50 lines
1.5 KiB
Racket
50 lines
1.5 KiB
Racket
#lang typed/racket
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(require (rename-in typed/rackunit [check-equal? test])
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(only-in typed/rackunit check-exn))
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(define-syntax-rule (test/exn expr msg)
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(check-exn exn:fail? (lambda () expr) msg))
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(define (msg0 [obj : Node] [selector : Symbol]) (obj selector))
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(define (node [v : Number] [l : Node] [r : Node]) : Node
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(lambda (m)
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(case m
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[(value) v]
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[(left) l]
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[(right) r]
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[(sum) (+ v (+ (msg-num l 'sum) (msg-num r 'sum)))]
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[else (error 'node (symbol->string m))])))
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(define (mt)
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(lambda ([m : Symbol])
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(case m
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[(sum) 0]
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[else (error 'mt (symbol->string m))])))
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(define-type Node (Symbol -> (U (U Number Node) Zero)))
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(define (msg-num [obj : Node] [selector : Symbol]) : Number
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(define result (msg0 obj selector))
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(cond
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[(number? result) result]
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[else (error 'msg-num "Expected a number message")]))
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(define (msg-node [obj : Node] [selector : Symbol]) : Node
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(define result (msg0 obj selector))
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(cond
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[(not (number? result)) result]
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[else (error 'msg-node "Expected a node message")]))
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(module+ test
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(define tree1 (node 1 (mt) (mt)))
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(test (msg0 tree1 'sum) 1)
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(test (msg0 tree1 'value) 1)
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(define tree2 (node 1 (node 2 (mt) (mt)) (mt)))
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(test (msg0 tree2 'value) 1)
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(test (msg0 (msg-node tree2 'left) 'value) 2)
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(test (msg0 (msg-node tree2 'right) 'sum) 0)
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(test/exn (msg0 (mt) 'left) "left")
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(test/exn (msg0 tree2 'wrong) "wrong")
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(test/exn (msg-num tree2 'left) "number")
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(test/exn (msg-node tree2 'value) "node"))
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