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-- G N A T C O L L --
-- --
-- Copyright (C) 2017, AdaCore --
-- --
-- This library is free software; you can redistribute it and/or modify it --
-- under terms of the GNU General Public License as published by the Free --
-- Software Foundation; either version 3, or (at your option) any later --
-- version. This library is distributed in the hope that it will be useful, --
-- but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHAN- --
-- TABILITY or FITNESS FOR A PARTICULAR PURPOSE. --
-- --
-- --
-- --
-- --
-- --
-- You should have received a copy of the GNU General Public License and --
-- a copy of the GCC Runtime Library Exception along with this program; --
-- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
-- <http://www.gnu.org/licenses/>. --
-- --
------------------------------------------------------------------------------
-- A promise (also known sometimes as a future or deferred) is a
-- a synchronization mechanism between asynchronous routines.
--
-- Such routines could be implemented as tasks, but also be handled
-- via the system's asynchronous I/O primitives, or an event loop in a
-- GUI program for instance.
--
-- A promise is a value that such a routine can return immediately to the
-- caller, before it even starts its processing or the actual value is
-- available. The caller can then subscribe to the promise, so that when
-- the value becomes actually known, it gets notified and a callback is
-- executed.
--
-- Here is a simple example:
--
-- package Str_Promises is new Promises (String);
--
-- type Process_Page is new Str_Promises.Callback with null record;
-- overriding procedure On_Next
-- (Self : in out Process_Page; Page : String)
-- is
-- begin
-- Put_Line ("Page contents is known: " & Page);
-- end On_Next;
--
-- P : Str_Promises.Promise;
--
-- P := Fetch_URL_Asynchronously ("http://...");
-- P.Subscribe (new Process_Page);
--
-- Where Fetch_URL_Asynchronously could run in a task, connect to a
-- web server and query a document.
--
-- But promises are more interesting when they are chained, i.e. the
-- action executed when the first promise is resolved will itself
-- return a promise. Here is an example:
--
-- package Str_Promises is new Promises (String);
-- package Int_Promises is new Promises (Integer);
-- package Str_To_Int is new Chains (Str_Promises, Int_Promises);
-- use Str_To_Int, Int_Promises;
--
-- type Count_Elements is new Str_To_Int.Callback with null record;
-- overriding procedure On_Next
-- (Self : in out Process_Page;
-- Page : String;
-- Output : in out Int_Promises.Promise);
-- -- For instance, count the number of elements in the XML, and
-- -- would call Output.Set_Value
--
-- type Report_Count is new Int_Promises.Callback with null record;
-- overriding procedure On_Next
-- (Self : in out Report_Count;
-- Count : Integer);
-- -- For instance display the number of elements in a GUI
--
-- Subscribe
-- (Fetch_URL_Asynchronously ("http://...")
-- and new Count_Elements
-- and new Report_Count);
--
-- The code above returns immediately, even though the URL will be fetched
-- in the background (which could take a few seconds), then parsed to count
-- the number of elements (which could be done in a separate task and take
-- a few milliseconds), and finally this count will be displayed in a GUI.
--
-- The advantage of this code is that it is composed of small, independent
-- building blocks that are executed when data becomes available. The caller
-- does not have to take care of the synchronization, since the promises
-- handle that.
--
-- Behavior
-- ========
--
-- A Promise is a concept that has been adopted by multiple programming
-- languages, starting with javascript and C++ (promises and futures).
--
-- There are some standard behaviors associated with promises, which this
-- package tries to conform with:
--
-- * A promise can be in one of three states:
-- - Pending: the promise has no associated value yet. Some subprogram is
-- still running in the background to fetch that value.
-- - Resolved: the routine has successfully finished running, and given
-- an actual value to the promise (via a call to Set_Value)
-- - Failed: the routine failed, and no value will ever be provided to the
-- routine. A reason for the failure is provided via a call
-- to Set_Error)
--
-- * Any number of callbacks can be set on a routine. They will all be
-- executed once when the promise is resolved or failed. They are never
-- executed again afterwards.
--
-- * A promise can be resolved at any time. Whenever it is resolved, all
-- callbacks currently set on the promise are executed and then
-- disconnected. It is an error to resolve a promise more than once.
--
-- * A callback can be added to a promise at any time. If the promise has
-- already been resolved, the callback is executed immediately with the
-- value set on that promise.
--
-- Tasks
-- =====
--
-- Promises are task safe. They can be used from multiple threads (and, as
-- always, a single call to Set_Value or Set_Error can be done), subscribed
-- to from multiple threads,...
-- However, the value itself is under your control. Although the promise
-- will only execute one callback at a time, to which is passes the value,
-- you should ensure that the value is not used from another thread in
-- parallel, or provide appropriate locking.
-- Using simple types like Integers or Strings should be safe.
--
-- Chaining and callbacks
-- ======================
--
-- Promises can be chained, so that the callback for the first promise will
-- itself return a promise, whose callback might in turn return a promise,
-- and so on.
--
-- The syntax to chain promises is:
--
-- Subscribe (P and new A and new B and new C;
--
-- Let's take the following chain:
--
-- P and new A -- A is the callback on P
-- and new B -- B is the callback on the promise returned by A
-- and new C; -- C is the callback on the promise returned by C
--
-- The following callbacks might occur (where *.P is the promise output
-- by the routine, *.V is the value of that promise, and *.R is the reason
-- for the failure of that promise, which defaults to the reason received
-- from the previous release unless overridden):
--
-- promises calls
-- If P is resolved: A.On_Next (P.V)
-- If A.P is resolved: B.On_Next (A.V)
-- if B.P is resolved: C.On_Next (B.V)
-- else B.P failed: C.On_Error (B.R)
-- else A.P failed: B.On_Error (A.R), C.On_Error (B.R)
-- else P failed: A.On_Error (P.R), B.On_Error (A.R),
-- C.On_Error (B.R)
--
-- Q: What if I want multiple callbacks on the same promise ?
-- A: You need to use intermediate variables, as in:
-- Q := P and new A;
-- Subscribe (Q and new B);
-- Subscribe (Q and new C);
-- Where both B and C are callbacks on A's return promise (and not
-- chained together).
--
-- A more convenient syntax exists, as in the following example:
--
-- P and (new A & new B) -- A.P is passed on to the next step
-- and (new C & new D) -- C.P is passed on to the next step
-- and new E;
--
-- If P is resolved: A.On_Next (P.V), B.On_Next (P.V)
-- if A.P is resolved: C.On_Next (A.V), D.On_Next (A.V)
-- if C.P is resolved: E.On_Next (C.V)
-- else C.P failed: E.On_Error (C.R)
-- else A.P failed: C.On_Error (A.R), D.On_Error (A.R),
-- E.On_Error (C.R)
-- else P failed: A.On_Error (P.R), B.On_Error (P.R),
-- C.On_Error (A.R), D.On_Error (A.R),
-- E.On_Error (C.R)
--
-- Note that there is no guaranteed order in which the callbacks are
-- executed, so for instance it is possible that C.On_Next and
-- E.On_Next are called before B.On_Next.
--
-- Q: What if I want different success and failure callbacks ?
-- A: A callback is an object with both a On_Next and a On_Error primitive
-- operations. So you could set two different callbacks on the same
-- promise (as we did above in the first question)
with GNATCOLL.Atomic;
with GNATCOLL.Refcount;
package GNATCOLL.Promises is
use type GNATCOLL.Atomic.Atomic_Counter;
subtype Promise_State is GNATCOLL.Atomic.Atomic_Counter;
Pending : constant Promise_State := 0;
Resolved : constant Promise_State := 1;
Failed : constant Promise_State := 2;
Resolving : constant Promise_State := 3;
Failing : constant Promise_State := 4;
Subscribing : constant Promise_State := 5;
subtype Actual_Promise_State is Promise_State range Pending .. Subscribing;
-- The various states that a promise can have.
-- We use atomic operations when possible to manipulate it, to make
-- promises task safe.
type Promise_Chain is tagged private;
procedure Subscribe (Self : Promise_Chain) with Inline => True;
-- A dummy type used when chaining promises with the "and"
-- operator. See below for an example of code.
--
-- Do not mark this procedure as "is null", since otherwise GNAT
-- does not even call the last "and" in the chain.
--------------
-- IFreeable --
--------------
type IFreeable is interface;
type Freeable_Access is access all IFreeable'Class;
-- a general interface for objects that have an explicit Free
-- primitive operation.
procedure Free (Self : in out IFreeable) is null;
-- Free internal data of Self
procedure Free (Self : in out Freeable_Access);
-- Free self, via its primitive operation, and then free the pointer
----------
-- Impl --
----------
-- This package is for implementation details
package Impl is
type IPromise_Data is interface;
procedure Free (Self : in out IPromise_Data) is null;
procedure Dispatch_Free (Self : in out IPromise_Data'Class);
type IAbstract_Promise is interface;
package Promise_Pointers is new GNATCOLL.Refcount.Shared_Pointers
(Element_Type => IPromise_Data'Class,
Release => Dispatch_Free,
Atomic_Counters => True); -- thread-safe
type Root_Promise is
new Promise_Pointers.Ref and IAbstract_Promise with null record;
type IPromise_Callback is interface and IFreeable;
type Promise_Callback_Access is access all IPromise_Callback'Class;
procedure On_Error
(Self : in out IPromise_Callback; Reason : String) is null;
-- Called when a promise has failed and will never be resolved.
end Impl;
--------------
-- Promises --
--------------
generic
type T (<>) is private;
package Promises is
type Promise is new Impl.IAbstract_Promise with private;
-- A promise is a smart pointer: it is a wrapper around shared
-- data that is freed when no more reference to the promise
-- exists.
subtype Result_Type is T;
---------------
-- Callbacks --
---------------
type Callback is interface and Impl.IPromise_Callback;
type Callback_Access is access all Callback'Class;
procedure On_Next (Self : in out Callback; R : Result_Type) is null;
-- Executed when a promise is resolved. It provides the real value
-- associated with the promise.
type Callback_List (<>) is private;
-- Multiple callbacks, all subscribed to the same promise (or
-- will be subscribed to the same promise).
--------------
-- Promises --
--------------
function Create return Promise
with
Post => Create'Result.Is_Created
and Create'Result.Get_State = Pending;
-- Create a new promise, with no associated value.
procedure Set_Value (Self : in out Promise; R : T)
with
Pre => Self.Is_Created
and Self.Get_State /= Resolved
and Self.Get_State /= Failed,
Post => Self.Get_State = Resolved;
-- Give a result to the promise.
-- The callbacks' On_Next methods are executed.
-- This can only be called once on a promise.
procedure Set_Error (Self : in out Promise; Reason : String)
with
Pre => Self.Is_Created
and Self.Get_State /= Resolved
and Self.Get_State /= Failed,
Post => Self.Get_State = Failed;
-- Mark the promise has failed. It will never be resolved.
-- The callbacks' On_Error method are executed.
procedure Subscribe
(Self : Promise;
Cb : not null access Callback'Class)
with Pre => Self.Is_Created;
function "and"
(Self : Promise;
Cb : not null access Callback'Class) return Promise_Chain
with Pre => Self.Is_Created;
function "and"
(Self : Promise; Cb : Callback_List) return Promise_Chain
with Pre => Self.Is_Created;
-- Will call Cb when Self is resolved or failed (or immediately if Self
-- has already been resolved or failed).
-- Any number of callbacks can be set on each promise.
-- If you want to chain promises (i.e. your callback itself returns
-- a promise), take a look at the Chains package below.
--
-- Cb must be allocated specifically for this call, and will be
-- freed as needed. You must not reuse the same pointer for multiple
-- calls to Subscribe.
-- ??? This is unsafe
--
-- Self is modified, but does not need to be "in out" since a promise
-- is a pointer. This means that Subscribe can be directly called on
-- the result of a function call, for instance.
function "&"
(Cb : not null access Callback'Class;
Cb2 : not null access Callback'Class) return Callback_List;
function "&"
(List : Callback_List;
Cb2 : not null access Callback'Class) return Callback_List;
-- Create a list of callbacks that will all be subscribed to the same
-- promise.
function Is_Created
(Self : Promise'Class) return Boolean with Inline_Always;
-- Whether the promise has been created
function Get_State
(Self : Promise'Class) return Actual_Promise_State with Inline_Always;
-- Used for pre and post conditions
private
type Promise is new Impl.Root_Promise with null record;
type Callback_List is
array (Natural range <>) of not null access Callback'Class;
function Is_Created (Self : Promise'Class) return Boolean
is (not Self.Is_Null);
end Promises;
------------
-- Chains --
------------
generic
with package Input_Promises is new Promises (<>);
with package Output_Promises is new Promises (<>);
package Chains is
type Callback is abstract new Input_Promises.Callback
with private;
procedure On_Next
(Self : in out Callback;
Input : Input_Promises.Result_Type;
Output : in out Output_Promises.Promise)
is abstract
with
Post'Class =>
Output.Get_State = Resolved
or Output.Get_State = Failed;
-- This is the procedure that needs overriding, not the one inherited
-- from Input_Promises. When chaining, a callback returns another
-- promise, to which the user can attach further callbacks, and so on.
--
-- Failures in a promise are by default propagated to the output
-- promise, unless you override the Failed primitive operation of
-- Self.
type Callback_List (<>) is private;
function Is_Registered
(Self : not null access Callback'Class) return Boolean
with Inline;
function Is_Registered (Self : Callback_List) return Boolean
with Inline;
-- Whether the callback has already been set on a promise. It is
-- invalid to use the same callback on multiple promises (or even
-- multiple times on the same promise).
function "and"
(Input : Input_Promises.Promise;
Cb : not null access Callback'Class)
return Output_Promises.Promise
with
Pre => not Is_Registered (Cb) and Input.Is_Created,
Post => Is_Registered (Cb)
and "and"'Result.Is_Created;
-- Chains two properties.
-- When Input is resolved, Cb is executed and will in turn resolve
-- the output promise
-- These functions return immediately a promise that will be resolved
-- later.
function "&"
(Cb : not null access Callback'Class;
Cb2 : not null access Input_Promises.Callback'Class)
return Callback_List
with
Pre => not Is_Registered (Cb);
-- ??? Results in GNAT bug box
-- Post => "and"'Result = Cb
-- and not Is_Registered ("and"'Result);
function "&"
(List : Callback_List;
Cb2 : not null access Input_Promises.Callback'Class)
return Callback_List;
-- Used to set multiple callbacks on the same promise, as in:
-- P & (new A and new B) & new C
-- Only Cb is expected to output a promise, which will be
-- forwarded to the next step (C in this example). Cb2 only
-- gets notified via its On_Next and On_Error primitives.
function "and"
(Input : Input_Promises.Promise;
Cb : Callback_List)
return Output_Promises.Promise
with
Pre => not Is_Registered (Cb) and Input.Is_Created,
Post => Is_Registered (Cb)
and "and"'Result.Is_Created;
-- Chaining multiple callbacks on the same promise
private
type Callback is abstract new Input_Promises.Callback with record
Promise : aliased Output_Promises.Promise;
end record;
overriding procedure On_Next
(Self : in out Callback; P : Input_Promises.Result_Type);
overriding procedure On_Error (Self : in out Callback; Reason : String);
type Callback_Array is array (Natural range <>)
of not null access Input_Promises.Callback'Class;
type Callback_List (N : Natural) is record
Cb : not null access Callback'Class;
Cb2 : Callback_Array (1 .. N);
end record;
end Chains;
private
type Promise_Chain is tagged null record;
end GNATCOLL.Promises;
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