(alist(x = , y = 0, z = NULL)))
## [1] TRUE
We condition on n >= 1L because a zero-length pairlist is NULL and assigning names or values to NULL is an error.
We condition on missing(v) || !is.null(v) because we don't want to assign NULL to r[[i]] (which would decrease the length of r by 1) and we don't want to call is.null with no argument:
v <- substitute()
missing(v)
## [1] TRUE
is.null(v)
## Error: argument "v" is missing, with no default