Get and put

[Sgeo]

I think it would make sense to include get, put, and modify for the state monad, or perhaps call them state-get state-put state-modify. Could also make state-put and state-modify not return useful values, but not sure if there's a reason to do that.

(define state-get
  (lambda (state)
    (state-doublet state state)))

(define (state-put new-state)
  (lambda (old-state)
    (state-doublet new-state new-state)))

(define (state-modify modifier)
  (lambda (old-state)
    (define result (modifier old-state))
    (state-doublet result result)))