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(define-module (data optional)
#:use-module (oop goops)
#:use-module (ice-9 match)
#:use-module (control monad)
#:use-module (data functor)
#:export (nothing just
nothing? just?
from-just
return-optional)
#:re-export (>>= >> fmap))
(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 (from-just default maybe-val)
"Returns default if maybe-val is nothing, otherwise
the value embedded in maybe-val"
(if (just? maybe-val)
(slot-ref maybe-val 'slot)
default))
(define-method (write (this <optional>) port)
(if (just? this)
(format port "[Just ~s]" (slot-ref this 'slot))
(format port "[Nothing]")))
(define return-optional just)
(define-method (>>= (this <optional>)
(proc <procedure>))
(cond ((nothing? this) (nothing))
((just? this)
(match this
(($ <optional> slot) (proc slot))))))
(define-method (>> (a <optional>)
(b <optional>))
(if (or (nothing? a)
(nothing? b))
(nothing)
b))
(define-method (fmap (f <procedure>)
(m <optional>))
(do x <- m
(return-optional (f x))))
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