Racket 只接受布尔值的结构

如果一个结构是布尔型的，我如何创建一个只接受一个值的结构

(struct mystruct [val]  //only take val if it's a boolean )

你试过了吗

#lang typed/racket

(struct point ([q : Boolean] [x : Real] [y : Real]))

你试过了吗

#lang typed/racket

(struct point ([q : Boolean] [x : Real] [y : Real]))

---

至少有三种方法：

使用合同定义结构

使用with

#：guard

定义带有保护的结构

a） 使用合同的警卫

b） 使用

除非

，

布尔值？

，

引发参数错误

手动定义的保护

在中编写程序而不是使用Racket，并使用类似

Boolean

的类型

1：带合同的结构 您可以这样使用：

(define-struct/contract mystruct ([val boolean?]))

(struct mystruct [val]
  #:guard <guard-procedure>)

正确和不正确地使用它：

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
make-mystruct: contract violation
  expected: boolean?
  given: 3
  in: the 1st argument of
      (-> boolean? symbol? any)
  contract from: (struct mystruct)
  blaming: .../so-54901923.rkt
   (assuming the contract is correct)

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
mystruct: contract violation
  expected: boolean?
  given: 3
  in: boolean?
  contract from: 
      .../so-54901923.rkt
  blaming: .../so-54901923.rkt
   (assuming the contract is correct)

2：带防护装置的结构 您可以使用的

#：guard

关键字如下：

(define-struct/contract mystruct ([val boolean?]))

(struct mystruct [val]
  #:guard <guard-procedure>)

正确和不正确地使用它：

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
make-mystruct: contract violation
  expected: boolean?
  given: 3
  in: the 1st argument of
      (-> boolean? symbol? any)
  contract from: (struct mystruct)
  blaming: .../so-54901923.rkt
   (assuming the contract is correct)

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
mystruct: contract violation
  expected: boolean?
  given: 3
  in: boolean?
  contract from: 
      .../so-54901923.rkt
  blaming: .../so-54901923.rkt
   (assuming the contract is correct)

如果您无法使用类似的功能，或者您需要的电源/控制比提供的更多，则可以手动定义保护程序。如果结构有n个字段，则guard过程应采用n+1个参数，对应于字段和结构名称

例如，对于您的

mystruct

，它应该是一个接受

val

字段值和名称的函数：

(lambda (val name)
  ???)

guard过程应使用n个值返回多值结果，对应于字段

(lambda (val name)
  (values val))

在传递值之前，它可以对值进行任意检查：

(lambda (val name)
  (unless (boolean? val)
    (raise-argument-error name "boolean" val))
  (values val))

将其放在

：guard

：

(struct mystruct [val]
  #:guard (lambda (val name)
            (unless (boolean? val)
              (raise-argument-error name "boolean" val))
            (values val)))

使用它：

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
mystruct: contract violation
  expected: boolean
  given: 3

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
- : Boolean
#true
> (mystruct-val xf)
- : Boolean
#false
> (mystruct 3)
Type Checker: type mismatch
  expected: Boolean
  given: Positive-Byte in: 3

3：打字球拍 你可以用。其形式对每个字段都有一个类型：

#lang typed/racket

(struct mystruct ([val : Boolean]))

使用它：

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
mystruct: contract violation
  expected: boolean
  given: 3

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
- : Boolean
#true
> (mystruct-val xf)
- : Boolean
#false
> (mystruct 3)
Type Checker: type mismatch
  expected: Boolean
  given: Positive-Byte in: 3

---

至少有三种方法：

使用合同定义结构

使用with

#：guard

定义带有保护的结构

a） 使用合同的警卫

b） 使用

除非

，

布尔值？

，

引发参数错误

手动定义的保护

在中编写程序而不是使用Racket，并使用类似

Boolean

的类型

1：带合同的结构 您可以这样使用：

(define-struct/contract mystruct ([val boolean?]))

(struct mystruct [val]
  #:guard <guard-procedure>)

正确和不正确地使用它：

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
make-mystruct: contract violation
  expected: boolean?
  given: 3
  in: the 1st argument of
      (-> boolean? symbol? any)
  contract from: (struct mystruct)
  blaming: .../so-54901923.rkt
   (assuming the contract is correct)

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
mystruct: contract violation
  expected: boolean?
  given: 3
  in: boolean?
  contract from: 
      .../so-54901923.rkt
  blaming: .../so-54901923.rkt
   (assuming the contract is correct)

2：带防护装置的结构 您可以使用的

#：guard

关键字如下：

(define-struct/contract mystruct ([val boolean?]))

(struct mystruct [val]
  #:guard <guard-procedure>)

正确和不正确地使用它：

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
make-mystruct: contract violation
  expected: boolean?
  given: 3
  in: the 1st argument of
      (-> boolean? symbol? any)
  contract from: (struct mystruct)
  blaming: .../so-54901923.rkt
   (assuming the contract is correct)

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
mystruct: contract violation
  expected: boolean?
  given: 3
  in: boolean?
  contract from: 
      .../so-54901923.rkt
  blaming: .../so-54901923.rkt
   (assuming the contract is correct)

如果您无法使用类似的功能，或者您需要的电源/控制比提供的更多，则可以手动定义保护程序。如果结构有n个字段，则guard过程应采用n+1个参数，对应于字段和结构名称

例如，对于您的

mystruct

，它应该是一个接受

val

字段值和名称的函数：

(lambda (val name)
  ???)

guard过程应使用n个值返回多值结果，对应于字段

(lambda (val name)
  (values val))

在传递值之前，它可以对值进行任意检查：

(lambda (val name)
  (unless (boolean? val)
    (raise-argument-error name "boolean" val))
  (values val))

将其放在

：guard

：

(struct mystruct [val]
  #:guard (lambda (val name)
            (unless (boolean? val)
              (raise-argument-error name "boolean" val))
            (values val)))

使用它：

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
mystruct: contract violation
  expected: boolean
  given: 3

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
- : Boolean
#true
> (mystruct-val xf)
- : Boolean
#false
> (mystruct 3)
Type Checker: type mismatch
  expected: Boolean
  given: Positive-Byte in: 3

3：打字球拍 你可以用。其形式对每个字段都有一个类型：

#lang typed/racket

(struct mystruct ([val : Boolean]))

使用它：

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
#true
> (mystruct-val xf)
#false
> (mystruct 3)
mystruct: contract violation
  expected: boolean
  given: 3

> (define xt (mystruct #true))
> (define xf (mystruct #false))
> (mystruct-val xt)
- : Boolean
#true
> (mystruct-val xf)
- : Boolean
#false
> (mystruct 3)
Type Checker: type mismatch
  expected: Boolean
  given: Positive-Byte in: 3