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?

>>2
That doesn't matter, the problem is when it returns b it overwrites b with the result of the forced promise. Wth would they do this if it's supposed to be based on scheme.