40 lines
1.4 KiB
Scheme
40 lines
1.4 KiB
Scheme
#lang scheme/base
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(require (for-syntax scheme/base
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scheme/struct-info))
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(provide make)
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;; (make struct-name field-expr ...)
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;; Checks that correct number of fields given.
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(define-syntax (make stx)
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(define (bad-struct-name x)
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(raise-syntax-error #f "expected struct name" stx x))
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(define (get-struct-info id)
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(unless (identifier? id)
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(bad-struct-name id))
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(let ([value (syntax-local-value id (lambda () #f))])
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(unless (struct-info? value)
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(bad-struct-name id))
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(extract-struct-info value)))
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(syntax-case stx ()
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[(make S expr ...)
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(let ()
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(define info (get-struct-info #'S))
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(define constructor (list-ref info 1))
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(define accessors (list-ref info 3))
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(unless (identifier? #'constructor)
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(raise-syntax-error #f "constructor not available for struct" stx #'S))
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(unless (andmap identifier? accessors)
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(raise-syntax-error #f "incomplete info for struct type" stx #'S))
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(let ([num-slots (length accessors)]
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[num-provided (length (syntax->list #'(expr ...)))])
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(unless (= num-provided num-slots)
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(raise-syntax-error
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#f
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(format "wrong number of arguments for struct ~s (expected ~s)"
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(syntax-e #'S)
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num-slots)
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stx)))
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(with-syntax ([constructor constructor])
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#'(constructor expr ...)))]))
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