Martin Fowlers State Machine in meta#

Martin Fowlers State Machine is quickly becoming the 99 bottles of beer on the wall for DSLs. So I implemented it in meta# and will be using it in my presentation at the next Twin Cities Code Camp.

The state machine sample project consists of two grammars, a parser and a transformer. It also has a state machine AST (semantic model) and a state machine runtime. The accompanying sample app uses an almost identical syntax as the one Fowler has in his book and is driven using a very simple console app.

StateMachineParser.g

namespace StateMachineCompiler:
    import MetaSharp.Transformation;
    import MetaSharp.Transformation.Lang;
    import StateMachineCompiler.Ast;
    import System;
    import System.CodeDom;
    import System.Linq;
    import System.Reflection;
    import System.Collections.Generic;

    grammar StateMachineParser < LangParser:

        override TypeDeclaration 
            = StateMachineDeclaration
            | super;

        StateMachineDeclaration
            =    StateMachine name:Identifier BlockBegin
                e:EventsDeclaration?
                r:ResetsDeclaration?
                c:CommandsDeclaration?
                s:StateDeclarations*
                error until BlockEnd -> {
                StateMachineDeclaration sm = new StateMachineDeclaration();
                sm.Name = name as string;
                sm.Events = e.Cast<EventDeclaration>();
                sm.Resets = r.Cast<ResetDeclaration>();
                sm.Commands = c.Cast<CommandDeclaration>();
                sm.States = s.Cast<StateDeclaration>();
                return sm;
            }

        EventsDeclaration
            =    Events BlockBegin
                e:EventMember*
                error until BlockEnd
                -> e;

        EventMember = name:Identifier code:Identifier StatementEnd -> {
                EventDeclaration e = new EventDeclaration();
                e.Name = name as string;
                e.Code = code as string;
                return e;
            }

        ResetsDeclaration
            =    Resets BlockBegin
                r:ResetMember*
                error until BlockEnd
                -> r;

        ResetMember = name:Identifier StatementEnd -> {
            ResetDeclaration r = new ResetDeclaration();
            r.Name = name as string;
            return r;
        }
            
        CommandsDeclaration
            =    Commands BlockBegin
                c:CommandMember*
                error until BlockEnd 
                -> c;

        CommandMember = name:Identifier code:Identifier StatementEnd -> {
            CommandDeclaration c = new CommandDeclaration();
            c.Name = name as string;
            c.Code = code as string;
            return c;
        }
            
        StateDeclarations
            =    State name:Identifier BlockBegin
                a:ActionsDeclaration?
                t:TransitionMember*
                error until BlockEnd -> {
                StateDeclaration s = new StateDeclaration();
                s.Name = name as string;
                s.Transitions = t.Cast<TransitionDeclaration>();
                if(a != null):
                    List<string> actions = new List<string>();
                    for(Node n in a.Cast<Node>()):
                        actions.Add(Node.Unwrap(n) as string);
                    end
                    s.Actions = actions;
                end
                return s;
            }

        ActionsDeclaration = Actions "{" a:List(Identifier, ",") "}" StatementEnd -> a;

        TransitionMember = e:Identifier "=>" s:Identifier StatementEnd -> {
            TransitionDeclaration t = new TransitionDeclaration();
            t.EventName = e as string;
            t.StateName = s as string;
            return t;
        }

        [Keyword] StateMachine = "statemachine";
        [Keyword] Events = "events";
        [Keyword] Resets = "resets";
        [Keyword] Commands = "commands";
        [Keyword] State = "state";
        [Keyword] Actions = "actions";

        override CustomTokens
            = '=' '>'
            | super;
    end
end

The parser inherits from LangParser and extends Fowlers DSL by being both inside of a namespace and also wrapped in a statemachine declaration block. Also I changed the block named “resetEvents” to just be “resets” which seemed more consistent to me.

StateMachineTransformer.g

namespace StateMachineCompiler:
    import MetaSharp.Transformation;
    import MetaSharp.Transformation.Lang.Ast;
    import StateMachineCompiler.Ast;
    import System;
    import System.CodeDom;
    import System.Linq;
    import System.Collections.Generic;

    grammar StateMachineTransformer < StateMachineParser:
        Main = UnitVisitor;

        UnitVisitor = Unit { Namespaces = [NamespaceVisitor*] };

        NamespaceVisitor = Namespace { Types = [TypeVisitor*] };

        TypeVisitor
            = StateMachineVisitor
            | CodeTypeDeclaration { };

        StateMachineVisitor = smd:StateMachineDeclaration {
                e:Events = [EventVisitor*],
                r:Resets = [ResetVisitor*],
                c:Commands = [CommandVisitor*],
                s:States = [StateVisitor*] -> Parser.Flatten(match)
            } -> {
                StateMachineDeclaration d = smd as StateMachineDeclaration;
                d.BaseTypes.Add(typeof(StateMachineBuilder));
                Constructor cons = new Constructor();
                cons.Attributes = MemberAttributes.Public;
                cons.Parameters.Add(new ParameterDeclarationExpression(typeof(CommandChannel), "commandChannel"));
                cons.BaseConstructorArgs.Add(new ReferenceExpression("commandChannel"));
                d.Members.Add(cons);

                Method createMachineMethod = new Method();
                createMachineMethod.Name = "CreateMachine";
                createMachineMethod.Attributes = MemberAttributes.Family | MemberAttributes.Override;
                createMachineMethod.ReturnType = new TypeReference(typeof(StateMachine));
                d.Members.Add(createMachineMethod);

                createMachineMethod.Statements.AddRange(e.Cast<CodeStatement>().ToArray());
                createMachineMethod.Statements.AddRange(c.Cast<CodeStatement>().ToArray());
                createMachineMethod.Statements.AddRange(s.Cast<CodeStatement>().OfType<VariableDeclarationStatement>().ToArray());

                StateDeclaration first = d.States.First();
                VariableDeclarationStatement v = new VariableDeclarationStatement(
                    typeof(StateMachine),
                    "machine",
                    new NewExpression(
                        typeof(StateMachine),
                        new ReferenceExpression("state_" + first.Name)));

                createMachineMethod.Statements.Add(v);
                createMachineMethod.Statements.AddRange(r.Cast<CodeStatement>().ToArray());
                createMachineMethod.Statements.AddRange(s.Cast<CodeStatement>().OfType<ExpressionStatement>().ToArray());

                ReturnStatement ret = new ReturnStatement(new ReferenceExpression("machine"));
                createMachineMethod.Statements.Add(ret);

                return d;
            }

        EventVisitor = EventDeclaration { } -> {
            EventDeclaration e = match as EventDeclaration;
            VariableDeclarationStatement v = new VariableDeclarationStatement(
                typeof(Event),
                "event_" + e.Name,
                new NewExpression(
                    typeof(Event),
                    new PrimitiveExpression(e.Name), 
                    new PrimitiveExpression(e.Code)));

            return v;
        }

        ResetVisitor = ResetDeclaration { } -> {
            ResetDeclaration r = match as ResetDeclaration;
            MethodInvokeExpression addResetEvent = new MethodInvokeExpression(
                new ReferenceExpression("machine"),
                "AddResetEvents",
                new ReferenceExpression("event_" + r.Name));

            return new ExpressionStatement(addResetEvent);
        }

        CommandVisitor = CommandDeclaration { } -> {
            CommandDeclaration c = match as CommandDeclaration;
            VariableDeclarationStatement v = new VariableDeclarationStatement(
                typeof(Command),
                "command_" + c.Name,
                new NewExpression(
                    typeof(Command),
                    new PrimitiveExpression(c.Name), 
                    new PrimitiveExpression(c.Code)));

            return v;
        }

        StateVisitor = StateDeclaration { } -> {
            StateDeclaration s = match as StateDeclaration;
            Nodes statements = new Nodes();
            List<CodeExpression> parameters = new List<CodeExpression>();
            parameters.Add(new PrimitiveExpression(s.Name));
            for(string a in s.Actions):
                parameters.Add(new ReferenceExpression("command_" + a));
            end

            statements.Add(new VariableDeclarationStatement(
                typeof(State),
                "state_" + s.Name,
                new NewExpression(
                    typeof(State),
                    parameters.ToArray())));

            for(TransitionDeclaration td in s.Transitions):
                statements.Add(new ExpressionStatement(
                    new MethodInvokeExpression(
                        new ReferenceExpression("state_" + s.Name),
                        "AddTransition",
                        new ReferenceExpression("event_" + td.EventName),
                        new ReferenceExpression("state_" + td.StateName))));
            end

            return statements;
        }

    end
end

This grammar inherits from StateMachineParser and therefore it receives the parsers output as input. It is an example of implementing a visitor pattern as a meta# grammar. It visits the state machine AST nodes and expands them into code objects, which a subsequent step uses to generate code.

Here is the modified ubiquitous state machine DSL, MissGrants.sm

namespace App:

    statemachine MissGrants:

        events:
            doorClosed D1CL;
            drawerOpened D20P;
            lightOn L10N;
            doorOpened D10P;
            panelClosed PNCL;
        end

        resets:
            doorOpened;
        end

        commands:
            unlockPanel PNUL;
            lockPanel PNLK;
            lockDoor D1LK;
            unlockDoor D1UL;
        end

        state idle:
            actions {unlockDoor, lockPanel};
            doorClosed => active;
        end

        state active:
            drawerOpened => waitingForLight;
            lightOn => waitingForDrawer;
        end

        state waitingForLight:
            lightOn => unlockedPanel;
        end

        state waitingForDrawer:
            drawerOpened => unlockedPanel;
        end

        state unlockedPanel:
            actions {unlockPanel, lockDoor};
            panelClosed => idle;
        end

    end

end

Here is the code generated by the meta# transformers…

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by a tool.
//     Runtime Version:4.0.30319.237
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

namespace App
{
    
    public class MissGrants : StateMachineCompiler.StateMachineBuilder
    {
        public MissGrants(StateMachineCompiler.CommandChannel commandChannel) : 
                base(commandChannel)
        {
        }
        protected override StateMachineCompiler.StateMachine CreateMachine()
        {
            StateMachineCompiler.Event event_doorClosed = new StateMachineCompiler.Event("doorClosed", "D1CL");
            StateMachineCompiler.Event event_drawerOpened = new StateMachineCompiler.Event("drawerOpened", "D20P");
            StateMachineCompiler.Event event_lightOn = new StateMachineCompiler.Event("lightOn", "L10N");
            StateMachineCompiler.Event event_doorOpened = new StateMachineCompiler.Event("doorOpened", "D10P");
            StateMachineCompiler.Event event_panelClosed = new StateMachineCompiler.Event("panelClosed", "PNCL");
            StateMachineCompiler.Command command_unlockPanel = new StateMachineCompiler.Command("unlockPanel", "PNUL");
            StateMachineCompiler.Command command_lockPanel = new StateMachineCompiler.Command("lockPanel", "PNLK");
            StateMachineCompiler.Command command_lockDoor = new StateMachineCompiler.Command("lockDoor", "D1LK");
            StateMachineCompiler.Command command_unlockDoor = new StateMachineCompiler.Command("unlockDoor", "D1UL");
            StateMachineCompiler.State state_idle = new StateMachineCompiler.State("idle", command_unlockDoor, command_lockPanel);
            StateMachineCompiler.State state_active = new StateMachineCompiler.State("active");
            StateMachineCompiler.State state_waitingForLight = new StateMachineCompiler.State("waitingForLight");
            StateMachineCompiler.State state_waitingForDrawer = new StateMachineCompiler.State("waitingForDrawer");
            StateMachineCompiler.State state_unlockedPanel = new StateMachineCompiler.State("unlockedPanel", command_unlockPanel, command_lockDoor);
            StateMachineCompiler.StateMachine machine = new StateMachineCompiler.StateMachine(state_idle);
            machine.AddResetEvents(event_doorOpened);
            state_idle.AddTransition(event_doorClosed, state_active);
            state_active.AddTransition(event_drawerOpened, state_waitingForLight);
            state_active.AddTransition(event_lightOn, state_waitingForDrawer);
            state_waitingForLight.AddTransition(event_lightOn, state_unlockedPanel);
            state_waitingForDrawer.AddTransition(event_drawerOpened, state_unlockedPanel);
            state_unlockedPanel.AddTransition(event_panelClosed, state_idle);
            return machine;
        }
    }
}

And finally the console app to drive and simulate the state machine, Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using StateMachineCompiler;

namespace App
{
    class Program
    {
        static void Main(string[] args)
        {
            CommandChannel channel = new CommandChannel(a => Console.WriteLine("Action: " + a));
            var builder = new MissGrants(channel);

            bool done = false;
            while (!done)
            {
                Console.WriteLine("State: " + builder.CurrentState);
                Console.Write("> ");
                var cmd = Console.ReadLine();
                builder.Handle(cmd);
            }
        }
    }
}

Language Workbench Competition

I’m actually pretty close to having all of the MetaSharp grammars translated into .g files and Attila is working on the msbuild compiler components. Once we have them both done and merged we will have achieved dogfood status. There are plenty of Grammar and Lang features I want to work on still but the next major feature will be the Language Workbench, or in other words the interactive editor (I want to have some type of grammar REPL for the tclangug presentation I will be giving next month at least).

I just recently learned about the Code Generation Conference and now I am learning that there was also a Language Workbench website and that they sponsored a competition that went along with the conference. The most interesting part of this is the description of what a language workbench should do and how to rate them:

http://www.languageworkbenches.net/LWCTask-1.0.pdf

Which almost reads like a workbench spec! So this is very exciting, I have been thinking about what a MetaSharp workbench should be like and I think that I have a pretty good idea of what the external expectations are now, so this is good.

What’s funny about MetaSharp is that it is designed to solve all of the “Advanced” scenarios but currently doesn’t solve any of the “Basic” scenarios, lol. I’m not sure if that’s a good sign or a bad sign yet…