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

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

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
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

Can I just mention the quick-and-dirty way that will get the job done?
Not as elegant as use the special features of the language *meant* for
this kind of job,
but suitable for a bear of very little brain.

1. Write a function that constructs the code you want as text, written
to a file.
2. source() the file.'

On Sat, 25 Oct 2025 at 15:23, 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
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Define the names and a prototype and then use formals(f) <- ... as shown.

  # inputs
  arg_names <- c("x", "y", paste0("a", 1:3),
paste0("b", 1:3))
  f <- function() {}

  arg <- alist(x = )
  formals(f) <- setNames(c(arg, rep(arg, length(arg_names)-1)), arg_names)
  f

giving

  function (x, y, a1, a2, a3, b1, b2, b3)
  {
  }

On Fri, Oct 24, 2025 at 10:23?PM 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
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.


-- 
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com