Evaluator.cs

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using System;

namespace Mercury.Interpreting
{
//=============================================================================
//  Evaluator
//=============================================================================

    internal sealed class Evaluator<T> : Argument
        where T : class
    {
        //--[  Private fields  ]-----------------------------------------------
        private bool             evaluated;
        private Argument[]       args;
        private RecursiveEval<T> eval;

        private InterpreterAction<T>  action;
        private InterpreterContext<T> context;

        //--[  Constructors  ]-------------------------------------------------

        public Evaluator(InterpreterAction<T> action, Argument[]               args, 
                         RecursiveEval<T>       eval, InterpreterContext<T> context)
            : base(action.ReturnType, null)
        {
            if (action == null) throw new ArgumentNullException("action");
            if (args   == null) throw new ArgumentNullException("args");
            if (eval   == null) throw new ArgumentNullException("eval");

            this.evaluated = false;
            this.action    = action;
            this.args      = args;
            this.eval      = eval;
            this.context   = context;
        }

        //--[  Properties  ]---------------------------------------------------

        internal override object Data
        {
            get
            {
                if (evaluated) return base.Data;
                evaluated = true;

                var type = context.reqtp;
                int tpix  = 0;
                for(int argix = 0; argix < args.Length; ++argix)
                {
                    ArgumentType argtp = action.ArgumentTypes[tpix];

                    if (argtp.HasFlag(ArgumentType.Lazy)) continue;

                    context.reqtp = argtp;
                    if ((args[argix].Data == null) && (!argtp.HasFlag(ArgumentType.Null)))
                        return null;

                    tpix = Math.Min(action.ArgumentTypes.Length - 1, ++tpix);
                }

                context.reqtp  = type;
                context.action = action.Name;
                context.LogEntryEx("(Evaluator)", "invoking {0}", action.Name);

                base.Data = action.Invoke(args, eval, context);
                
                context.LogEntryEx("(Evaluator)", "action {0} returned [{1}]", 
                    action.Name, base.Data  == null ? "(null)" : base.Data.ToString());
                return base.Data;
            }
        }

    }
}