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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 option-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.
(require-library datatype) (import option-functor datatype) ;; apply functor to instantiate module (module item-options = option-functor (import scheme) (define (item? xpr) ...) ; this defines modules item-options and implicitly _item-options ;; don't forget to patch item-options.import.scm when compiling item-options by replacing the name of the functor parameter with the name of the functor argument _item-options ;; import the module (import (prefix item-options item-)) ;; now use the generated routines
- options #!optional symprocedure
the fundamental datatype-constructor of an empty option
- some itemprocedure
the fundamental datatype-constructor for a nonempty option. In typed modules, the argument item must pass the item? check.
- option? xprprocedure
- none? xprprocedure
evaluates xpr to an empty option?
- some-ref optprocedure
fetches the value out of the option argument, it there is one, otherwise signals an error.
(use options) (define opt (some 5)) (option? opt) ; -> #t (none? opt) ; -> #f (none? (none)) ; -> #t (some-ref opt) ; -> 5 (option? (none)) ; -> #t (some-ref (none)) ; -> error
Dec 04, 2017
Copyright (c) 2014-2017, Juergen Lorenz All rights reserved.
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- patch removed
- functor renamed
- initial import