How to generate a pairlist with no defaults without using alist?

Question

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.

Answer 1

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

Answer 2

There isn't much scope for avoiding alist, other than stealing the pairlist from an anonymous function definition using formals:

as.function(c(formals(\(x){}), quote(x)))
#> function (x) 
#> x

Or stealing from a built-in function:

as.function(c(formals(mean)[1], quote(x)))
#> function (x) 
#> x