擴展C#與元編程

擴展C#與元編程

https://www.cnblogs.com/knat/p/4580393.html

https://www.cnblogs.com/knat/p/4584023.html

擴展C#與元編程（一）

衆所周知，Roslyn project已經開源一年多了。簡單的說，Roslyn是：1）用C#/VB寫的C#/VB的編譯器，以及與IDE集成；2）編譯器的功能以API的方式暴露出來（即一組DLL）。
Roslyn對大多數開發者來講沒啥用處，你關心的是你的應用程序的邏輯與構建而不須要關心編譯器是怎麼運行的。有時你以爲C#/VB有須要加強的地方，因而你興致勃勃的跑到Roslyn論壇去發表一個proposal，MS的guy會給你的issue打上Area-Language Design的標籤，網友們也許會來討論一下，而後呢？就沒有而後了，這有點像在求雨，下不下、何時下、下多大全憑神仙們（MS guys）的"心情"。
求人不如求己，既然Roslyn已經以MIT方式開源，我們就站在巨人的肩膀上本身動手豐衣足食吧，也就是，擴展C#，作（屌絲級的）complier guy！
衆所周知，C#是門general-purpose的編程語言，你想添加的feature既能夠是general的，也能夠是specific的。好比，XML literal是個general的C# feature，下兩圖展現了一個specific的feature，讓C#支持Windows Workflow Foundation（WF）的activity：

其實，這不應叫"擴展C#"，而該叫作"建立一門衍生自C#的DSL"，若是你對WF感興趣，請訪問
Metah.W: A Workflow Metaprogramming Language。
姑且就用"擴展C#"這個叫法。如上兩圖所示，Metah.W（MW）在C#中加入了activity的構造，在概念上，activity是C# class和function的合體，在圖一中，public sealed class搖身一變成爲public sealed activity，接着宣告activity的名字，小括號中宣告parameters，如string BookmarkName, string Text，接着宣告可選的return type，如activity Prompt的as int宣告返回類型是int，若無as XXX宣告則返回void。Activity的body中是變量的宣告和statement的使用，statement能夠是：1）C# expression statement，如圖一中的target = new Random().Next(1, MaxNumber) + 1;；2）well-known statement，如if-else, while, do-while, foreach, try-catch-finally等；3）special statement，如圖二中的statemachine, delay。由於activity是class和function的合體，能夠調用它並將返回值賦給變量/參數，如guess = new Prompt().Invoke(...)。
將FirstLook.mw送進MW編譯器，將產生下面的C#代碼：

//FirstLook.mw.cs, generated by the MW compiler

namespace HelloMW.FirstLook

{

public sealed class SequentialNumberGuess : global::System.Activities.Activity

{

public global::System.Activities.InArgument<int> MaxNumber { get; set; }

public global::System.Activities.OutArgument<int> Turns { get; set; }

 

private global::System.Activities.Activity __GetImplementation__()

{

global::System.Activities.Activity __vroot__;

{

var __v__0 = new global::System.Activities.Statements.Sequence();

var target = new global::System.Activities.Variable<int>();

__v__0.Variables.Add(target);

var guess = new global::System.Activities.Variable<int>();

__v__0.Variables.Add(guess);

__v__0.Activities.Add(new global::MetahWActionActivity(__ctx__ =>

{

target.SetEx(__ctx__, new Random().Next(1, MaxNumber.Get(__ctx__)) + 1);

}

));

var __v__1 = new global::System.Activities.Statements.DoWhile();

__v__1.Condition = new global::MetahWFuncActivity<bool>(__ctx__ => guess.Get(__ctx__) != target.Get(__ctx__));

{

var __v__2 = new global::System.Activities.Statements.Sequence();

__v__2.Activities.Add(new Prompt().Initialize(__activity2__ =>

{

__activity2__.BookmarkName = new global::System.Activities.InArgument<string>(new global::MetahWFuncActivity<string>(__ctx__ => "EnterGuess"));

__activity2__.Text = new global::System.Activities.InArgument<string>(new global::MetahWFuncActivity<string>(__ctx__ => "Please enter a number between 1 and " + MaxNumber.Get(__ctx__)));

__activity2__.Result = new global::System.Activities.OutArgument<int>(new global::MetahWLocationActivity<int>(guess));

}

));

__v__2.Activities.Add(new global::MetahWActionActivity(__ctx__ =>

{

Turns.SetEx(__ctx__, __val__ => ++__val__, true);

}

));

var __v__3 = new global::System.Activities.Statements.If();

__v__3.Condition = new global::System.Activities.InArgument<bool>(new global::MetahWFuncActivity<bool>(__ctx__ => guess.Get(__ctx__) != target.Get(__ctx__)));

var __v__4 = new global::System.Activities.Statements.If();

__v__4.Condition = new global::System.Activities.InArgument<bool>(new global::MetahWFuncActivity<bool>(__ctx__ => guess.Get(__ctx__) < target.Get(__ctx__)));

__v__4.Then = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Your guess is too low.");

}

);

__v__4.Else = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Your guess is too high.");

}

);

__v__3.Then = __v__4;

__v__2.Activities.Add(__v__3);

__v__1.Body = __v__2;

}

__v__0.Activities.Add(__v__1);

__vroot__ = __v__0;

}

return __vroot__;

}

 

private global::System.Func<global::System.Activities.Activity> __implementation__;

protected override global::System.Func<global::System.Activities.Activity> Implementation

{

get

{

return __implementation__ ?? (__implementation__ = __GetImplementation__);

}

set

{

throw new global::System.NotSupportedException();

}

}

}

 

public sealed class Prompt : global::System.Activities.Activity<int>

{

public global::System.Activities.InArgument<string> BookmarkName { get; set; }

public global::System.Activities.InArgument<string> Text { get; set; }

 

private global::System.Activities.Activity __GetImplementation__()

{

global::System.Activities.Activity __vroot__;

var __v__0 = new global::System.Activities.Statements.Sequence();

__v__0.Activities.Add(new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine(Text.Get(__ctx__));

}

));

__v__0.Activities.Add(new ReadInt().Initialize(__activity2__ =>

{

__activity2__.BookmarkName = new global::System.Activities.InArgument<string>(new global::MetahWFuncActivity<string>(__ctx__ => BookmarkName.Get(__ctx__)));

__activity2__.Result = new global::System.Activities.OutArgument<int>(new global::MetahWLocationActivity<int>(Result));

}

));

__vroot__ = __v__0;

return __vroot__;

}

 

private global::System.Func<global::System.Activities.Activity> __implementation__;

protected override global::System.Func<global::System.Activities.Activity> Implementation

{

get

{

return __implementation__ ?? (__implementation__ = __GetImplementation__);

}

set

{

throw new global::System.NotSupportedException();

}

}

}

}

這就是元編程，把語法糖解糖的過程，即把高級抽象的描述翻譯成低級具體的實現。我以爲，"語法糖"是個膚淺的認識，實際上，多數的"語法糖"都涉及到語義，不只僅是簡單的語法轉換。元編程的另外一個例子，早期某些C++編譯器能將C++代碼翻譯成等價的C代碼，即C++是門元編程語言，它是C的"語法糖"。
日光之下，並沒有新事。元編程是個很是"古老"的概念，但在每一個"時代"它都能玩出耳目一新的花樣。
欲知後事如何，請聽下回分解。

擴展C#與元編程（二）

若是你對Windows Workflow Foundation（WF）一無所知，當看到擴展C#與元編程（一）中由MW編譯器生成的FirstLook.mw.cs時，也許這麼在想：我KAO，這是C#版的彙編語言！
WF究竟是什麼？能夠這麼認爲：WF runtime是高級版的CLR（CLR上的CLR），activity是高級版的MSIL指令（能夠勉強這麼比喻），Metah.W是高級版的C#。
Activity能夠分爲兩類：primitive activity和composite activity。Primitive activity繼承自System.Activities.NativeActivity, System.Activities.NativeActivity<T>等，用來實現流程控制容器，如sequence, if-else, while, foreach, try-catch-finally等：

namespace System.Activities.Statements {

public sealed class Sequence : NativeActivity {

public Sequence();

public Collection<Variable> Variables { get; }

public Collection<Activity> Activities { get; }

//...

}

 

public sealed class If : NativeActivity {

public If();

public InArgument<bool> Condition { get; set; }

public Activity Then { get; set; }

public Activity Else { get; set; }

//...

}

 

public sealed class While : NativeActivity {

public While();

public Collection<Variable> Variables { get; }

public Activity<bool> Condition { get; set; }

public Activity Body { get; set; }

//...

}

 

public sealed class ForEach<T> : NativeActivity {

public ForEach();

public InArgument<IEnumerable<T>> Values { get; set; }

public ActivityAction<T> Body { get; set; }

//...

}

 

public sealed class TryCatch : NativeActivity {

public TryCatch();

public Activity Try { get; set; }

public Collection<Catch> Catches { get; }

public Activity Finally { get; set; }

//...

}

}

WF這個高級CLR使人拍手稱快的特性之一是，你能夠自定義流程控制容器（能夠想象成自定義MSIL指令），好比高大上的狀態機：

namespace System.Activities.Statements {

public sealed class StateMachine : NativeActivity {

public StateMachine();

public Collection<Variable> Variables { get; }

public State InitialState { get; set; }

public Collection<State> States { get; }

//...

}

public sealed class State {

public State();

public bool IsFinal { get; set; }

public Collection<Variable> Variables { get; }

public Activity Entry { get; set; }

public Activity Exit { get; set; }

public Collection<Transition> Transitions { get; }

//...

}

public sealed class Transition {

public Transition();

public Activity Trigger { get; set; }

public Activity<bool> Condition { get; set; }

public Activity Action { get; set; }

public State To { get; set; }

//...

}

}

SecondLook.mw展現瞭如何使用狀態機：

一個statemachine至少包含一個common state及一個final state，每一個state由惟一的label標識，如InPark, InNeutral等，break關鍵字標明這是一個final state，statemachine關鍵字後的goto clause標明initial common state，當流程進入某common state後，首先執行~>所標識的entry statement，接着執行on關鍵字標識的trigger statement，而後依順序評估if關鍵字標識的condition expression，若是某condition評估爲true，則執行<~所標識的exit statement和do關鍵字標識的action statement（SecondLook.mw中未使用），接着跳轉到goto關鍵字所標識的state，若是全部的condition expression都評估爲false，則從新執行該state的trigger statement。若是某final state執行完畢，則該statemachine執行完畢。
MW編譯器將activity Drive翻譯成下面的C#代碼：

//SecondLook.mw.cs, generated by MW compiler

namespace HelloMW.SecondLook

{

class Drive : global::System.Activities.Activity

{

private global::System.Activities.Activity __GetImplementation__()

{

global::System.Activities.Activity __vroot__;

{

var __v__0 = new global::System.Activities.Statements.Sequence();

var isMoved = new global::System.Activities.Variable<bool>();

__v__0.Variables.Add(isMoved);

__v__0.Activities.Add(new global::MetahWActionActivity(__ctx__ =>

{

isMoved.SetEx(__ctx__, false);

}

 

));

var __v__1 = new global::System.Activities.Statements.While();

__v__1.Condition = new global::MetahWFuncActivity<bool>(__ctx__ => !isMoved.Get(__ctx__));

{

var __v__2 = new global::System.Activities.Statements.Sequence();

{

var __v__3 = new global::System.Activities.Statements.StateMachine();

var action = new global::System.Activities.Variable<DriveAction>();

__v__3.Variables.Add(action);

var __v__4 = new global::System.Activities.Statements.State();

var __v__5 = new global::System.Activities.Statements.Transition();

var __v__6 = new global::System.Activities.Statements.State();

var __v__7 = new global::System.Activities.Statements.Transition();

var __v__8 = new global::System.Activities.Statements.Transition();

var __v__9 = new global::System.Activities.Statements.Transition();

var __v__10 = new global::System.Activities.Statements.State();

var __v__11 = new global::System.Activities.Statements.Transition();

var __v__12 = new global::System.Activities.Statements.State();

var __v__13 = new global::System.Activities.Statements.Transition();

var __v__14 = new global::System.Activities.Statements.State();

{

__v__3.States.Add(__v__4);

__v__4.Entry = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Enter InPark");

}

 

);

__v__4.Exit = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Exit InPark");

}

 

);

__v__5.Trigger = new GetDriveAction().Initialize(__activity2__ =>

{

__activity2__.Result = new global::System.Activities.OutArgument<global::HelloMW.SecondLook.DriveAction>(new global::MetahWLocationActivity<global::HelloMW.SecondLook.DriveAction>(action));

}

 

);

__v__5.Condition = new global::MetahWFuncActivity<bool>(__ctx__ => action.Get(__ctx__) == DriveAction.Neutral);

__v__4.Transitions.Add(__v__5);

}

 

{

__v__3.States.Add(__v__6);

__v__6.Entry = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Enter InNeutral");

}

 

);

__v__6.Exit = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Exit InNeutral");

}

 

);

__v__7.Trigger = new GetDriveAction().Initialize(__activity2__ =>

{

__activity2__.Result = new global::System.Activities.OutArgument<global::HelloMW.SecondLook.DriveAction>(new global::MetahWLocationActivity<global::HelloMW.SecondLook.DriveAction>(action));

}

 

);

__v__7.Condition = new global::MetahWFuncActivity<bool>(__ctx__ => action.Get(__ctx__) == DriveAction.Forward);

__v__6.Transitions.Add(__v__7);

__v__8.Trigger = __v__7.Trigger;

__v__8.Condition = new global::MetahWFuncActivity<bool>(__ctx__ => action.Get(__ctx__) == DriveAction.Reverse);

__v__6.Transitions.Add(__v__8);

__v__9.Trigger = __v__7.Trigger;

__v__9.Condition = new global::MetahWFuncActivity<bool>(__ctx__ => action.Get(__ctx__) == DriveAction.TurnOff);

__v__6.Transitions.Add(__v__9);

}

 

{

__v__3.States.Add(__v__10);

__v__10.Entry = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Enter InForward");

isMoved.SetEx(__ctx__, true);

}

 

);

__v__10.Exit = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Exit InForward");

}

 

);

__v__11.Trigger = new GetDriveAction().Initialize(__activity2__ =>

{

__activity2__.Result = new global::System.Activities.OutArgument<global::HelloMW.SecondLook.DriveAction>(new global::MetahWLocationActivity<global::HelloMW.SecondLook.DriveAction>(action));

}

 

);

__v__11.Condition = new global::MetahWFuncActivity<bool>(__ctx__ => action.Get(__ctx__) == DriveAction.Neutral);

__v__10.Transitions.Add(__v__11);

}

 

{

__v__3.States.Add(__v__12);

__v__12.Entry = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Enter InReverse");

isMoved.SetEx(__ctx__, true);

}

 

);

__v__12.Exit = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("Exit InReverse");

}

 

);

__v__13.Trigger = new GetDriveAction().Initialize(__activity2__ =>

{

__activity2__.Result = new global::System.Activities.OutArgument<global::HelloMW.SecondLook.DriveAction>(new global::MetahWLocationActivity<global::HelloMW.SecondLook.DriveAction>(action));

}

 

);

__v__13.Condition = new global::MetahWFuncActivity<bool>(__ctx__ => action.Get(__ctx__) == DriveAction.Neutral);

__v__12.Transitions.Add(__v__13);

}

 

{

__v__3.States.Add(__v__14);

__v__14.IsFinal = true;

__v__14.Entry = new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("TurnedOff");

}

 

);

}

 

__v__3.InitialState = __v__4;

__v__5.To = __v__6;

__v__7.To = __v__10;

__v__8.To = __v__12;

__v__9.To = __v__14;

__v__11.To = __v__6;

__v__13.To = __v__6;

__v__2.Activities.Add(__v__3);

}

 

__v__2.Activities.Add(new global::MetahWActionActivity(__ctx__ =>

{

Console.WriteLine("isMoved: " + isMoved.Get(__ctx__));

}

 

));

__v__1.Body = __v__2;

}

 

__v__0.Activities.Add(__v__1);

__vroot__ = __v__0;

}

 

return __vroot__;

}

 

private global::System.Func<global::System.Activities.Activity> __implementation__;

protected override global::System.Func<global::System.Activities.Activity> Implementation

{

get

{

return __implementation__ ?? (__implementation__ = __GetImplementation__);

}

 

set

{

throw new global::System.NotSupportedException();

}

}

}

}

"彙編語言代碼"老是冗長無趣的，因此有了Metah.W這樣的高級語言。下面是可能的執行結果：

Enter InPark

!action: Neutral

Exit InPark

Enter InNeutral

!action: Reverse

Exit InNeutral

Enter InReverse

!action: TurnOff

!action: Forward

!action: Neutral

Exit InReverse

Enter InNeutral

!action: Forward

Exit InNeutral

Enter InForward

!action: Forward

!action: Forward

!action: Reverse

!action: Neutral

Exit InForward

Enter InNeutral

!action: TurnOff

Exit InNeutral

TurnedOff

isMoved: True

請按任意鍵繼續. . .

Composite activity直接繼承自System.Activities.Activity或System.Activities.Activity<T>，它由其它primitive activity和/或composite activity組合而成。Metah.W和WF designer只能創做composite activity。

我不知道你們怎麼看待微軟這家公司，多數時候MS是家專業但缺少想象力的公司，有時它能創造出一些使人眼前一亮的做品，C#從第二版開始就一直閃亮，WF是個極富想象力的技術，不過，WF如今仍是個藏在深山中的璞玉，但願此文能激起你研究WF的興趣。關於
Metah.W: A Workflow Metaprogramming的更多信息請訪問：https://github.com/knat/Metah 。待續。