[R] Converting expressions to functions

[Prof. John C Nash]

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