sharplib/imm/FSM.cs

using System;
using System.Runtime.CompilerServices;
using Optional;



namespace imm;



public record class Context : imm.Recorded<Context>
{

}

public record class State<TSUB, CTX>( CTX Context ) : imm.Recorded<TSUB>
	where TSUB : State<TSUB, CTX>
	where CTX : Context
{
	virtual public (CTX, TSUB) onEnter(CTX ctx, State<TSUB, CTX> oldState)
	{
		return (ctx, (TSUB)this);
	}

	virtual public (CTX, TSUB) onExit(CTX ctx, State<TSUB, CTX> newState)
	{
		return (ctx, (TSUB)this);
	}
}



public record class FSM<TSUB, ST, CTX> : imm.Recorded<TSUB>
	where TSUB : FSM<TSUB, ST, CTX>
	where ST : State<ST, CTX>
	where CTX : Context
{
	public CTX Context { get; init; }
	public ST State { get; init; }

	public FSM( CTX context, ST stStart )
	{
		Context = context;
		State = stStart;
	}

	public TSUB Transition(ST newState, string reason,
													[CallerMemberName] string memberName = "",
													[CallerFilePath] string filePath = "",
													[CallerLineNumber] int lineNumber = 0,
													[CallerArgumentExpression("fn")]
													string expression = default
	)
	{
		log.debug( $"Trans from {State.GetType().Name} to {newState.GetType().Name} for {reason}" );

		var origState = State;

		var (newCtx, oldState) = State.onExit(Context, newState);
                               
		var (newCTX, storeState) = newState.onEnter(newCtx, oldState);

		var newFSM = this.Process( (v) => (this as TSUB) with
		{
			Context = newCTX,
			State = storeState,
		}, $"{reason}"  );

		return newFSM;
	}

	/*
	public TSUB ( Func<ST, ST> fn,  string reason, 
													[CallerMemberName] string member = "",
													[CallerFilePath] string file = "",
													[CallerLineNumber] int line = 0,
													[CallerArgumentExpression("fn")]
													string expression = default 
 	)
	{
		var newState = fn( State );

		if( object.ReferenceEquals( newState, State ) ) return (TSUB)this;

		TSUB newFSM = this.Process( this with
		{
			Context = Context,
			State = newState,
		}, $"Processing: {newState.GetType().Name} for {reason}", 
		member, file, line, expression );

		return (TSUB)newFSM;
	}
	*/

}