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(.module:
[lux #*
[control
[functor (#+ Functor)]
[apply (#+ Apply)]
["." monad (#+ do Monad)]]
["." io (#+ IO io)]
[data
["." product]
["." maybe]
[collection
["." list]]]
[type
abstract]]
[//
["." atom (#+ Atom atom)]
["." promise (#+ Promise Resolver)]
["." frp ("#/." functor)]])
(type: #export (Observer a)
(-> a (IO Any)))
(abstract: #export (Var a)
{#.doc "A mutable cell containing a value, and observers that will be alerted of any change to it."}
(Atom [a (List (Observer a))])
(def: #export (var value)
{#.doc "Creates a new STM var, with a default value."}
(All [a] (-> a (Var a)))
(:abstraction (atom.atom [value (list)])))
(def: read!!
(All [a] (-> (Var a) a))
(|>> :representation atom.read io.run product.left))
(def: #export (read! (^:representation var))
{#.doc "Reads var immediately, without going through a transaction."}
(All [a] (-> (Var a) (IO a)))
(|> var
atom.read
(:: io.functor map product.left)))
(def: (write! new-value (^:representation var))
(All [a] (-> a (Var a) (IO Any)))
(do io.monad
[(^@ old [_value _observers]) (atom.read var)
succeeded? (atom.compare-and-swap old [new-value _observers] var)]
(if succeeded?
(do @
[_ (monad.map @ (function (_ f) (f new-value)) _observers)]
(wrap []))
(write! new-value (:abstraction var)))))
(def: #export (follow target)
{#.doc "Creates a channel that will receive all changes to the value of the given var."}
(All [a] (-> (Var a) (IO (frp.Channel a))))
(do io.monad
[#let [[channel source] (frp.channel [])
target (:representation target)]
_ (atom.update (function (_ [value observers])
[value (#.Cons (:: source feed) observers)])
target)]
(wrap channel)))
)
(type: (Tx-Frame a)
{#var (Var a)
#original a
#current a})
(type: Tx
(List (Ex [a] (Tx-Frame a))))
(type: #export (STM a)
{#.doc "A computation which updates a transaction and produces a value."}
(-> Tx [Tx a]))
(def: (find-var-value var tx)
(All [a] (-> (Var a) Tx (Maybe a)))
(|> tx
(list.find (function (_ [_var _original _current])
(is? (:coerce (Var Any) var)
(:coerce (Var Any) _var))))
(:: maybe.monad map (function (_ [_var _original _current])
_current))
(:assume)
))
(def: #export (read var)
(All [a] (-> (Var a) (STM a)))
(function (_ tx)
(case (find-var-value var tx)
(#.Some value)
[tx value]
#.None
(let [value (read!! var)]
[(#.Cons [var value value] tx)
value]))))
(def: (update-tx-value var value tx)
(All [a] (-> (Var a) a Tx Tx))
(case tx
#.Nil
#.Nil
(#.Cons [_var _original _current] tx')
(if (is? (:coerce (Var Any) var)
(:coerce (Var Any) _var))
(#.Cons {#var (:coerce (Var Any) _var)
#original (:coerce Any _original)
#current (:coerce Any value)}
tx')
(#.Cons {#var _var
#original _original
#current _current}
(update-tx-value var value tx')))
))
(def: #export (write value var)
{#.doc "Writes value to var."}
(All [a] (-> a (Var a) (STM Any)))
(function (_ tx)
(case (find-var-value var tx)
(#.Some _)
[(update-tx-value var value tx)
[]]
#.None
[(#.Cons [var (read!! var) value] tx)
[]])))
(structure: #export _ (Functor STM)
(def: (map f fa)
(function (_ tx)
(let [[tx' a] (fa tx)]
[tx' (f a)]))))
(structure: #export apply (Apply STM)
(def: &functor ..functor)
(def: (apply ff fa)
(function (_ tx)
(let [[tx' f] (ff tx)
[tx'' a] (fa tx')]
[tx'' (f a)]))))
(structure: #export monad (Monad STM)
(def: &functor ..functor)
(def: (wrap a)
(function (_ tx) [tx a]))
(def: (join mma)
(function (_ tx)
(let [[tx' ma] (mma tx)]
(ma tx')))))
(def: #export (update f var)
{#.doc "Will update a Var's value, and return a tuple with the old and the new values."}
(All [a] (-> (-> a a) (Var a) (STM [a a])))
(do ..monad
[a (read var)
#let [a' (f a)]
_ (write a' var)]
(wrap [a a'])))
(def: (can-commit? tx)
(-> Tx Bit)
(list.every? (function (_ [_var _original _current])
(is? _original (read!! _var)))
tx))
(def: (commit-var! [_var _original _current])
(-> (Ex [a] (Tx-Frame a)) (IO Any))
(if (is? _original _current)
(io [])
(write! _current _var)))
(def: fresh-tx Tx (list))
(type: (Commit a)
[(STM a)
(Promise a)
(Resolver a)])
(def: pending-commits
(Atom (Rec Commits
[(Promise [(Ex [a] (Commit a)) Commits])
(Resolver [(Ex [a] (Commit a)) Commits])]))
(atom (promise.promise [])))
(def: commit-processor-flag
(Atom Bit)
(atom #0))
(def: (issue-commit commit)
(All [a] (-> (Commit a) (IO Any)))
(let [entry [commit (promise.promise [])]]
(do io.monad
[|commits|&resolve (atom.read pending-commits)]
(loop [[|commits| resolve] |commits|&resolve]
(case (promise.poll |commits|)
#.None
(do io.monad
[resolved? (resolve entry)]
(if resolved?
(atom.write (product.right entry) pending-commits)
(recur |commits|&resolve)))
(#.Some [head tail])
(recur tail))))))
(def: (process-commit commit)
(All [a] (-> (Commit a) (IO Any)))
(let [[stm-proc output resolve] commit
[finished-tx value] (stm-proc fresh-tx)]
(if (can-commit? finished-tx)
(do io.monad
[_ (monad.map @ commit-var! finished-tx)]
(resolve value))
(issue-commit commit))))
(def: init-processor!
(IO Any)
(do io.monad
[flag (atom.read commit-processor-flag)]
(if flag
(wrap [])
(do @
[was-first? (atom.compare-and-swap flag #1 commit-processor-flag)]
(if was-first?
(do @
[[promise resolve] (atom.read pending-commits)]
(promise.await (function (recur [head [tail _resolve]])
(do @
[_ (process-commit head)]
(promise.await recur tail)))
promise))
(wrap [])))
)))
(def: #export (commit stm-proc)
{#.doc (doc "Commits a transaction and returns its result (asynchronously)."
"Note that a transaction may be re-run an indeterminate number of times if other transactions involving the same variables successfully commit first."
"For this reason, it's important to note that transactions must be free from side-effects, such as I/O.")}
(All [a] (-> (STM a) (Promise a)))
(let [[output resolver] (promise.promise [])]
(exec (io.run (do io.monad
[_ init-processor!]
(issue-commit [stm-proc output resolver])))
output)))
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