urbit/lib/async.hoon
2019-05-31 14:59:36 -07:00

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|* [input-type=mold card-type=mold contract-type=mold]
|%
+$ async-input [=bowl:gall in=(unit [=wire sign=input-type])]
+$ async-move (pair bone card-type)
::
:: cards: cards to send immediately. These will go out even if a
:: later stage of the computation fails, so they shouldn't have
:: any semantic effect on the rest of the system.
:: Alternately, they may record an entry in contracts with
:: enough information to undo the effect if the computation
:: fails.
:: effects: moves to send after the computation ends.
:: contracts: stuff to cancel at end of computation.
:: wait: don't move on, stay here. The next sign should come back
:: to this same callback.
:: cont: continue computation with new callback.
:: fail: abort computation; don't send effects
:: done: finish computation; send effects
::
++ async-output-raw
|* a=mold
$~ [~ ~ ~ %done *a]
$: cards=(list card-type)
effects=(list async-move)
contracts=(set [add=? contract=contract-type])
$= next
$% [%wait ~]
[%cont self=(async-form-raw a)]
[%fail err=(pair term tang)]
[%done value=a]
==
==
::
++ async-form-raw
|* a=mold
$-(async-input (async-output-raw a))
::
:: Abort asynchronous computation with error message
::
++ async-fail
|= err=(pair term tang)
|= async-input
[~ ~ ~ %fail err]
::
:: Asynchronous transcaction monad.
::
:: Combo of four monads:
:: - Reader on input-type
:: - Writer on card-type
:: - Continuation
:: - Exception
::
++ async
|* a=mold
|%
++ output (async-output-raw a)
::
:: Type of an asynchronous computation.
::
++ form (async-form-raw a)
::
:: Monadic pure. Identity computation for bind.
::
++ pure
|= arg=a
^- form
|= async-input
[~ ~ ~ %done arg]
::
:: Monadic bind. Combines two computations, associatively.
::
++ bind
|* b=mold
|= [m-b=(async-form-raw b) fun=$-(b form)]
^- form
|= input=async-input
=/ b-res=(async-output-raw b)
(m-b input)
^- output
:^ cards.b-res effects.b-res contracts.b-res
?- -.next.b-res
%wait [%wait ~]
%cont [%cont ..$(m-b self.next.b-res)]
%fail [%fail err.next.b-res]
%done [%cont (fun value.next.b-res)]
==
::
:: The async monad must be evaluted in a particular way to maintain
:: its monadic character. +take:eval implements this.
::
++ eval
|%
:: Indelible state of a async
::
+$ eval-form
$: effects=(list async-move)
contracts=(set contract-type)
=form
==
::
:: Convert initial form to eval-form
::
++ from-form
|= =form
^- eval-form
[~ ~ form]
::
:: The cases of results of +take
::
+$ eval-result
$% [%next ~]
[%fail contracts=(set contract-type) err=(pair term tang)]
[%done contracts=(set contract-type) value=a]
==
::
:: Take a new sign and run the async against it
::
++ take
:: moves: accumulate throughout recursion the moves to be
:: produced now
=| moves=(list async-move)
|= [=eval-form =bone =async-input]
^- [[(list async-move) =eval-result] _eval-form]
=* take-loop $
:: run the async callback
::
=/ =output (form.eval-form async-input)
:: add cards to moves
::
=. moves
%+ welp
moves
%+ turn cards.output
|= card=card-type
^- async-move
[bone card]
:: add effects to list to be produced when done
::
=. effects.eval-form
(weld effects.eval-form effects.output)
:: add or remove contracts
::
=. .
=* loop-result .
=/ new=(list [add=? contract=contract-type])
~(tap in contracts.output)
|- ^+ loop-result
=* loop $
?~ new
loop-result
?: add.i.new
?: (~(has in contracts.eval-form) contract.i.new)
%= loop-result
next.output [%fail %contract-already-exists >contract.i.new< ~]
==
%= loop
contracts.eval-form (~(put in contracts.eval-form) contract.i.new)
new t.new
==
?: (~(has in contracts.eval-form) contract.i.new)
%= loop
contracts.eval-form (~(del in contracts.eval-form) contract.i.new)
new t.new
==
%= loop-result
next.output [%fail %contract-doesnt-exist >contract.i.new< ~]
==
:: if done, produce effects
::
=? moves ?=(%done -.next.output)
%+ welp
moves
effects.eval-form
:: case-wise handle next steps
::
?- -.next.output
%wait [[moves %next ~] eval-form]
%fail [[moves %fail contracts.eval-form err.next.output] eval-form]
%done [[moves %done contracts.eval-form value.next.output] eval-form]
%cont
:: recurse to run continuation with initialization input
::
%_ take-loop
form.eval-form self.next.output
async-input [bowl.async-input ~]
==
==
--
--
--