maybe.zp

(load "monads/monads.zp")

(define (monads:just value) "build a just instance of the maybe monad"
  (monads:generic value :just (lambda (val) (monads:just val)) (lambda (f) (f value))))

(define (monads:nothing) "build a nothing instance of the maybe monad"
  (monads:generic (nil) :nothing (lambda (val) (monads:nothing)) (lambda (f) (monads:nothing))))

(define (monads:just? monad) "check whether monad instance is just"
  (eq? (monads:get-type monad) :just))

(define (monads:nothing? monad) "check whether monad instance is nothing"
  (eq? (monads:get-type monad) :nothing))

(define (monads:from-maybe monad default) "returns value of just or default value"
  (if (monads:just? monad) (monads:get-val! monad) default))

(define (monads:list-to-maybe l) "returns nothing on empty list or just with head of list"
  (if (null? l) (monads:nothing) (monads:just (head l))))

(define (monads:maybe-to-list monad) "returns empty list on nothing or a singleton list with element of maybe"
  (if (monads:just? monad) (list (monads:get-val! monad)) []))

(define (monads:cat-maybe monads) "returns list of all just values from list of monads"
  (map monads:get-val! (filter monads:just? monads)))

(define (monads:map-maybe f l)
  "applies function to list and returns all instances where the function returned a just value"
  (monads:cat-maybe (map f l)))