A less ceremonial approach to “explicit endomorphism” presented by Mark Seemann in “Builder as a monoid”

BuilderAsMonoid.csx

// This script is related the following blog entry by Mark Seeman titled,
// "Builder as a monoid":
//
// https://blog.ploeh.dk/2020/02/17/builder-as-a-monoid/
//
// It demonstrates a simpler approach to avoid the ceremony of "explicit
// endomorphism" by using merely delegates. It does so by replacing
// "IEndomorphism<T>" with "Func<T, T>" and then using "+" in lieu of
// "AppendEndomorphism<T>" to achieve the same effect.
//
// Run using dotnet-script (https://github.com/filipw/dotnet-script).
// First, ensure you have it installed:
//
//     dotnet tool install -g dotnet-script
//
// then execute this script using the following command-line:
//
//     dotnet script BuilderAsMonoid.csx

#r "nuget: Newtonsoft.Json, 12.0.3"

#nullable enable

using System;
using System.Linq;
using System.Net.Http;
using Newtonsoft.Json;
using static System.Console;
using HttpRequestMessageBuilderEndomorphism = System.Func<HttpRequestMessageBuilder, HttpRequestMessageBuilder>;

var setup
    = HttpRequestMessageSetup.Method(HttpMethod.Post)
    + HttpRequestMessageSetup.Json(new
    {
        id = Guid.NewGuid(),
        date = "2020-03-22 19:30:00",
        name = "Ælfgifu",
        email = "ælfgifu@example.net",
        quantity = 1
    });

var msg = setup(new HttpRequestMessageBuilder("http://example.com/")).Build();

WriteLine($"{msg.Method} {msg.RequestUri} HTTP/{msg.Version.ToString(2)}");
foreach (var (name, values) in msg.Headers.Concat(msg.Content.Headers))
    WriteLine($"{name}: {string.Join(", ", values)}");
WriteLine();
WriteLine(await msg.Content.ReadAsStringAsync());

static class HttpRequestMessageSetup
{
    public static HttpRequestMessageBuilderEndomorphism Method(HttpMethod method) => b => b.WithMethod(method);
    public static HttpRequestMessageBuilderEndomorphism Json(object body) => b => b.AddJsonBody(body);
}

// Source: https://blog.ploeh.dk/2020/02/10/builder-isomorphisms/#ed99bc3715f541be8b6b9239848395e3
// Copyright Mark Seemann 2020

sealed class HttpRequestMessageBuilder
{
    private readonly Uri url;
    private object? jsonBody;

    public HttpRequestMessageBuilder(string url) : this(new Uri(url)) { }

    public HttpRequestMessageBuilder(Uri url)
    {
        this.url = url;
        Method = HttpMethod.Get;
    }

    public HttpMethod Method { get; private set; }

    public HttpRequestMessageBuilder WithMethod(HttpMethod newMethod)
    {
        Method = newMethod;
        return this;
    }

    public HttpRequestMessageBuilder AddJsonBody(object jsonBody)
    {
        this.jsonBody = jsonBody;
        return this;
    }

    public HttpRequestMessage Build()
    {
        var message = new HttpRequestMessage(Method, url);
        BuildBody(message);
        return message;
    }

    private void BuildBody(HttpRequestMessage message)
    {
        if (jsonBody is null)
            return;

        string json = JsonConvert.SerializeObject(jsonBody);
        message.Content = new StringContent(json);
        message.Content.Headers.ContentType.MediaType = "application/json";
    }
}

LICENSE

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