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    Expression Example

    This example demostrates how expression processing can be broken down into smaller and simpler handlers.

    The Expression classes in this example.

    /// <summary>Expression</summary>
    public record Expression() : IRecord;
    /// <summary>Binary operation expression</summary>
    public record BinaryExpression([Request] object operation, [Request] object left, [Request] object right) : Expression;
    /// <summary>Constant expression</summary>
    public record ConstantExpression([Request] object value) : Expression;
    

    Evaluate

    EvaluateRequest is request to evaluate the value of an Expression.

    /// <summary>Request to evaluate expression</summary>
    public record struct EvaluateRequest([Request] object Expression) : IRequestFor<object>
    {
        /// <summary>Create from <paramref name="expression"/></summary>
        public static implicit operator EvaluateRequest(Expression expression) => new EvaluateRequest(expression);
    }
    

    Each handler provides a simple service, and together they can resolve trees of expressions.

    /// <summary>Evaluates sum-expressions of integers.</summary>
    [Order(100), Export<IHandlerBase>]
    public class SumEvaluator : IHandler<EvaluateRequest, int>
    {
        /// <summary>Evaluate sum expression.</summary>
        public void Handle(IQuery<EvaluateRequest, int> query)
        {
            // Query already handled by another handler
            if (query.Handled()) return;
    
            // Not Binary Expression
            if (query.Request.Expression is not BinaryExpression e) return;
            // Operation not "+"
            if (e.operation is not string op || op != "+") return;
    
            // Evaluate left
            if (!query.Service.TryGet(new EvaluateRequest(e.left), out int? lv)) return;
            // Evaluate right
            if (!query.Service.TryGet(new EvaluateRequest(e.right), out int? rv)) return;
            // Calculate sum
            int sum = lv!.Value + rv!.Value;
            // Assign result
            query.Response.SetValue(sum);
        }
    }
    
    /// <summary>Evaluates constant expressions.</summary>
    [Order(1_000), Export<IHandlerBase>]
    public class ConstantEvaluator : IHandler<EvaluateRequest, int>
    {
        /// <summary>Evaluate sum expression.</summary>
        public void Handle(IQuery<EvaluateRequest, int> query)
        {
            // Query already handled by another handler
            if (query.Handled()) return;
            // Not constant
            if (query.Request.Expression is not ConstantExpression ce) return;
            // Assign constant
            query.Response.SetValue(ce.value);
        }
    }
    

    Evaluate value of expression.

    // Find exported handlers
    IHandlerBase[] handlers = HandlerExports.Of<Anchor>();
    // Create service
    IService service = Services.Create((handlers, ServiceHandlers.Instance));
    // Type-cast
    IService<EvaluateRequest, int> evaluateService = service.Cast<EvaluateRequest, int>();
    
    // Create expression: (6+2)+100
    Expression e =
        new BinaryExpression("+",
            new BinaryExpression("+",
                    new ConstantExpression(6),
                    new ConstantExpression(2)),
            new ConstantExpression(100));
    
    // Evaluate value
    int value = evaluateService.GetRequired(e);
    // Print value
    WriteLine(value); // "108"
    

    Coalesce

    CoalesceRequest is similiar request, but it tries to optimize expressions into smaller form.

    /// <summary>Request to coalesce expression into shorter format</summary>
    public record struct CoalesceRequest([Request] object Expression) : IRequestFor<Expression>
    {
        /// <summary>Create from <paramref name="expression"/></summary>
        public static implicit operator CoalesceRequest(Expression expression) => new CoalesceRequest(expression);
    }
    

    Each handler does their own part, and together they handle expression trees.

    /// <summary>Coalesces sum expessions.</summary>
    [Order(10), Export<IHandlerBase>]
    public class SumCoalescer : IHandler<CoalesceRequest, Expression>
    {
        /// <summary>Evaluate sum expression.</summary>
        public void Handle(IQuery<CoalesceRequest, Expression> query)
        {
            // Query already handled by another handler
            if (query.Handled()) return;
    
            // Not Binary Expression
            if (query.Request.Expression is not BinaryExpression e) return;
            // Not "+" operation
            if (e.operation is not string op || op != "+") return;
    
            // Get child expressions
            object? le = e.left, re = e.right;
            // Try resolve left
            if (le is not ConstantExpression) query.Service.TryGet(new CoalesceRequest(le), out le);
            // Left is not constant
            if (le is not ConstantExpression lc || lc.value is not int lv) return;
    
            // Try resolve right
            if (re is not ConstantExpression) query.Service.TryGet(new CoalesceRequest(re), out re);
            // Right is not constant
            if (re is not ConstantExpression rc || rc.value is not int rv) return;
    
            // Calculate sum
            int sum = lv + rv;
            // Replace as constant
            ConstantExpression result = new ConstantExpression(sum);
    
            // Assign result
            query.Response.SetValue(result);
        }
    }
    
    /// <summary>Return expression as is.</summary>
    [Order(1_000_000_000), Export<IHandlerBase>]
    public class PassthroughCoaleser : IHandler<CoalesceRequest, Expression>
    {
        /// <summary>Evaluate sum expression.</summary>
        public void Handle(IQuery<CoalesceRequest, Expression> query)
        {
            // Query already handled by another handler
            if (query.Handled()) return;
            // As-is
            query.Response.SetValue(query.Request.Expression);
        }
    
    }
    

    Coalesce expression.

    // Find exported handlers
    IHandlerBase[] handlers = HandlerExports.Of<Anchor>();
    // Create service
    IService service = Services.Create((handlers, ServiceHandlers.Instance));
    // Type-cast
    IService<CoalesceRequest, Expression> coalesceService = service.Cast<CoalesceRequest, Expression>();
    
    // Create expression: (6+2)+100
    Expression e =
        new BinaryExpression("+",
            new BinaryExpression("+",
                    new ConstantExpression(6),
                    new ConstantExpression(2)),
            new ConstantExpression(100));
    
    // Coalese expression into shorter format
    Expression ce = coalesceService.GetRequired(e);
    // Print expression
    WriteLine(ce); // "ConstantExpression { value = 108 }"
    

    Full Example

    Full example
    using System;
    using Avalanche.Module;
    using Avalanche.Service;
    using Avalanche.Utilities;
    using Avalanche.Utilities.Record;
    using static System.Console;
    
    public class example_expression
    {
        // <01>
        /// <summary>Expression</summary>
        public record Expression() : IRecord;
        /// <summary>Binary operation expression</summary>
        public record BinaryExpression([Request] object operation, [Request] object left, [Request] object right) : Expression;
        /// <summary>Constant expression</summary>
        public record ConstantExpression([Request] object value) : Expression;
        // </01>
    
        public static void Run()
        {
            {
                // <10>
                // Find exported handlers
                IHandlerBase[] handlers = HandlerExports.Of<Anchor>();
                // Create service
                IService service = Services.Create((handlers, ServiceHandlers.Instance));
                // Type-cast
                IService<EvaluateRequest, int> evaluateService = service.Cast<EvaluateRequest, int>();
    
                // Create expression: (6+2)+100
                Expression e =
                    new BinaryExpression("+",
                        new BinaryExpression("+",
                                new ConstantExpression(6),
                                new ConstantExpression(2)),
                        new ConstantExpression(100));
    
                // Evaluate value
                int value = evaluateService.GetRequired(e);
                // Print value
                WriteLine(value); // "108"
                // </10>
            }
    
            {
                // <20>
                // Find exported handlers
                IHandlerBase[] handlers = HandlerExports.Of<Anchor>();
                // Create service
                IService service = Services.Create((handlers, ServiceHandlers.Instance));
                // Type-cast
                IService<CoalesceRequest, Expression> coalesceService = service.Cast<CoalesceRequest, Expression>();
    
                // Create expression: (6+2)+100
                Expression e =
                    new BinaryExpression("+",
                        new BinaryExpression("+",
                                new ConstantExpression(6),
                                new ConstantExpression(2)),
                        new ConstantExpression(100));
    
                // Coalese expression into shorter format
                Expression ce = coalesceService.GetRequired(e);
                // Print expression
                WriteLine(ce); // "ConstantExpression { value = 108 }"
                // </20>
            }
        }
    
        // <11>
        /// <summary>Request to evaluate expression</summary>
        public record struct EvaluateRequest([Request] object Expression) : IRequestFor<object>
        {
            /// <summary>Create from <paramref name="expression"/></summary>
            public static implicit operator EvaluateRequest(Expression expression) => new EvaluateRequest(expression);
        }
        // </11>
    
        // <12>
        /// <summary>Evaluates sum-expressions of integers.</summary>
        [Order(100), Export<IHandlerBase>]
        public class SumEvaluator : IHandler<EvaluateRequest, int>
        {
            /// <summary>Evaluate sum expression.</summary>
            public void Handle(IQuery<EvaluateRequest, int> query)
            {
                // Query already handled by another handler
                if (query.Handled()) return;
    
                // Not Binary Expression
                if (query.Request.Expression is not BinaryExpression e) return;
                // Operation not "+"
                if (e.operation is not string op || op != "+") return;
    
                // Evaluate left
                if (!query.Service.TryGet(new EvaluateRequest(e.left), out int? lv)) return;
                // Evaluate right
                if (!query.Service.TryGet(new EvaluateRequest(e.right), out int? rv)) return;
                // Calculate sum
                int sum = lv!.Value + rv!.Value;
                // Assign result
                query.Response.SetValue(sum);
            }
        }
    
        /// <summary>Evaluates constant expressions.</summary>
        [Order(1_000), Export<IHandlerBase>]
        public class ConstantEvaluator : IHandler<EvaluateRequest, int>
        {
            /// <summary>Evaluate sum expression.</summary>
            public void Handle(IQuery<EvaluateRequest, int> query)
            {
                // Query already handled by another handler
                if (query.Handled()) return;
                // Not constant
                if (query.Request.Expression is not ConstantExpression ce) return;
                // Assign constant
                query.Response.SetValue(ce.value);
            }
        }
        // </12>
    
    
        // <21>
        /// <summary>Request to coalesce expression into shorter format</summary>
        public record struct CoalesceRequest([Request] object Expression) : IRequestFor<Expression>
        {
            /// <summary>Create from <paramref name="expression"/></summary>
            public static implicit operator CoalesceRequest(Expression expression) => new CoalesceRequest(expression);
        }
        // </21>
    
        // <22>
        /// <summary>Coalesces sum expessions.</summary>
        [Order(10), Export<IHandlerBase>]
        public class SumCoalescer : IHandler<CoalesceRequest, Expression>
        {
            /// <summary>Evaluate sum expression.</summary>
            public void Handle(IQuery<CoalesceRequest, Expression> query)
            {
                // Query already handled by another handler
                if (query.Handled()) return;
    
                // Not Binary Expression
                if (query.Request.Expression is not BinaryExpression e) return;
                // Not "+" operation
                if (e.operation is not string op || op != "+") return;
    
                // Get child expressions
                object? le = e.left, re = e.right;
                // Try resolve left
                if (le is not ConstantExpression) query.Service.TryGet(new CoalesceRequest(le), out le);
                // Left is not constant
                if (le is not ConstantExpression lc || lc.value is not int lv) return;
    
                // Try resolve right
                if (re is not ConstantExpression) query.Service.TryGet(new CoalesceRequest(re), out re);
                // Right is not constant
                if (re is not ConstantExpression rc || rc.value is not int rv) return;
    
                // Calculate sum
                int sum = lv + rv;
                // Replace as constant
                ConstantExpression result = new ConstantExpression(sum);
    
                // Assign result
                query.Response.SetValue(result);
            }
        }
    
        /// <summary>Return expression as is.</summary>
        [Order(1_000_000_000), Export<IHandlerBase>]
        public class PassthroughCoaleser : IHandler<CoalesceRequest, Expression>
        {
            /// <summary>Evaluate sum expression.</summary>
            public void Handle(IQuery<CoalesceRequest, Expression> query)
            {
                // Query already handled by another handler
                if (query.Handled()) return;
                // As-is
                query.Response.SetValue(query.Request.Expression);
            }
    
        }
        // </22>
    
        /// <summary>Dummy class that is used to get reference to the Assembly.</summary>
        public class Anchor { }
    }
    
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