[R] evaluating expression within nls()

[Timur Elzhov]

Dear R experts,

I'd like to fit data with weighted function fcn(data, p1, p2). At
first I used the standard form:

    nls(~ fcn(data, p1, p2),
         start = list(p1 = p01, p2 = p02),
         data  = data)

Then, I decided to change fcn() that it would be able to accept expression
as an additional parameter,

    nls(~ fcn(data, expr, p1, p2), ...)
    # ('expr' is defined just before 'nls' calling)

    Error in eval(expr, envir, enclos) : Object "expr" not found

Yes, in help(nls) is pointed, that
    Arguments:
      data: an optional data frame in which to evaluate the variables in
            `formula'

But, I'd like 'nls' to see my expression 'expr'... :-/  How can I do that?
Thank you!

--
WBR,
Timur.