R help - Oct 2025 - Construct a function with argments specified by an R expression

I would start by making a list of the desired length, where each element is
the missing argument:

f <- rep(list(quote(expr = )), 2 + length(argnms))

then add names to the list:

names(f) <- c("x", "y", argnms)

and then use formals<-

formals(foo) <- f

Although, I would ask if you're certain that this is actually what you want
to do. It's usually a bad idea to have variable names that you don't
know.
I'd to think it'd be better to have the default argument:

a = list()

and supply a list of `a` values instead of several `a` arguments.

On Fri, Oct 24, 2025, 22:23 Rolf Turner <rolfturner at posteo.net> wrote:
>
> I want to build a function (say "buildFn") to *return* a function
of the
> form
>
>     foo(x,y,a1, ... an, b1, ..., bn)
>
> where the arguments ai and bi are given in the form of a list created
> in R.  E.g. I'd like to be able to say
>
>      argnms <-
c(paste0("a",1:3),paste0("b",1:3"))
>      foo <- buildFn(argnms)
>
> with the resulting "foo" having arguments x, y, a1, a2, a3, b1,
b2, b3.
>
> I thought I might be able to do this using "formals<-" but I
cannot get
> this to work.  i could provide more detail of what I've tried, but
it's
> probably not worth it.
>
> Can anyone steer me in the right direction?  Thanks.
>
> cheers,
>
> Rolf Turner
>
> --
> Honorary Research Fellow
> Department of Statistics
> University of Auckland
> Stats. Dep't. (secretaries) phone:
>          +64-9-373-7599 ext. 89622
> Home phone: +64-9-480-4619
>
> ______________________________________________
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On Fri, 24 Oct 2025 22:31:53 -0400
Iris Simmons <ikwsimmo at gmail.com> wrote:
> I would start by making a list of the desired length, where each
> element is the missing argument:
> 
> f <- rep(list(quote(expr = )), 2 + length(argnms))
> 
> then add names to the list:
> 
> names(f) <- c("x", "y", argnms)
> 
> and then use formals<-
> 
> formals(foo) <- f
Wow! That was fast!  And it seems to do exactly what I want.

What I was missing, I guess, was the quote(expr = ) structure.  I have
never really understood quote(), nor expr() either.  I'll just treat it
as a black box for now.
> Although, I would ask if you're certain that this is actually what
> you want to do. It's usually a bad idea to have variable names that
> you don't know. I'd to think it'd be better to have the default
> argument:
> 
> a = list()
> 
> and supply a list of `a` values instead of several `a` arguments.
Yes I understand that that would be the "sensible" approach but it
doesn't work in the context with which I am confronted.

I have more work to do before I get the approach that I've envisaged up
and running.

I may inflict more questions upon you later.  (No good deed goes
unpunished! ??)


cheers,

Rolf

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
Stats. Dep't. (secretaries) phone:
         +64-9-373-7599 ext. 89622
Home phone: +64-9-480-4619