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# exav - EXecution As a Value
## Why?
The fundamental building block of Clojure is the immutable value. Values can be shared, values can be saved, values can be inspected, values can be transformed.
Programs often must ask for input from the external world. exav models this much like core.async; we write code which looks very much like a simple function which is producing a value, except that sprinkled throughout are operations which cause execution to be suspended until some external event takes place to supply us with a new value.
core.async is wonderful, but its fundamental building block is the channel, which is not a value. We have no way of looking inside the computation which is suspended. We have no way of rewinding it. We have no way of persisting it. core.async code builds up a black box, to be fed with data.
I want the ability to rewind time, hotload new code, then run time forward again. I want the ability to script complex interactions in a game that take place over arbitrary amounts of time, and for my entire game state to exist in a map that can be saved as EDN or Transit. exav gives me the tools to do this.
## Usage
exav currently has one basic building block: the `proc` macro. The `proc` macro works very much like the `go` macro in core.async, except that instead of returning a channel, it returns a function, and instead of blocking on `>!` or `<!`, it blocks on calls to `wait`. The function returned by proc takes zero, one, or two arguments.
(let [p (proc (str "Hello, " (wait :name)))
; calling a proc with no arguments starts the process
state (p)
; a proc will always return a map with a standard structure
; if the :waitfor key exists, the value is whatever was passed to (wait)
waitfor (:waitfor state) ; => :name
; calling a proc with two arguments continues the process
; you must pass the state returned by the proc, and the value to be returned by (wait)
state2 (p state "Bob")
; if the :result key exists, it will be the only thing in the map, and is equal to the
; final return value of the proc
result (:result state2) ; => "Hello, Bob"
; note that it is completely legal to hold onto and re-use old states!
result2 (:result (p state "Phil")) ; => "Hello, Phil"
; a proc can also be called with just one argument, which is the same as passing nil
; for the second argument
result3 (:result (p state)) ; => "Hello, "
])
`wait` always takes a single argument; a value that describes what the proc is waiting for.
## License
Copyright © 2014 Jeremy Penner
Distributed under the Eclipse Public License either version 1.0 or (at
your option) any later version.

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# Introduction to exav
TODO: write [great documentation](http://jacobian.org/writing/what-to-write/)

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(defproject exav "0.1.0-SNAPSHOT"
:description "EXecution As a Value, flexible tools for async"
:url "http://example.com/FIXME"
:license {:name "Eclipse Public License"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:dependencies [[org.clojure/clojure "1.6.0"]
[org.clojure/core.async "0.1.346.0-17112a-alpha"]])

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(ns exav.core
(:require [clojure.core.async.impl.ioc-macros :as ioc]))
;; Goal:
;; Convert an arbitrary clojure form that contains "blocking" calls, to a function which
;; accepts a state and an argument (the result of the blocking call) and returns the next
;; state and a "blocking" value. This state can then be persisted, arbitrarily rolled back,
;; and re-run; it is a simple clojure value. No hidden local state is generated.
; "machine" is a mutable array of objects used by the core.async ioc-macros to implement
; the state machine. standard indices into the array are used to implement various fields
; (what state is running, the value of the previous expression, etc)
; We extend this with two extra fields - WAITFOR-IDX, which stores the value passed to (wait),
; and WAITFOR-FLAG-IDX, which is set to true when (wait) is called, so that we can tell the
; difference between a final return and (wait nil).
(def ^:const WAITFOR-IDX ioc/USER-START-IDX)
(def ^:const WAITFOR-FLAG-IDX (+ ioc/USER-START-IDX 1))
(def ^:const LOCALS-START-IDX (+ ioc/USER-START-IDX 2))
(defn wait [ev]
(assert nil "wait must be used in an proc block"))
(defn wait* [machine nextblock ev]
"The function called to implement our custom terminator (last statement in a 'block')
when the state machine is running. Returns nil to signal that the state machine should
suspend execution (the other option is :recur, which would immediately move on to the
next block)"
(ioc/aset-all! machine WAITFOR-IDX ev
WAITFOR-FLAG-IDX true
ioc/STATE-IDX nextblock)
nil)
(defn config-machine [machine state value]
"Takes apart a state map as returned from a proc, and sets up all the values in an empty
machine array to allow it to continue. Returns the state's thread bindings, basically just
to pass to run-machine, because they are the only thing we need to configure whether or not
we have a state."
(assert (and (integer? (:state state))
(vector? (:locals state)))
"state is not valid")
; copy locals
(doseq [[i local] (map vector (iterate inc LOCALS-START-IDX) (:locals state))]
(ioc/aset-object machine i local))
(ioc/aset-all! machine
; copy next state
ioc/STATE-IDX (:state state)
; copy argument
ioc/VALUE-IDX value)
(:bindings state))
(defn run-machine [machine bindings]
"Given a machine array and thread bindings, runs the machine until it finishes or blocks,
and returns a new state map with the results."
(let [bindings (or bindings (clojure.lang.Var/getThreadBindingFrame))]
(ioc/aset-all! machine ioc/BINDINGS-IDX bindings)
(ioc/run-state-machine machine)
(if (ioc/aget-object machine WAITFOR-FLAG-IDX)
; blocked
{:bindings bindings
:locals (vec (for [i (range LOCALS-START-IDX (.length machine))]
(ioc/aget-object machine i)))
:state (ioc/aget-object machine ioc/STATE-IDX)
:waitfor (ioc/aget-object machine WAITFOR-IDX)}
; complete
{:result (ioc/aget-object machine ioc/VALUE-IDX)})))
(defmacro proc [& body]
`(let [machine-fn# ~(ioc/state-machine `(do ~@body) 2 (keys &env) {`wait `wait*})]
(fn proc#
([] (run-machine (machine-fn#) nil))
([state#] (proc# state# nil))
([state# value#]
(let [machine# (machine-fn#)
bindings# (config-machine machine# state# value#)]
(run-machine machine# bindings#))))))

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(ns exav.perfunct)
; Persistable functions:
; A function in clojure is a first-class value that can be passed around and composed.
; It can't, however, be sensibly persisted, except in ad-hoc ways. In general, there
; is not much of a use-case for persisting arbitrary functions. We don't really ever
; want to be given an arbitrary block of clojure code, compile it, and run it - that's
; a security disaster. It would often be nice, however, to have a first-class way of
; _referring_ to a particular set of whitelisted functions, and be able to write down
; and read those _references_.
; One could concievably just persist a clojure var, but that has its own set of problems.
; Chief among them is that you would still require a whitelist to ensure that you're not
; being tricked into calling "evil" functions in places that you didn't expect.
; Another significant problem is that you lose any particular ability to use or persist
; a closure.
; perfunct provides a simple solution to these problems. We introduce an object called
; a peref (for "persistable reference"), which contains a simple value used to refer to
; the function, can be directly called like a function (much like a var), and derefed (if
; it is referring to a non-function unpersistable thing, such as an atom or channel).
; We provide built-in hooks for pr-str and transit serialization, reader literals, and
; functions for extracting a peref's value as clojure data.
; In addition to persisting references to functions, we can use this flexible
; functionality to persist references to atoms, to core.async channels, to expensive
; objects that should be instantiated lazily, or even to resources meant to be loaded
; from disk.
; definvokable adapted from code by Meikel Brandmeyer
; https://groups.google.com/forum/#!topic/clojure/pl4HgR9L_lg
(def max-arities 20)
(defmacro definvokable
[type fields f & deftype-tail]
(let [args (repeatedly max-arities gensym)
arity (fn [n]
(let [args (take n args)]
`(invoke [this# ~@args] (~f this# ~@args))))
vararg `(invoke [this# ~@args more#]
(apply ~f this# ~@args more#))
apply-to `(applyTo [this# args#] (apply ~f this# args#))]
`(deftype ~type
~fields
clojure.lang.IFn
~@(map arity (range (inc max-arities)))
~vararg
~apply-to
~@deftype-tail)))
(defn invoke-peref [peref & args] (apply @peref args))
(definvokable Peref [lookup name params] invoke-peref
clojure.lang.IDeref
(deref [_] (apply (.lookup-fn lookup) name params)))
(defn invoke-peref-lookup [lookup name & args] (->Peref lookup name args))
(definvokable PerefLookup [id lookup-fn] invoke-peref-lookup)
(defn peref-tag [p]
(let [id (.id (.lookup p))]
(str (namespace id) "/" (name id))))
(defn peref-data [p]
(let [name (.name p)
params (.params p)]
(vec (cons name params))))
(defmethod print-method Peref [v ^java.io.Writer w]
"Print an edn literal for a Peref"
(let [tag (str "#" (peref-tag v) " ")]
(.write w tag)
(print-method (peref-data v) w)))
(defn cached [lookup-fn]
"Takes the result of a lookup-fn and memoizes it. Also provides a function,
accessed by looking up :clear-cache, which clears the cache completely if called
with no arguments, and clears the cache for a particular peref when called with
its arguments."
(let [cache (atom {})
clear-cache
(fn ([] (reset! cache {}))
([peref] (swap! cache #(dissoc % (peref-data peref)))))]
(fn [& args]
(if (= (first args) :clear-cache)
(apply clear-cache (rest args))
(if (contains? @cache args)
(get @cache args)
(let [v (apply lookup-fn (vec args))]
(swap! cache #(assoc % args v))
v))))))
(defn partial-map [fn-map]
"Takes a map of the form {name fn} and returns a lookup function which
takes a name and the arguments to pass to fn and returns the fn.
It is legal for the value to not be a function so long as the lookup
is not passed any extra arguments besides the name."
(fn [name & args]
(let [fn (get fn-map name)]
(if fn
(apply partial fn args)
nil))))
(defn compose-lookup [& lookup-fns]
"Returns a lookup-fn which calls each of the passed-in lookup-fns in turn,
returning the first non-nil result."
(fn [& args]
(loop [lookup-fns lookup-fns]
(if (seq lookup-fns)
(let [lookup-fn (first lookup-fns)
v (apply lookup-fn args)]
(if (nil? v)
(recur (rest lookup-fns))
v))))))
(defn lookup [id & lookup-fns]
"Creates a peref lookup function, identified by a namespaced symbol, which is
used to create peref objects. Simply call the result with the arguments to pass
to your lookup-fns, and it will return a peref object that will call your lookup-fns
when derefed."
(->PerefLookup id (apply compose-lookup lookup-fns)))

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(ns exav.core-test
(:require [clojure.test :refer :all]
[exav.core :refer :all]))
(deftest a-test
(testing "FIXME, I fail."
(is (= 0 1))))