source: project/release/4/options/trunk/options.scm @ 31334

#|[
Author: Juergen Lorenz
ju (at) jugilo (dot) de

Copyright (c) 2014, Juergen Lorenz
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]|#

;;; The option datatype is inspired by the equally named type in ML.  It
;;; simply stores one or no value at all. To be able to type options, we
;;; implement it as a functor, named options-functor, and supply an
;;; untyped version as a module named options, which simply applies the
;;; functor to a module with an any? type test, i.e. no check at all.
;;; This argument module is generated implicitly by instantiation of the
;;; functor and named _options.
;;; Unfortunately, Chicken's functor implementation contains a bug,
;;; which appears only when compiling the generated module: The functor
;;; parameter, M, must be replaced by the functor argument, _options, in
;;; the Chicken generated file options.import.scm. Hence we need to
;;; patch that latter file.
;;;
(require-library datatype)

(functor (options-functor (M (item?)))
         (options option? none some none? some-ref)
(import scheme datatype M
        (only chicken error case-lambda))

(define-datatype option option?
  (none)
  (some (item item?)))

(define (none? xpr)
  (cases option xpr
    (none () #t)
    (else #f)))

(define (some-ref xpr)
  (cases option xpr
    (none () (error 'option "no value available"))
    (some (item) item)))

(define options
  (let (
    (signatures '(
      (none)
      (some item)
      (option? xpr)
      (none? xpr)
      (some-ref opt)
      ))
    )
    (case-lambda
      (() (map car signatures))
      ((sym) (assq sym signatures)))))

) ; options-functor

(module options = options-functor
  (import scheme)
  (define (item? xpr) #t)
) ; options