Scheme 避免在结构中显示3次
我定义了一个结构,如下所示Scheme 避免在结构中显示3次,scheme,racket,generic-interface,Scheme,Racket,Generic Interface,我定义了一个结构,如下所示 (struct vector (x y z) #:methods gen:custom-write [(define (write-proc vector port mode) (let ([print (if mode write display)]) (write-string "<") (print (vector-x vector)) (write-string ", ") (pr
(struct vector (x y z)
#:methods gen:custom-write
[(define (write-proc vector port mode)
(let ([print (if mode write display)])
(write-string "<")
(print (vector-x vector))
(write-string ", ")
(print (vector-y vector))
(write-string ", ")
(print (vector-z vector))
(write-string ">")))])
(结构向量(x y z)
#:方法gen:自定义写入
[(定义(写入过程向量端口模式)
(let([打印(如果模式写入显示)])
(写入字符串“”)])
但是我在REPL中发现了一个奇怪的行为,其中结构被显示了3次:
> (define a (vector 1 2 3))
> a
<1, 2, 3><1, 2, 3><1, 2, 3>
>(定义一个(向量1、2、3))
>a
我一定是做错了什么,但找不到我的问题。有人能解释一下为什么我有3次输出吗?将输出定向到输出端口,一切正常:
#lang racket
(struct vector (x y z)
#:methods gen:custom-write
[(define (write-proc vector port mode)
(let ([print (if mode write display)])
(write-string "<" port)
(print (vector-x vector) port)
(write-string ", " port)
(print (vector-y vector) port)
(write-string ", " port)
(print (vector-z vector) port)
(write-string ">" port)))])
#朗球拍
(结构向量(x y z)
#:方法gen:自定义写入
[(定义(写入过程向量端口模式)
(let([打印(如果模式写入显示)])
(写入字符串“端口”)])
您需要使用写入过程提供的端口
:
(struct vector (x y z)
#:methods gen:custom-write
[(define (write-proc vector port mode)
(let ([print (if mode write display)])
(write-string "<" port)
(print (vector-x vector) port)
(write-string ", " port)
(print (vector-y vector) port)
(write-string ", " port)
(print (vector-z vector) port)
(write-string ">" port)))])
谢谢,我没有注意文档:)哎呀,我看到@soegaard在我打字的时候发布了一个答案。我想我会把这个保留下来,作为参数化
提示。
(struct vector (x y z)
#:methods gen:custom-write
[(define (write-proc vector port mode)
(let ([print (if mode write display)])
(parameterize ([current-output-port port]) ;; <== new
(write-string "<")
(print (vector-x vector))
(write-string ", ")
(print (vector-y vector))
(write-string ", ")
(print (vector-z vector))
(write-string ">"))))])