4.2 Creating Structure Types

The resulting structure type has init-field-cnt+auto-field-cnt fields (in addition to any fields from super-type), but only init-field-cnt constructor arguments (in addition to any constructor arguments from super-type). The remaining fields are initialized with auto-v. The total field count (including super-type fields) must be no more than 32768.

The props argument is a list of pairs, where the car of each pair is a structure type property descriptor, and the cdr is an arbitrary value. A property can be specified multiple times in props (including properties that are automatically added by properties that are directly included in props) only if the associated values are eq?, otherwise the exn:fail:contract exception is raised. See Structure Type Properties for more information about properties. When inspector is 'prefab, then props must be null.

The inspector argument normally controls access to reflective information about the structure type and its instances; see Structure Inspectors for more information. If inspector is 'prefab, then the resulting prefab structure type and its instances are always transparent.

If proc-spec is an integer or procedure, instances of the structure type act as procedures. See prop:procedure for further information. Providing a non-#f value for proc-spec is the same as pairing the value with prop:procedure at the end of props, plus including proc-spec in immutables when proc-spec is an integer.

The immutables argument provides a list of field positions. Each element in the list must be unique, otherwise exn:fail:contract exception is raised. Each element must also fall in the range 0 (inclusive) to init-field-cnt (exclusive), otherwise exn:fail:contract exception is raised.

The guard argument is either a procedure of n+1 arguments or #f, where n is the number of arguments for the new structure type’s constructor (i.e., init-field-cnt plus constructor arguments implied by super-type, if any). If guard is a procedure, then the procedure is called whenever an instance of the type is constructed, or whenever an instance of a subtype is created. The arguments to guard are the values provided for the structure’s first n fields, followed by the name of the instantiated structure type (which is name, unless a subtype is instantiated). The guard result must be n values, which become the actual values for the structure’s fields. The guard can raise an exception to prevent creation of a structure with the given field values. If a structure subtype has its own guard, the subtype guard is applied first, and the first n values produced by the subtype’s guard procedure become the first n arguments to guard. When inspector is 'prefab, then guard must be #f.

If constructor-name is not #f, it is used as the name of the generated constructor procedure as returned by object-name or in the printed form of the constructor value.

The generate argument is used to define a new generator for this structure type, which can be used to create random instances of the structure type. For more information see contract-generate.

The exercise argument allows you to define a function to verify that a given value is an instance of your contract. This will also be used for random generation.

The result of make-struct-type is five values:

• a structure type descriptor,

• a constructor procedure,

• a predicate procedure,

• an accessor procedure, which consumes a structure and a field index between 0 (inclusive) and init-field-cnt+auto-field-cnt (exclusive), and

• a mutator procedure, which consumes a structure, a field index, and a field value.

Examples:

> (define-values (struct:a make-a a? a-ref a-set!)     (make-struct-type 'a #f 2 1 'uninitialized)) > (define an-a (make-a 'x 'y)) > (a-ref an-a 1) 'y > (a-ref an-a 2) 'uninitialized > (define a-first (make-struct-field-accessor a-ref 0)) > (a-first an-a) 'x

> (define-values (struct:b make-b b? b-ref b-set!)     (make-struct-type 'b struct:a 1 2 'b-uninitialized))

> (define a-b (make-b 'x 'y 'z))

> (a-ref a-b 1)

'y

> (a-ref a-b 2)

'uninitialized

> (b-ref a-b 0)

'z

> (b-ref a-b 1)

'b-uninitialized

> (b-ref a-b 2)

'b-uninitialized

> (define-values (struct:c make-c c? c-ref c-set!)     (make-struct-type      'c struct:b 0 0 #f null (make-inspector) #f null      ; guard checks for a number, and makes it inexact      (lambda (a1 a2 b1 name)        (unless (number? a2)          (error (string->symbol (format "make-~a" name))                 "second field must be a number"))        (values a1 (exact->inexact a2) b1))))

> (make-c 'x 'y 'z)

make-c: second field must be a number

> (define a-c (make-c 'x 2 'z))

> (a-ref a-c 1)

2.0

> (define p1 #s(p a b c))

> (define-values (struct:p make-p p? p-ref p-set!)     (make-struct-type 'p #f 3 0 #f null 'prefab #f '(0 1 2)))

> (p? p1)

#t

> (p-ref p1 0)

'a

> (make-p 'x 'y 'z)

'#s(p x y z)

For examples, see make-struct-type.

For examples, see make-struct-type.