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(read-enable 'curly-infix)
(use-modules (oop goops)
(oop goops describe)
(ice-9 match)
(srfi srfi-1))
(define-class <monoid> ())
(define-generic null)
(define-generic mappend)
(define (<> . args)
(fold mappend (null (car args)) (reverse args)))
(define-method (mappend (a <pair>)
(b <pair>))
(append a b))
(define-method (mappend (a <pair>) (b <null>)) a)
(define-method (mappend (a <null>) (b <pair>)) b)
(define-method (mappend (a <null>) (b <null>)) '())
(define-method (null (a <pair>)) '())
(define-method (null (a <null>)) '())
(define-macro (null type)
(symbol-append type '-null))
(define-class <applicative> ())
(define-generic map)
(define-class <monad> (<applicative> <monoid>))
(define-generic bind)
#; (define-generic return)
(define-class <optional> ()
(slot #:init-value #f
#:init-keyword #:slot)
(just #:init-value #t
#:init-keyword #:just))
(define (nothing) (make <optional> #:just #f))
(define (just obj) (make <optional>
#:just #t
#:slot obj))
(define (nothing? this)
(not (slot-ref this 'just)))
(define (just? this)
(not (nothing? this)))
(define-method (write (this <optional>) port)
(if (just? this)
(format port "[Just ~s]" (slot-ref this 'slot))
(format port "[Nothing]")))
(define-class <either> ()
(slot #:init-keyword #:slot)
(dir #:init-keyword #:dir #:init-value 'left))
(define (left val)
"Error values"
(make <either> #:slot val #:dir 'left))
(define (right val)
"Good values"
(make <either> #:slot val #:dir 'right))
(define-method (write (this <either>) port)
(format port "[~a ~s]"
(slot-ref this 'dir)
(slot-ref this 'slot)))
(define-method (>>= (this <either>)
(proc <procedure>))
(case (slot-ref this 'dir)
((left) this)
((right) (match this (($ <either> slot) (proc slot))))))
(define return-either right)
(define return-optional just)
;; bind :: Monad m => m a -> (a -> m b) -> m b
;; return :: Monad m => a -> m a
;; map :: Functor f => (a -> b) -> f a -> f b
(define-method (>>= (this <optional>)
(proc <procedure>))
(cond ((nothing? this) (nothing))
((just? this)
(match this
(($ <optional> slot) (proc slot))))))
(define-method (>>= (this <null>)
proc)
'())
(define-method (>>= (this <pair>)
(proc <procedure>))
(concatenate! (map proc this)))
(define-class <writer> ()
(value #:init-keyword #:value)
(monoid #:init-keyword #:monoid))
(define (writer value context)
(make <writer>
#:value value
#:monoid context))
(define-method (>>= (this <writer>)
(proc <procedure>))
(match this (($ <writer> value monoid)
(match (proc value)
(($ <writer> nval ncontext)
(writer nval { monoid <> ", " <> ncontext }))))))
(define-method (write (this <writer>) port)
(match this (($ <writer> value monoid)
(format port "[Writer ~s, ~s]" value monoid))))
(define-syntax do
(syntax-rules (<- let =)
((_ let var = val rest ...)
(let ((var val)) (do rest ...)))
((_ ptrn <- val rest ...)
(<- ptrn val rest ...))
((_ a) a)
((_ token rest ...)
(begin token (do rest ...)))))
(define-syntax <-
(lambda (x)
(syntax-case x (just left right writer)
((_ (just var) val rest ...)
#'(>>= val (lambda (var) rest ...)))
((_ (writer var) val rest ...)
#'(>>= val (lambda (var) rest ...)))
#;
((_ (left var) val rest ...) ;
#'(>>= val (lambda (var) rest ...)))
#;
((_ (right var) val rest ...) ;
#'(>>= val (lambda (var) rest ...))))))
;;; Examples:
;;; These are do in the conext of the optional monad.
(do (just x) <- (just 10)
x) ; => 10
(do let y = (just 10)
(just x) <- y
(+ x 5)) ; => 15
(do (just x) <- (nothing)
(+ x 5)) ; => [Nothing]
;;;
(do let either = left 10
(left x) <- either
x) ; EVALUATION ERROR
;; Int -> Writer Int String
(define (log-number n)
(writer n (format #f "Got nuber: ~a" n)))
(define (mult-with-log)
(do (writer a) <- (log-number 3)
(writer b) <- (log-number 5)
(* a b)))
(do (writer a) <- (log-number 1)
a) ; EVALUATION ERROR
(begin { (log-number 1) >>= log-number })
;; => [Writer 1, "Got nuber: 1, Got nuber: 1"]
(do (writer a) <- (log-number 3)
(writer b) <- (log-number 5)
(writer (* a b) ""))
;; EVALUATION ERROR
;; => [Writer 3, "Got nuber: 3, base"]
(log-number 3) ; => [Writer 3, "Got nuber: 3"]
(just 1) ; => [Just 1]
(do let y = 5
(just x) <- (just 10)
(just (* x y)))
;; => [Just 50]
;;; TODO
;;; '<- and 'let can't be used after '<-
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