blob: 12275d1d8c108cdc03c655e7e65ffdac32244e63 (
about) (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
|
(define-module (monad state)
#:use-module (monad core)
#:re-export (with-monad)
#:export (state-monad run-state
get put modify))
(define (return x)
(lambda (state)
(cons x state)))
(define (>>= m f)
(lambda (old-state)
(let ((state-pair (m old-state)))
((f (car state-pair)) (cdr state-pair)))))
(define state-monad (make-monad >>= return))
(define (run-state initial-state m)
(car (m initial-state)))
;; Examine the state at this point in the computation.
(define (get)
(lambda (s)
(cons s s)))
;; replace the state
(define (put s)
(lambda (_)
(cons #f s)))
;; update the state
(define (modify f)
(lambda (s)
(cons #f (f s))))
|
