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(use-modules (ice-9 curried-definitions)
(ice-9 match))
;;; This is a minimal implementation of the state monad.
;;; It is incompatible with the rest of my monad system,
;;; since everything either has type <pair> or <procedure>.
;;; But it should work as a nice base for the actual
;;; implementation.
;;; This implementation works, as far as I can tell, exactly
;;; like the Haskell version (of MonadState). But obviously
;;; without all the nice syntax.
;;; Anything that takes an st-list and returns an st-list is
;;; considered a State value.
;;; An st-list is a list where the car is the last returned
;;; value, and the cadr is the "state".
;; newtype State = st-list -> st-list
;; State
(define ((get) st-list)
"Sets the return value to the state value"
(match st-list
((v st)
(list st st))))
;; v -> State
(define ((put v) st-list)
"Sets the state value to v, sets the return value to ()"
(list '() v))
;; State -> (v -> State) -> State
(define ((bind st-proc proc) st-list)
(let ((new-st-list (st-proc st-list)))
(match new-st-list
((v _)
((proc v) new-st-list)))))
;; State -> State -> State
(define ((then st-proc-1 st-proc-b) st-list-a)
(let ((st-list-b (st-proc-1 st-list-a)))
(st-proc-b st-list-b)))
;; v -> State
(define ((return v) st-list)
"Sets the return value to v"
(cons v (cdr st-list)))
;; State -> v -> (r v)
(define (run-state st-proc init)
"Exec state with init as starting state value"
(st-proc (list init init)))
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