Optional.cs

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Waher.Script.Abstraction.Elements;
using Waher.Script.Model;
using Waher.Script.Objects;
 
namespace Waher.Script.Functions.Runtime
{
    public class Optional : FunctionOneVariable
    {
        public Optional(ScriptNode Argument, int Start, int Length, Expression Expression)
            : base(Argument, Start, Length, Expression)
        {
        }
 
        public override string FunctionName => nameof(Optional);
 
        public override IElement Evaluate(Variables Variables)
        {
            try
            {
                return this.Argument.Evaluate(Variables);
            }
            catch (Exception)
            {
                return ObjectValue.Null;
            }
        }
 
        public override async Task<IElement> EvaluateAsync(Variables Variables)
        {
            try
            {
                return await this.Argument.EvaluateAsync(Variables);
            }
            catch (Exception)
            {
                return ObjectValue.Null;
            }
        }
 
        public override IElement Evaluate(IElement Argument, Variables Variables)
        {
            return Argument;
        }
 
        public override PatternMatchResult PatternMatch(IElement CheckAgainst, Dictionary<string, IElement> AlreadyFound)
        {
            object Obj = CheckAgainst.AssociatedObjectValue;
 
            if ((Obj is null) || (Obj is string s && string.IsNullOrEmpty(s)))
            {
                return this.ForAllChildNodes((ScriptNode Node, out ScriptNode NewNode, object State) =>
                {
                    NewNode = null;
 
                    if (Node is VariableReference Ref)
                    {
                        if (AlreadyFound.TryGetValue(Ref.VariableName, out IElement Value) &&
                            !(Value.AssociatedObjectValue is null))
                        {
                            return false;
                        }
 
                        AlreadyFound[Ref.VariableName] = null;
                    }
 
                    return true;
                }, null, SearchMethod.TreeOrder) ? PatternMatchResult.Match : PatternMatchResult.NoMatch;
            }
            else
                return this.Argument.PatternMatch(CheckAgainst, AlreadyFound);
        }
    }
}