Lazy in racket

How2lazy in DrRacket?

There's the lazy library, but it's not functional, i.e. I define a to be 1, define b as a + a, force b, it gives 2, but then I force set a to be 5, and it gives b as 2 again.

Any way around this?

>>12
One of the big things in Scheme was the whole "you should be able to do thing without the language doing shit in the background"

Isn't it bad programming to assume the programmer is ignorant of side effects? Is there even ANY possible way in Racket to have the expression (+ a b) hold valid, where a is a value and b is an expression?