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 { }
}