R help - Jul 2010 - Converting expressions to functions

Dear R colleagues,

I'm looking for some examples or vignettes or similar to suggest good ways
to convert an
expression to a function. In particular, I'd like to (semi-) automate the
conversion of
nls() expressions to residual functions. Example

Given variables y, T and parameters b1, b2, b3 in

exprtext<-"y~b1/(1+b2*exp(-b3*T))"

I'd like to generate a function

resids<-function(b, y, T){
	b1<-b[1]
	b2<-b[2]
	b3<-b[3] # Pedestrian but hopefully clear
	res<-b1/(1+b2*exp(-b3*T))-y
	return(res)
}

Preferably, I want to use only R facilities rather than call Perl or Python etc.
And as
much as possible, I'd like to automate the process. There are likely several
ways to proceed.

Please contact me off list. I will report my success or lack thereof to the list
in the
near future.

Thanks,

John Nash