3.1 Defining Classes and Instances

A class is an abstract collection of names with optional default values.

Algebraic Racket’s classes are a lot like Haskell’s type classes without the typing constraints. They allow abstract design patterns like monads with do-notation to be expressed simply and in a generic way that makes sense for Racket.

The class form declares an abstract class and its members.

> (class Eq     [== (.. not /=)]     [/= (.. not ==)]     minimal ([==] [/=]))

Using an uninstantiated member is a syntax error.

> ==

eval:43:14: ==: no instance for Eq

in: ==

> (/= 1 0)

eval:44:14: /=: no instance for Eq

in: (/= 1 0)

The instance form produces an object that holds class member definitions.

> (define-syntax StringEq (instance Eq [== string=?]))

> (define-syntax NumberEq (instance Eq [/= (|| < >)]))

Attempting to define an incomplete instance is a syntax error.

> (define-syntax BadEq (instance Eq))

eval:58:35: instance: Not a minimal definition

in: (instance Eq extends ())

The with-instance form temporarily instantiates the members of an instance.

> (with-instance StringEq     (and (== "abcde" "abcde")          (/= "hello" "world")))

#t

> (with-instance NumberEq     (and (== 1 1)          (/= 2 3)))

#t

The with-instance form can prefix the names it binds so multiple instances of the same class can be used together.

> (with-instance [S: StringEq]     (with-instance [N: NumberEq]       (and (S:== "abc" "abc")            (N:== 123 123))))

#t

The with-instances form offers a cleaner syntax for working with multiple instances.

> (define-syntax EqEq (instance Eq [== eq?]))

> (with-instances (EqEq                    [S: StringEq]                    [N: NumberEq])     (and (== + +) (S:== "?" "?") (N:== 0 0)))

#t

A class form with n members defines n+1 names:

• class-id, a class descriptor that represents the class.

• for each member-id, a transformer binding that raises a syntax error.

The minimal directive, when present, constrains valid instances to only those that provide at least one of the minimal member-id sets.

The extends directive, when present, recursively imports the members of the instance-ids and the instances they extend.

> (define-syntax EscapeContinuation     (instance Continuation       [call (φ f (call/ec (φ esc (parameterize ([current-esc esc]) (f)))))]       [abort (current-abort current-esc "no escape continuation")]))

> (define-syntax CurrentContinuation     (instance Continuation       [call (φ f (call/cc (φ con (parameterize ([current-con con]) (f)))))]       [abort (current-abort current-con "no current continuation")]))

If any prefix-ids are given, they are prepended to the names of the members defined by the corresponding instance-ids.

> (with-instance CurrentContinuation     (call (λ () (abort 1 2) 3)))

1 2

> (splicing-with-instance [C: CurrentContinuation]     (define (g x)       (C:call (λ () (C:abort `(ABORT ,x))))))

> (g 2)

'(ABORT 2)