(module promise '#%kernel
(#%require "small-scheme.rkt"
           "more-scheme.rkt"
           "define.rkt"
           (rename "define-struct.rkt" define-struct define-struct*)
           (for-syntax '#%kernel "stxcase-scheme.rkt" "name.rkt")
           '#%unsafe)
(#%provide force promise? promise-forced? promise-running?
           ;; provided to create extensions
           (struct promise ()) pref pset! prop:force reify-result
           promise-printer
           (struct running ()) (struct reraise ())
           (for-syntax make-delayer))

;; This module implements "lazy" (composable) promises and a `force'
;; that is iterated through them.

;; This is similar to the *new* version of srfi-45 -- see the
;; post-finalization discussion at http://srfi.schemers.org/srfi-45/ for
;; more details; specifically, this version is the `lazy2' version from
;; http://srfi.schemers.org/srfi-45/post-mail-archive/msg00013.html.
;; Note: if you use only `force'+`delay' it behaves as in Scheme (except
;; that `force' is identity for non promise values), and `force'+`lazy'
;; are sufficient for implementing the lazy language.

;; unsafe accessors
(define-syntax pref  (syntax-rules () [(_ p  ) (unsafe-struct-ref  p 0  )]))
(define-syntax pset! (syntax-rules () [(_ p x) (unsafe-struct-set! p 0 x)]))

;; ----------------------------------------------------------------------------
;; Forcers

;; `force/composable' iterates on composable promises
;; * (force X) = X for non promises
;; * does not deal with multiple values in the composable case
;; note: measuring time invested divided by the number of lines, this innocent
;; looking piece of code is by far the leader of that competition -- handle
;; with extreme care.
(define (force/composable root)
  (let ([v (pref root)])
    (cond
      [(procedure? v)
       ;; mark the root as running: avoids cycles, and no need to keep banging
       ;; the root promise value; it makes this non-r5rs, but the only
       ;; practical uses of these things could be ones that use state to avoid
       ;; an infinite loop.  (See the generic forcer below.)
       ;; (careful: avoid holding a reference to the thunk, to allow
       ;; safe-for-space loops)
       (pset! root (make-running (object-name v)))
       (call-with-exception-handler
        (lambda (e) (pset! root (make-reraise e)) e)
        (lambda ()
          ;; iterate carefully through chains of composable promises
          (let loop ([v (v)]) ; does not handle multiple values!
            (cond [(composable-promise? v)
                   (let ([v* (pref v)])
                     (pset! v root) ; share with root
                     (cond [(procedure? v*) (loop (v*))]
                           ;; it must be a list of one value (because
                           ;; composable promises never hold multiple values),
                           ;; or a composable promise
                           [(pair? v*) (pset! root v*) (unsafe-car v*)]
                           ;; note: for the promise case we could jump only to
                           ;; the last `let' (for `v*'), but that makes the
                           ;; code heavier, and runs slower (probably goes over
                           ;; some inlining/unfolding threshold).
                           [else (loop v*)]))]
                  ;; reached a non-composable promise: share and force it now
                  [(promise? v) (pset! root v) (force v)]
                  ;; error here for "library approach" (see above URL)
                  [else (pset! root (list v)) v]))))]
      ;; try to make the order efficient, with common cases first
      [(pair? v) (if (null? (unsafe-cdr v)) (unsafe-car v) (apply values v))]
      ;; follow all sharings (and shortcut directly to the right force)
      [(composable-promise? v) (force/composable v)]
      [(null? v) (values)]
      [(promise? v) (force v)] ; non composable promise is forced as usual
      [else (error 'force "composable promise with invalid contents: ~e" v)])))

;; convenient utility for any number of stored values or a raised value.
(define (reify-result v)
  (cond [(pair? v) (if (null? (unsafe-cdr v)) (unsafe-car v) (apply values v))]
        [(null? v) (values)]
        [(reraise? v) (v)]
        [else (error 'force "promise with invalid contents: ~e" v)]))

;; generic force for "old-style" promises -- they're still useful in
;; that they allow multiple values.  In general, this is slower, but has
;; more features.  (They could allow self loops, but this means holding
;; on to the procedure and its resources while it is running, and lose
;; the ability to know that it is running; the second can be resolved
;; with a new kind of `running' value that can be used again, but the
;; first cannot be solved.  I still didn't ever see any use for them, so
;; they're still forbidden -- throw a "reentrant promise" error.)
(define (force/generic promise)
  (reify-result
   (let ([v (pref promise)])
     (if (procedure? v)
       (begin
         (pset! promise (make-running (object-name v)))
         (call-with-exception-handler
          (lambda (e) (pset! promise (make-reraise e)) e)
          (lambda ()
            (let ([vs (call-with-values v list)]) (pset! promise vs) vs))))
       v))))

;; dispatcher for composable promises, generic promises, and other values
(define (force promise)
  (if (promise? promise)
    ((promise-forcer promise) promise) ; dispatch to specific forcer
    promise)) ; different from srfi-45: identity for non-promises

;; ----------------------------------------------------------------------------
;; Struct definitions

;; generic promise printer
(define (promise-printer promise port write?)
  (let loop ([v (pref promise)])
    (cond
      [(reraise? v)
       (let ([r (reraise-val v)])
         (if (exn? r)
           (fprintf port (if write? "#<promise!exn!~s>" "#<promise!exn!~a>")
                    (exn-message r))
           (fprintf port (if write? "#<promise!raise!~s>" "#<promise!raise!~a>")
                    r)))]
      [(running? v)
       (let ([r (running-name v)])
         (if r
           (fprintf port "#<promise:!running!~a>" r)
           (fprintf port "#<promise:!running>")))]
      [(procedure? v)
       (cond [(object-name v)
              => (lambda (n) (fprintf port "#<promise:~a>" n))]
             [else (display "#<promise>" port)])]
      [(promise? v) (loop (pref v))] ; hide sharing
      ;; values
      [(null? v) (fprintf port "#<promise!(values)>")]
      [(null? (cdr v))
       (fprintf port (if write? "#<promise!~s>" "#<promise!~a>") (car v))]
      [else (display "#<promise!(values" port)
            (let ([fmt (if write? " ~s" " ~a")])
              (for-each (lambda (x) (fprintf port fmt x)) v))
            (display ")>" port)])))

;; property value for the right forcer to use
(define-values [prop:force promise-forcer]
  (let-values ([(prop pred? get) ; no need for the predicate
                (make-struct-type-property 'forcer
                  (lambda (v info)
                    (unless (and (procedure? v)
                                 (procedure-arity-includes? v 1))
                      (raise-type-error 'prop:force "a unary function" v))
                    v))])
    (values prop get)))

;; A promise value can hold
;; - (list <value> ...): forced promise (possibly multiple-values)
;;        - composable promises deal with only one value
;; - <promise>: a shared (redirected) promise that points at another one
;;        - possible only with composable promises
;; - <thunk>: usually a delayed promise,
;;        - can also hold a `running' thunk that will throw a reentrant error
;;        - can also hold a raising-a-value thunk on exceptions and other
;;          `raise'd values (actually, applicable structs for printouts)
;; First, a generic struct, which is used for all promise-like values
(define-struct promise ([val #:mutable])
  #:property prop:custom-write promise-printer
  #:property prop:force force/generic)
;; Then, a subtype for composable promises
(define-struct (composable-promise promise) ()
  #:property prop:force force/composable)

;; template for all delay-like constructs
;; (with simple keyword matching: keywords is an alist with default exprs)
(define-for-syntax (make-delayer stx maker keywords)
  ;; no `cond', `and', `or', `let', `define', etc here
  (letrec-values
      ([(exprs+kwds)
        (lambda (stxs exprs kwds)
          (if (null? stxs)
            (values (reverse exprs) (reverse kwds))
            (if (not (keyword? (syntax-e (car stxs))))
              (exprs+kwds (cdr stxs) (cons (car stxs) exprs) kwds)
              (if (if (pair? (cdr stxs))
                    (if (assq (syntax-e (car stxs)) keywords)
                      (not (assq (syntax-e (car stxs)) kwds))
                      #f)
                    #f)
                (exprs+kwds (cddr stxs) exprs
                            (cons (cons (syntax-e (car stxs)) (cadr stxs))
                                  kwds))
                (values #f #f)))))]
       [(stxs) (syntax->list stx)]
       [(exprs kwds) (exprs+kwds (if stxs (cdr stxs) '()) '() '())]
       [(kwd-args) (if kwds
                     (map (lambda (k)
                            (let-values ([(x) (assq (car k) kwds)])
                              (if x (cdr x) (cdr k))))
                          keywords)
                     #f)]
       ;; some strange bug with `syntax-local-expand-expression' makes this not
       ;; work well with identifiers, so turn the name into a symbol to work
       ;; around this for now
       [(name0) (syntax-local-infer-name stx)]
       [(name) (if (syntax? name0) (syntax-e name0) name0)])
    (syntax-case stx ()
      [_ (pair? exprs) ; throw a syntax error if anything is wrong
         (with-syntax ([(expr ...) exprs]
                       [(kwd-arg ...) kwd-args])
           (with-syntax ([proc (syntax-property
                                (syntax/loc stx (lambda () expr ...))
                                'inferred-name name)]
                         [make maker])
             (syntax/loc stx (make proc kwd-arg ...))))])))

;; Creates a composable promise
;;   X = (force (lazy X)) = (force (lazy (lazy X))) = (force (lazy^n X))
(#%provide (rename lazy* lazy))
(define lazy make-composable-promise)
(define-syntax (lazy* stx) (make-delayer stx #'lazy '()))

;; Creates a (generic) promise that does not compose
;;   X = (force (delay X)) = (force (lazy (delay X)))
;;                         = (force (lazy^n (delay X)))
;;   X = (force (force (delay (delay X)))) != (force (delay (delay X)))
;; so each sequence of `(lazy^n o delay)^m' requires m `force's and a
;; sequence of `(lazy^n o delay)^m o lazy^k' requires m+1 `force's (for k>0)
;; (This is not needed with a lazy language (see the above URL for details),
;; but provided for regular delay/force uses.)
(#%provide (rename delay* delay))
(define delay make-promise)
(define-syntax (delay* stx) (make-delayer stx #'delay '()))

;; For simplicity and efficiency this code uses thunks in promise values for
;; exceptions: this way, we don't need to tag exception values in some special
;; way and test for them -- we just use a thunk that will raise the exception.
;; But it's still useful to refer to the exception value, so use an applicable
;; struct for them.  The same goes for a promise that is being forced: we use a
;; thunk that will throw a "reentrant promise" error -- and use an applicable
;; struct so it is identifiable.
(define-struct reraise (val)
  #:property prop:procedure (lambda (this) (raise (reraise-val this))))
(define-struct running (name)
  #:property prop:procedure (lambda (this)
                              (let ([name (running-name this)])
                                (if name
                                  (error 'force "reentrant promise ~.s" name)
                                  (error 'force "reentrant promise")))))

;; ----------------------------------------------------------------------------
;; Utilities

(define (promise-forced? promise)
  (if (promise? promise)
    (let ([v (pref promise)])
      (or (not (procedure? v)) (reraise? v))) ; #f when running
    (raise-type-error 'promise-forced? "promise" promise)))

(define (promise-running? promise)
  (if (promise? promise)
    (running? (pref promise))
    (raise-type-error 'promise-running? "promise" promise)))

)

#|
Simple code for timings:
  (define (c n) (lazy (if (zero? n) (delay 'hey!) (c (sub1 n)))))
  (for ([i (in-range 9)])
    (collect-garbage) (collect-garbage) (collect-garbage)
    (time (for ([i (in-range 10000)]) (force (c 2000)))))
Also, run (force (c -1)) and check constant space
|#
