R devel - May 2011 - Is it possible to define a function's arguments via a wildcard in 'substitute()'?

Dear List,

just out of pure curiosity: is it possible to define a function via 
'substitute()' such that the function's formal arguments are
specified
by a "wildcard" that is substituted when the expression is evaluated?

Simple example:

x.args <- formals("data.frame")
x.body <- expression(
     out <- myArg + 100,
     return(out)
)

expr <- substitute(
     myFoo <- function(
         ARGS,
         myArg
     ){
         print("hello world!")
         print(ARGS)
         eval(BODY)

     },
     list(ARGS=x.args, BODY=x.body)
)

eval(expr)
myFoo(myArg=5)
# works

myFoo(a=1:3, stringsAsFactors=FALSE, myArg=5)
# does not work

It works for wildcard 'BODY' in the function's body, but not for 
wildcard 'ARGS' in the argument definition part of the function
definition.

I thought that when writing a function that depends on some other 
function like 'data.frame()', it would maybe be possible not to 
'hardcode' the formal arguments of 'data.frame()' in the new
function
def but to have it mapped somewhat dynamically so that when 
'data.frame()' changes, the new function would change as well. This is 
probably a bad idea for countless reasons, nevertheless I'd be 
interested in learning if it's possible at all ;-)

TIA,
Janko

I think for the case where you want to built up a call from a function
name + list of arguments, it's best to use call or as.call:

call("f", a = 1, b = 2, c = 3)

or if you already have the list:
l <- list(as.name("f"), a = 1, b = 2, c = 3)
as.call(l)

Hadley

On Thu, May 26, 2011 at 10:15 AM, Janko Thyson
<janko.thyson.rstuff at googlemail.com> wrote:
> Dear List,
>
> just out of pure curiosity: is it possible to define a function via
> 'substitute()' such that the function's formal arguments are
specified by a
> "wildcard" that is substituted when the expression is evaluated?
>
> Simple example:
>
> x.args <- formals("data.frame")
> x.body <- expression(
> ? ?out <- myArg + 100,
> ? ?return(out)
> )
>
> expr <- substitute(
> ? ?myFoo <- function(
> ? ? ? ?ARGS,
> ? ? ? ?myArg
> ? ?){
> ? ? ? ?print("hello world!")
> ? ? ? ?print(ARGS)
> ? ? ? ?eval(BODY)
>
> ? ?},
> ? ?list(ARGS=x.args, BODY=x.body)
> )
>
> eval(expr)
> myFoo(myArg=5)
> # works
>
> myFoo(a=1:3, stringsAsFactors=FALSE, myArg=5)
> # does not work
>
> It works for wildcard 'BODY' in the function's body, but not
for wildcard
> 'ARGS' in the argument definition part of the function definition.
>
> I thought that when writing a function that depends on some other function
> like 'data.frame()', it would maybe be possible not to
'hardcode' the formal
> arguments of 'data.frame()' in the new function def but to have it
mapped
> somewhat dynamically so that when 'data.frame()' changes, the new
function
> would change as well. This is probably a bad idea for countless reasons,
> nevertheless I'd be interested in learning if it's possible at all
;-)
>
> TIA,
> Janko
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>


-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/