Inline the trail pointer functions
| author |
Steve Losh <steve@stevelosh.com> |
| date |
Mon, 11 Jul 2016 20:04:02 +0000 |
| parents |
96258fb7be70 |
| children |
d2ede3f3926a |
(in-package #:bones.wam)
;;;; WAM
(declaim
;; Inline all these struct accessors, otherwise things get REAL slow.
(inline wam-store
wam-code
wam-code-labels
wam-logic-stack
wam-logic-pool
wam-functors
wam-fail
wam-backtracked
wam-unification-stack
wam-trail
wam-number-of-arguments
wam-subterm
wam-program-counter
wam-heap-pointer
wam-continuation-pointer
wam-environment-pointer
wam-backtrack-pointer
wam-cut-pointer
wam-heap-backtrack-pointer
wam-mode))
(defun allocate-wam-code (size)
;; The WAM bytecode is all stored in this array. The first
;; `+maximum-query-size+` words are reserved for query bytecode, which will
;; get loaded in (overwriting the previous query) when making a query.
;; Everything after that is for the actual database.
(make-array (+ +maximum-query-size+ size)
:initial-element 0
:element-type 'code-word))
(defun allocate-wam-store (size)
;; The main WAM store contains three separate blocks of values:
;;
;; [0, +register-count+) -> the local X_n registers
;; [+stack-start+, +stack-end+) -> the stack
;; [+heap-start+, ...) -> the heap
;;
;; `+register-count+` and `+stack-start+` are the same number, and
;; `+stack-end+` and `+heap-start+` are the same number as well.
(make-array (+ +register-count+
+stack-limit+
size)
:initial-element (make-cell-null)
:element-type 'cell))
(defstruct (wam
(:print-function
(lambda (wam stream depth)
(declare (ignore depth))
(print-unreadable-object
(wam stream :type t :identity t)
(format stream "an wam"))))
(:constructor make-wam%))
(store
(allocate-wam-store 0)
:type store
:read-only t)
(code
(allocate-wam-code 0)
:type (simple-array code-word (*))
:read-only t)
(code-labels
(make-hash-table)
:read-only t)
(logic-stack
nil
:type list)
(logic-pool
nil
:type list)
(functors
(make-array 64
:fill-pointer 0
:adjustable t
:element-type 'functor)
:type (vector functor)
:read-only t)
(unification-stack
(make-array 16
:fill-pointer 0
:adjustable t
:element-type 'store-index)
:type (vector store-index)
:read-only t)
(trail
(make-array 64
:fill-pointer 0
:adjustable t
:initial-element 0
:element-type 'store-index)
:type (vector store-index)
:read-only t)
;; Unique registers
(number-of-arguments 0 :type arity) ; NARGS
(subterm +heap-start+ :type heap-index) ; S
(program-counter 0 :type code-index) ; P
(code-pointer +maximum-query-size+ :type code-index) ; CODE
(heap-pointer (1+ +heap-start+) :type heap-index) ; H
(stack-pointer +stack-start+ :type stack-index) ; SP
(continuation-pointer 0 :type code-index) ; CP
(environment-pointer +stack-start+ :type environment-pointer) ; E
(backtrack-pointer +stack-start+ :type backtrack-pointer) ; B
(cut-pointer +stack-start+ :type backtrack-pointer) ; B0
(heap-backtrack-pointer +heap-start+ :type heap-index) ; HB
;; Other global "registers"
(fail nil :type boolean)
(backtracked nil :type boolean)
(mode nil :type (or null (member :read :write))))
(defun* make-wam (&key (store-size (megabytes 10))
(code-size (megabytes 1)))
(:returns wam)
(make-wam% :code (allocate-wam-code code-size)
:store (allocate-wam-store store-size)))
;;;; Store
(declaim (inline wam-store-cell (setf wam-store-cell)))
(defun* wam-store-cell ((wam wam) (address store-index))
(:returns cell)
"Return the cell at the given address.
Please don't use this unless you absolutely have to. Prefer something more
specific like `wam-heap-cell` else so you've got some extra sanity checking...
"
(aref (wam-store wam) address))
(defun* (setf wam-store-cell) ((new-value cell)
(wam wam)
(address store-index))
(setf (aref (wam-store wam) address) new-value))
;;;; Heap
;;; The WAM heap is all the memory left in the store after the local registers
;;; and stack have been accounted for. Because the store is adjustable and the
;;; heap lives at the end of it, the heap can grow if necessary.
;;;
;;; We reserve the first address in the heap as a sentinel, as an "unset" value
;;; for various pointers into the heap.
(declaim (inline wam-heap-pointer-unset-p
wam-heap-cell
(setf wam-heap-cell)
wam-heap-pointer
(setf wam-heap-pointer)
wam-heap-push!))
(defun* wam-heap-pointer-unset-p ((wam wam) (address heap-index))
(:returns boolean)
(declare (ignore wam))
(= address +heap-start+))
(defun* wam-heap-push! ((wam wam) (cell cell))
(:returns (values cell heap-index))
"Push the cell onto the WAM heap and increment the heap pointer.
Returns the cell and the address it was pushed to.
"
(if (>= (wam-heap-pointer wam) +store-limit+) ; todo: respect actual size...
(error "WAM heap exhausted.")
(values cell (array-push cell (wam-store wam) (wam-heap-pointer wam)))))
(defun* wam-heap-cell ((wam wam) (address heap-index))
(:returns cell)
"Return the heap cell at the given address."
(when (wam-heap-pointer-unset-p wam address)
(error "Cannot read from heap address zero."))
(aref (wam-store wam) address))
(defun* (setf wam-heap-cell) ((new-value cell)
(wam wam)
(address heap-index))
(when (wam-heap-pointer-unset-p wam address)
(error "Cannot write to heap address zero."))
(setf (aref (wam-store wam) address) new-value))
;;;; Trail
(declaim (inline wam-trail-pointer (setf wam-trail-pointer)))
(defun* wam-trail-pointer ((wam wam))
(:returns trail-index)
"Return the current trail pointer of the WAM."
(fill-pointer (wam-trail wam)))
(defun* (setf wam-trail-pointer) ((new-value trail-index)
(wam wam))
(setf (fill-pointer (wam-trail wam)) new-value))
(defun* wam-trail-push! ((wam wam) (address store-index))
(:returns (values store-index trail-index))
"Push `address` onto the trail.
Returns the address and the trail address it was pushed to.
"
(let ((trail (wam-trail wam)))
(if (= +trail-limit+ (fill-pointer trail))
(error "WAM trail exhausted.")
(values address (vector-push-extend address trail)))))
(defun* wam-trail-pop! ((wam wam))
(:returns store-index)
"Pop the top address off the trail and return it."
(vector-pop (wam-trail wam)))
(defun* wam-trail-value ((wam wam) (address trail-index))
;; TODO: can we really not just pop, or is something else gonna do something
;; fucky with the trail?
(:returns store-index)
"Return the element (a heap index) in the WAM trail at `address`."
(aref (wam-trail wam) address))
(defun* (setf wam-trail-value) ((new-value store-index)
(wam wam)
(address trail-index))
(setf (aref (wam-trail wam) address) new-value))
;;;; Stack
;;; The stack is stored as a fixed-length hunk of the main WAM store array,
;;; between the local register and the heap, with small glitch: we reserve the
;;; first word of the stack (address `+stack-start`) to mean "uninitialized", so
;;; we have a nice sentinel value for the various pointers into the stack.
(declaim (inline assert-inside-stack
wam-stack-ensure-size
wam-stack-word
(setf wam-stack-word)
wam-backtrack-pointer-unset-p
wam-environment-pointer-unset-p))
(defun* assert-inside-stack ((wam wam) (address store-index))
(:returns :void)
(declare (ignorable wam address))
(policy-cond:policy-cond
((>= debug 2)
(progn
(assert (<= +stack-start+ address (1- +stack-end+)) ()
"Cannot access stack cell at address ~X (outside the stack range ~X to ~X)"
address +stack-start+ +stack-end+)
(assert (not (= +stack-start+ address)) ()
"Cannot access stack address zero.")))
((>= safety 1)
(when (not (< +stack-start+ address +stack-end+))
(error "Stack bounds crossed. Game over.")))
(t nil)) ; wew lads
(values))
(defun* wam-stack-ensure-size ((wam wam) (address stack-index))
(:returns :void)
"Ensure the WAM stack is large enough to be able to write to `address`."
(assert-inside-stack wam address)
(values))
(defun* wam-stack-word ((wam wam) (address stack-index))
(:returns stack-word)
"Return the stack word at the given address."
(assert-inside-stack wam address)
(aref (wam-store wam) address))
(defun* (setf wam-stack-word) ((new-value stack-word)
(wam wam)
(address stack-index))
(assert-inside-stack wam address)
(setf (aref (wam-store wam) address) new-value))
(defun* wam-backtrack-pointer-unset-p
((wam wam)
&optional
((backtrack-pointer backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns boolean)
(= backtrack-pointer +stack-start+))
(defun* wam-environment-pointer-unset-p
((wam wam)
&optional
((environment-pointer environment-pointer)
(wam-environment-pointer wam)))
(:returns boolean)
(= environment-pointer +stack-start+))
;;; Stack frames are laid out like so:
;;;
;;; |PREV|
;;; | CE | <-- environment-pointer
;;; | CP |
;;; | B0 |
;;; | N |
;;; | Y0 |
;;; | .. |
;;; | Yn |
;;; |NEXT| <-- fill-pointer
(declaim (inline wam-stack-frame-ce
wam-stack-frame-cp
wam-stack-frame-cut
wam-stack-frame-n
wam-stack-frame-arg
(setf wam-stack-frame-arg)
wam-stack-frame-size))
(defun* wam-stack-frame-ce
((wam wam)
&optional
((e environment-pointer)
(wam-environment-pointer wam)))
(:returns environment-pointer)
(wam-stack-word wam e))
(defun* wam-stack-frame-cp
((wam wam)
&optional
((e environment-pointer)
(wam-environment-pointer wam)))
(:returns continuation-pointer)
(wam-stack-word wam (1+ e)))
(defun* wam-stack-frame-cut
((wam wam)
&optional
((e environment-pointer)
(wam-environment-pointer wam)))
(:returns backtrack-pointer)
(wam-stack-word wam (+ 2 e)))
(defun* wam-stack-frame-n
((wam wam)
&optional
((e environment-pointer)
(wam-environment-pointer wam)))
(:returns stack-frame-argcount)
(wam-stack-word wam (+ 3 e)))
(defun* wam-stack-frame-arg
((wam wam)
(n register-index)
&optional
((e environment-pointer)
(wam-environment-pointer wam)))
(:returns cell)
(wam-stack-word wam (+ 4 n e)))
(defun* (setf wam-stack-frame-arg) ((new-value cell)
(wam wam)
(n register-index)
&optional ((e environment-pointer)
(wam-environment-pointer wam)))
(setf (wam-stack-word wam (+ e 4 n))
new-value))
(defun* wam-stack-frame-size
((wam wam)
&optional
((e environment-pointer)
(wam-environment-pointer wam)))
(:returns stack-frame-size)
"Return the size of the stack frame starting at environment pointer `e`."
(+ (wam-stack-frame-n wam e) 4))
;;; Choice point frames are laid out like so:
;;;
;;; |PREV|
;;; 0 | N | <-- backtrack-pointer
;;; 1 | CE |
;;; 2 | CP | This is a bit different than the book. We stick the
;;; 3 | CB | arguments at the end of the frame instead of the beginning,
;;; 4 | BP | so it's easier to retrieve the other values.
;;; 5 | TR |
;;; 6 | H |
;;; 7 | A0 |
;;; | .. |
;;; 7+n | An |
;;; |NEXT| <-- fill-pointer
(declaim (inline wam-stack-choice-n
wam-stack-choice-ce
wam-stack-choice-cp
wam-stack-choice-cb
wam-stack-choice-bp
wam-stack-choice-tr
wam-stack-choice-h
wam-stack-choice-arg
(setf wam-stack-choice-arg)
wam-stack-choice-size))
(defun* wam-stack-choice-n
((wam wam)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns arity)
(wam-stack-word wam b))
(defun* wam-stack-choice-ce
((wam wam)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns environment-pointer)
(wam-stack-word wam (+ b 1)))
(defun* wam-stack-choice-cp
((wam wam)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns continuation-pointer)
(wam-stack-word wam (+ b 2)))
(defun* wam-stack-choice-cb
((wam wam)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns backtrack-pointer)
(wam-stack-word wam (+ b 3)))
(defun* wam-stack-choice-bp
((wam wam)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns continuation-pointer)
(wam-stack-word wam (+ b 4)))
(defun* wam-stack-choice-tr
((wam wam)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns trail-index)
(wam-stack-word wam (+ b 5)))
(defun* wam-stack-choice-h
((wam wam)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns heap-index)
(wam-stack-word wam (+ b 6)))
(defun* wam-stack-choice-arg
((wam wam)
(n arity)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns cell)
(wam-stack-word wam (+ b 7 n)))
(defun* (setf wam-stack-choice-arg) ((new-value cell)
(wam wam)
(n arity)
&optional ((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(setf (wam-stack-word wam (+ b 7 n))
new-value))
(defun* wam-stack-choice-size
((wam wam)
&optional
((b backtrack-pointer)
(wam-backtrack-pointer wam)))
(:returns stack-choice-size)
"Return the size of the choice frame starting at backtrack pointer `b`."
(+ (wam-stack-choice-n wam b) 7))
(defun* wam-stack-top ((wam wam))
(:returns stack-index)
"Return the top of the stack.
This is the first place it's safe to overwrite in the stack.
"
;; The book is wrong here -- it looks up the "current frame size" to
;; determine where the next frame should start, but on the first allocation
;; there IS no current frame so it looks at garbage. Fuckin' great.
(let ((e (wam-environment-pointer wam))
(b (wam-backtrack-pointer wam)))
(cond
((and (wam-backtrack-pointer-unset-p wam b)
(wam-environment-pointer-unset-p wam e)) ; first allocation
(1+ +stack-start+))
((> e b) ; the last thing on the stack is a frame
(+ e (wam-stack-frame-size wam e)))
(t ; the last thing on the stack is a choice point
(+ b (wam-stack-choice-size wam b))))))
;;;; Resetting
(defun* wam-truncate-heap! ((wam wam))
(setf (wam-heap-pointer wam) (1+ +heap-start+)))
(defun* wam-truncate-trail! ((wam wam))
(setf (fill-pointer (wam-trail wam)) 0))
(defun* wam-truncate-unification-stack! ((wam wam))
(setf (fill-pointer (wam-unification-stack wam)) 0))
(defun* wam-reset-local-registers! ((wam wam))
(loop :for i :from 0 :below +register-count+ :do
(setf (wam-local-register wam i) (make-cell-null))))
(defun* wam-reset! ((wam wam))
(wam-truncate-heap! wam)
(wam-truncate-trail! wam)
(wam-truncate-unification-stack! wam)
(policy-cond:policy-if (>= debug 2)
(wam-reset-local-registers! wam)
nil) ; fuck it
(setf (wam-program-counter wam) 0
(wam-continuation-pointer wam) 0
(wam-environment-pointer wam) +stack-start+
(wam-backtrack-pointer wam) +stack-start+
(wam-cut-pointer wam) +stack-start+
(wam-heap-backtrack-pointer wam) +heap-start+
(wam-backtracked wam) nil
(wam-fail wam) nil
(wam-subterm wam) +heap-start+
(wam-mode wam) nil))
;;;; Code
(defun* retrieve-instruction (code-store (address code-index))
"Return the full instruction at the given address in the code store."
(make-array (instruction-size (aref code-store address))
:displaced-to code-store
:displaced-index-offset address
:adjustable nil
:element-type 'code-word))
(defun* wam-code-label ((wam wam)
(functor functor-index))
(:returns (or null code-index))
(gethash functor (wam-code-labels wam)))
(defun* (setf wam-code-label) ((new-value code-index)
(wam wam)
(functor symbol)
(arity arity))
;; Note that this takes a functor/arity and not a cons.
(setf (gethash (wam-ensure-functor-index wam (cons functor arity))
(wam-code-labels wam))
new-value))
(defun* wam-load-query-code! ((wam wam)
(query-code query-code-holder))
(:returns :void)
(setf (subseq (wam-code wam) 0) query-code)
(values))
;;;; Logic Stack
(defstruct logic-frame
(start 0 :type code-index)
(final nil :type boolean)
(predicates (make-hash-table) :type hash-table))
(defun* wam-logic-pool-release ((wam wam) (frame logic-frame))
(:returns :void)
(with-slots (start final predicates) frame
(clrhash predicates)
(setf start 0 final nil))
(push frame (wam-logic-pool wam))
(values))
(defun* wam-logic-pool-request ((wam wam))
(:returns logic-frame)
(or (pop (wam-logic-pool wam))
(make-logic-frame)))
(defun* wam-current-logic-frame ((wam wam))
(:returns (or null logic-frame))
(first (wam-logic-stack wam)))
(defun* wam-logic-stack-empty-p ((wam wam))
(:returns boolean)
(not (wam-current-logic-frame wam)))
(defun* wam-logic-open-p ((wam wam))
(:returns boolean)
(let ((frame (wam-current-logic-frame wam)))
(and frame (not (logic-frame-final frame)))))
(defun* wam-logic-closed-p ((wam wam))
(:returns boolean)
(not (wam-logic-open-p wam)))
(defun* wam-push-logic-frame! ((wam wam))
(:returns :void)
(assert (wam-logic-closed-p wam) ()
"Cannot push logic frame unless the logic stack is closed.")
(let ((frame (wam-logic-pool-request wam)))
(setf (logic-frame-start frame)
(wam-code-pointer wam))
(push frame (wam-logic-stack wam)))
(values))
(defun* wam-pop-logic-frame! ((wam wam))
(:returns :void)
(with-slots (logic-stack) wam
(assert logic-stack ()
"Cannot pop logic frame from an empty logic stack.")
(assert (logic-frame-final (first logic-stack)) ()
"Cannot pop unfinalized logic frame.")
(let ((frame (pop logic-stack)))
(setf (wam-code-pointer wam)
(logic-frame-start frame))
(loop :for label :being :the hash-keys :of (logic-frame-predicates frame)
:do (remhash label (wam-code-labels wam)))
(wam-logic-pool-release wam frame)))
(values))
(defun* assert-label-not-already-compiled ((wam wam) clause label)
(assert (not (wam-code-label wam label))
()
"Cannot add clause ~S because its predicate has preexisting compiled code."
clause))
(defun* wam-logic-frame-add-clause! ((wam wam) clause)
(assert (wam-logic-open-p wam) ()
"Cannot add clause ~S without an open logic stack frame."
clause)
(multiple-value-bind (functor arity) (find-predicate clause)
(let ((label (wam-ensure-functor-index wam (cons functor arity))))
(assert-label-not-already-compiled wam clause label)
(with-slots (predicates)
(wam-current-logic-frame wam)
(enqueue clause (gethash-or-init label predicates (make-queue))))))
(values))
(defun* wam-finalize-logic-frame! ((wam wam))
(assert (wam-logic-open-p wam) ()
"There is no logic frame waiting to be finalized.")
(with-slots (predicates final)
(wam-current-logic-frame wam)
(loop :for clauses :being :the hash-values :of predicates
;; circular dep on the compiler here, ugh.
:do (compile-rules wam (queue-contents clauses)))
(setf final t))
(values))
;;;; Registers
;;; The WAM has two types of registers:
;;;
;;; * Local/temporary/arguments registers live at the beginning of the WAM
;;; memory store.
;;;
;;; * Stack/permanent registers live on the stack, and need some extra math to
;;; find their location.
;;;
;;; Registers are typically denoted by their "register index", which is just
;;; their number. Hoever, the bytecode needs to be able to distinguish between
;;; local and stack registers. To do this we just make separate opcodes for
;;; each kind. This is ugly, but it lets us figure things out at compile time
;;; instead of runtime, and register references happen A LOT at runtime.
;;;
;;; As for the CONTENTS of registers: a register (regardless of type) always
;;; contains a cell. The book is maddeningly unclear on this in a bunch of
;;; ways. I will list them here so maybe you can feel a bit of my suffering
;;; through these bytes of text.
;;;
;;; The first thing the book says about registers is "registers have the same
;;; format as heap cells". Okay, fine. The *very next diagram* shows "register
;;; assignments" that appear to put things that are very much *not* heap cells
;;; into registers!
;;;
;;; After a bit of puttering you realize that the diagram is referring only to
;;; the compilation, not what's *actually* stored in these registers at runtime.
;;; You move on and see some pseudocode that contains `X_i <- HEAP[H]` which
;;; confirms that his original claim was accurate, and registers are actually
;;; (copies of) heap cells. Cool.
;;;
;;; Then you move on and see the definition of `deref(a : address)` and note
;;; that it takes an *address* as an argument. On the next page you see
;;; `deref(X_i)` and wait what the fuck, a register is an *address* now? You
;;; scan down the page and see `HEAP[H] <- X_i` which means no wait it's a cell
;;; again.
;;;
;;; After considering depositing your laptop into the nearest toilet and
;;; becoming a sheep farmer, you conclude a few things:
;;;
;;; 1. The book's code won't typecheck.
;;; 2. The author is playing fast and loose with `X_i` -- sometimes it seems to
;;; be used as an address, sometimes as a cell.
;;; 3. The author never bothers to nail down exactly what is inside the fucking
;;; things, which is a problem because of #2.
;;;
;;; If you're like me (painfully unlucky), you took a wild guess and decided to
;;; implement registers as containing *addresses*, i.e., indexes into the
;;; heap, figuring that if you were wrong it would soon become apparent.
;;;
;;; WELL it turns out that you can get all the way to CHAPTER FIVE with
;;; registers implemented as addresses, at which point you hit a wall and need
;;; to spend a few hours refactoring a giant chunk of your code and writing
;;; angry comments in your source code.
;;;
;;; Hopefully I can save someone else this misery by leaving you with this:
;;; ____ _____________________________________ _____ ___ ____ ______ ______________ __ _____
;;; / __ \/ ____/ ____/ _/ ___/_ __/ ____/ __ \/ ___/ / | / __ \/ ____/ / ____/ ____/ / / / / ___/
;;; / /_/ / __/ / / __ / / \__ \ / / / __/ / /_/ /\__ \ / /| | / /_/ / __/ / / / __/ / / / / \__ \
;;; / _, _/ /___/ /_/ // / ___/ // / / /___/ _, _/___/ / / ___ |/ _, _/ /___ / /___/ /___/ /___/ /______/ /
;;; /_/ |_/_____/\____/___//____//_/ /_____/_/ |_|/____/ /_/ |_/_/ |_/_____/ \____/_____/_____/_____/____/
(declaim (inline wam-local-register
(setf wam-local-register)
wam-stack-register
(setf wam-stack-register)))
(defun* wam-local-register ((wam wam) (register register-index))
(:returns cell)
"Return the value stored in the WAM local register with the given index."
(aref (wam-store wam) register))
(defun* (setf wam-local-register) ((new-value cell)
(wam wam)
(register register-index))
(setf (aref (wam-store wam) register) new-value))
(defun* wam-stack-register ((wam wam) (register register-index))
(:returns cell)
"Return the value stored in the WAM stack register with the given index."
(wam-stack-frame-arg wam register))
(defun* (setf wam-stack-register) ((new-value cell)
(wam wam)
(register register-index))
(setf (wam-stack-frame-arg wam register) new-value))
(defun* wam-s-cell ((wam wam))
"Retrieve the cell the S register is pointing at.
If S is unbound, throws an error.
"
(let ((s (wam-subterm wam)))
(if (wam-heap-pointer-unset-p wam s)
(error "Cannot dereference unbound S register.")
(wam-heap-cell wam s))))
;;;; Functors
;;; Functors are stored in an adjustable array. Cells refer to a functor using
;;; the functor's address in this array.
(declaim (inline wam-functor-lookup
wam-functor-symbol
wam-functor-arity))
(defun* wam-ensure-functor-index ((wam wam) (functor functor))
(:returns functor-index)
"Return the index of the functor in the WAM's functor table.
If the functor is not already in the table it will be added.
"
(let ((functors (wam-functors wam)))
(or (position functor functors :test #'equal)
(vector-push-extend functor functors))))
(defun* wam-functor-lookup ((wam wam) (functor-index functor-index))
(:returns functor)
"Return the functor with the given index in the WAM."
(aref (wam-functors wam) functor-index))
(defun* wam-functor-symbol ((wam wam) (functor-index functor-index))
(:returns symbol)
"Return the symbol of the functor with the given index in the WAM."
(car (wam-functor-lookup wam functor-index)))
(defun* wam-functor-arity ((wam wam) (functor-index functor-index))
(:returns arity)
"Return the arity of the functor with the given index in the WAM."
(cdr (wam-functor-lookup wam functor-index)))
;;;; Unification Stack
(defun* wam-unification-stack-push! ((wam wam) (address store-index))
(:returns :void)
(vector-push-extend address (wam-unification-stack wam))
(values))
(defun* wam-unification-stack-pop! ((wam wam))
(:returns store-index)
(vector-pop (wam-unification-stack wam)))
(defun* wam-unification-stack-empty-p ((wam wam))
(:returns boolean)
(zerop (length (wam-unification-stack wam))))