Create TODO_StateMachine.md

TODO_StateMachine.md

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+## Proposal (Workflow State Machine)
+
+### Goal
+To implement
+* Async operations
+* Delayed operations
+* State machine (visual state, animation)
+
+### Agenda
+- [x] State Machine Interface
+- [x] Raw state machine
+- [x] Extension Syntax
+- [x] Extension (Enumerable, $Yield)
+- [x] Extension (Task, $Await, return)
+- [ ] Extension (State Machine)
+
+### Extension (Syntax)
+
+* `return` is always mapped to `ReturnAndExit`
+    * If `return` has an expression, than `ReturnAndExit` should also have an argument
+* `$<OPERATOR> ...;` is available if `<OPERATOR>AndRead` or `<OPERATOR>AndPause` exists
+* `var NAME = $<OPERATOR> ...;` is available if `<OPERATOR>AndRead` exists
+    * The return type is object, except that there is a `static func CastResult(value : object):T` in one of the argument types
+    * Use the first available and correct CastResult that is discovered
+* A function body begins with `${` or `$<PROVIDER>{` means this is a coroutine function
+    * If the provider is not omitted, it will search for a type named `<PROVIDER>`, and than use `<PROVIDER>Coroutine` as the provider
+    * If the provider is omitted, it will search the return type and its base types in order, and find the first existing `<TYPE>Coroutine` class as the provider
+    * A provider is only choosen by name. If the class ending with `Coroutine` doesn't have appropriate functions, errors occur.
+    * For example:
+        * `func F() : int[] ${}` returns `int[]`, which is `system::Enumerable`, so `system::EnumerableCoroutine` is used.
+        * `func F() : IDownloadAsync^ ${}` returns `IDownloadAsync`, which has a base interface `system::Async`, so `system::AsyncCoroutine` is used.
+        * `func F() : void $Async{}` has a provider `Async`, so `system::AsyncCoroutine` is used.
+* If the function return type does not match the provider's `Create` function
+    * If the function returns void, then `CreateAndRun` is used
+    * In other cases:
+        * Cast from InterfaceA to InterfaceB
+        * InterfaceB is required to inherit from InterfaceA
+        * Fail if InterfaceB has extra methods
+
+### Extension (State Machine Interface)
+
+#### Declare input methods in interfaces
+* `$input Name(a:Ta, b:Tb);` **declaration**
+    * `func Name(a:Ta, b:Tb) : bool`
+        * Returns false if a failed input causes a retry
+        * No overloading
+    * `prop NameEnabled : bool {const}`
+        * `var <prop>NameEnabled : bool = false;`
+        * `func GetNameEnabled() : bool { ... }`
+        * `func SetNameEnabled(<value> : bool) : void { ... }`
+
+#### Statements in `$state` declarations
+* `$input[_else_exit] { case <INPUT-NAME>(arguments...): { ... } ... }` **statement**
+    * If the input is failed to match
+        * `$input_else_exit`: exit the current state scope
+        * `$input`: retry
+* `$input[_else_exit] <INPUT-NAME>(arguments...) (;|{ ... })` **statement**
+    * Short for `$input[_else_exit] { case <INPUT-NAME>(arguments...): { ... } }`
+* `$join <NAME>;` **statement**
+    * Enter the specified state
+* `$join { ... }` **statement**
+    * Create a state scope, if any `$input_else_exit` failed inside, exit the `$join` statement
+
+#### Declare states inside a new interface expression
+* `$state <NAME> { ... }`
+    * Define a sub state machine
+* `$state { ... }`
+    * Define a state machine as the entry
+    * Cannot create dead loop for any failed input
+        * Example: `$state { while (true) { $join { $input_else_exit DoSomething(); } } }`
+
+#### Sample
+```
+interface ICalculator : Coroutine
+{
+    $input Digit(i : int);
+    $input Dot();
+    $input Add();
+    $input Mul();
+    $input Equal();
+    $input Clear();
+    
+    prop Value : string {const}
+}
+
+var calculator = new ICalculator^
+{
+    var valueFirst : string = "";
+    var op : string = "";
+    override prop Value : string = "0";
+    
+    func Update(value : string) : void
+    {
+        SetValue(value);
+        valueFirst = value;
+    }
+    
+    func Calculate() : void
+    {
+        if (valueFirst == "")
+        {
+            valueFirst = value;
+        }
+        else if (op == "+")
+        {
+            Update((cast double valueFirst) + (cast double Value));
+        }
+        else if (op == "*")
+        {
+            Update((cast double valueFirst) * (cast double Value));
+        }
+        else
+        {
+            raise $"Unrecognized operator: $(op)";
+        }
+    }
+    
+    $state Digits()
+    {
+        while (true)
+        {
+            $input_else_exit Digit(i)
+            {
+                Value = Value & i;
+            }
+        }
+    }
+    
+    $state Integer()
+    {
+        $input_else_exit Digit(i)
+        {
+            Value = i;
+            $join Digits();
+        }
+    }
+    
+    $state Number()
+    {
+        $join Integer();
+        $input_else_exit Dot()
+        {
+            Value = Value & ".";
+        }
+        $input Digit(i)
+        {
+            Value = Value & i;
+        }
+        $join Digits();
+    }
+
+    $state
+    {
+        while (true)
+        {
+            $join Number();
+            $input
+            {
+                case Add():     {Calculate(); op = "+";}
+                case Mul():     {Calculate(); op = "-";}
+                case Equal():   {Calculate(); op = "=";}
+                case Clear():
+                {
+                    valueFirst = "";
+                    op = "";
+                    Value = "0";
+                }
+            }
+        }
+    }
+};
+```