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46 lines
1.5 KiB
Racket
46 lines
1.5 KiB
Racket
#lang debug racket/base
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(require "helper.rkt" racket/class)
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(provide (all-defined-out))
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#|
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approximates
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https://github.com/mbutterick/restructure/blob/master/src/Optional.coffee
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|#
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(define xoptional%
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(class xenobase%
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(super-new)
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(init-field [(@type type)] [(@condition condition)])
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(unless (xenomorphic-type? @type)
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(raise-argument-error '+xoptional"xenomorphic type" @type))
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(define (resolve-condition parent)
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(define maybe-proc @condition)
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(if (procedure? maybe-proc) (maybe-proc parent) maybe-proc))
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(define/augment (xxdecode port parent)
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(when (resolve-condition parent)
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(send @type xxdecode port parent)))
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(define/augment (xxencode val port [parent #f])
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(when (resolve-condition parent)
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(send @type xxencode val port parent)))
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(define/augment (xxsize [val #f] [parent #f])
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(if (resolve-condition parent) (send @type xxsize val parent) 0))))
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(define no-val (gensym))
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(define (+xoptional [type-arg #f] [cond-arg no-val]
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#:type [type-kwarg #f]
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#:condition [cond-kwarg no-val]
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#:pre-encode [pre-proc #f]
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#:post-decode [post-proc #f])
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(define type (or type-arg type-kwarg))
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(define condition (cond
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[(and (eq? cond-arg no-val) (eq? cond-kwarg no-val)) #true]
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[(not (eq? cond-arg no-val)) cond-arg]
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[(not (eq? cond-kwarg no-val)) cond-kwarg]))
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(new (generate-subclass xoptional% pre-proc post-proc) [type type] [condition condition]))
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