Generics

[bksaiki]

This PR adds generic functions. Generic function interfaces are defined by the define-generics syntactic form:

(define-generics <id>
                 [<method-id> . <formals>]
                 ...)

<formals> ::= (<arg> ...)
          ::= (<arg> ... . <rest-id>)
          ::= <rest-id>

The syntactic form binds the identifier gen:<id> to an object that encodes a generics interfaces and the identifier gen:<id>? to a predicate that returns #t if a record type implements the interface. Records implement generic function interfaces via the methods clause. See the definition of set as an example:

(define-record-type set
  (opaque #t)
  (sealed #t)
  (fields store)
  (protocol
    (lambda (p)
      (lambda xs
        (define h (make-hashtable))
        (for-each (lambda (x) (hashtable-set! h x #f)) xs)
        (p h))))
  (methods gen:equal+hash
    (lambda (a b rec-equal?)
      (rec-equal? (set-store a) (set-store b)))
    (lambda (a rec-hash)
      (+ (rec-hash (set-store a)) 401235121)))
  (methods gen:custom-write
    (lambda (s port mode)
      (fprintf port "#<set ~s>" (hashtable-keys (set-store s))))))

where gen:equal+hash and gen:custom-write are interfaces defined by

(define-generics equal+hash
                 [equal-proc a b rec-equal?]
                 [hash-proc a rec-hash])

(define-generics custom-write
                 [write-proc v port mode])

Fixes bugs in the evaluator that causes the garbage collector to GC prematurely.