R devel - Feb 2006 - Retrieving an unevaluated argument

I'm trying to retrieve an unevalated argument (a list in particular). 
I can do this easily when I call the function directly:

a1 <- function(x) match.call()$x
> a1(list(y=x^2))
list(y = x^2)

But when the function is called by another function, it gets trickier

b <- function(x, f) f(x)
> b(list(x^2), a1)
x

The best I've been able to do is:

a2 <- function(x) parse(text=deparse(substitute(x, parent.frame())))[[1]]
> b(list(x^2), a2)
list(x^2)

But I'm sure there must be a better way!

Hadley

Why isn't substitute(x, parent.frame()) enough?  parse(deparse()) seems
redundant...

Andy

From: hadley wickham
> 
> I'm trying to retrieve an unevalated argument (a list in particular). 
> I can do this easily when I call the function directly:
> 
> a1 <- function(x) match.call()$x
> 
> > a1(list(y=x^2))
> list(y = x^2)
> 
> But when the function is called by another function, it gets trickier
> 
> b <- function(x, f) f(x)
> 
> > b(list(x^2), a1)
> x
> 
> The best I've been able to do is:
> 
> a2 <- function(x) parse(text=deparse(substitute(x, 
> parent.frame())))[[1]]
> 
> > b(list(x^2), a2)
> list(x^2)
> 
> But I'm sure there must be a better way!
> 
> Hadley
> 
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