hmm

Result.cs

using System;

/*
     * f0 :: a -> R<F,T0>
     * f1 :: t0 -> a -> R<F,T1>
     * F :: a -> R<F,T1>
     * F = f0 * f1 みたいなことしたい.
*/

namespace TT
{
    public class Result<TF,TS>
    {
        public bool Status;

        public TF Left { get; set; }

        public TS Right { get; set; }

        public Result (bool b)
        {
            this.Status = b;
        }

        public static Result<TF,TS> Success (TS s)
        {
            return new Result<TF,TS> (true){ Right = s };
        }

        public static Result<TF,TS> Failure (TF f)
        {
            return new Result<TF,TS> (false){ Left = f };
        }

        public Result<TF,TS2> Merge<TS2> (Func<TS,Result<TF,TS2>>use)
        {
            if (!this.Status) {
                return Result<TF,TS2>.Failure (this.Left);
            }
            return use (this.Right); 
        }

        public override string ToString ()
        {
            if (this.Status) {
                return String.Format ("Success[{0}]", this.Right);
            } else {
                return String.Format ("Failure[{0}]", this.Left);
            }
        }
    }

    public class LoginStatus
    {
        public string Token { get; set; }
    }

    public class UserInfo
    {
        public string Name { get; set; }
    }

    public enum APIStatus
    {
        ok,
        ng,
        timeout,
        notfound
    }

    public class MessageUtil
    {
        public static string MessageFromStatus (APIStatus s)
        {
            switch (s) {
            case APIStatus.ok:
                throw new InvalidOperationException ("don't call");
            case APIStatus.ng:
                return "arya- ng.";
            case APIStatus.timeout:
                return "oo timeout.";
            case APIStatus.notfound:
                return "oops not found.";
            default:
                throw new NotImplementedException ();
            }
        }

        public static Result<string, T> MessageFromStatus<T> (APIStatus s, string prefix)
        {
            return Result<string,T>.Failure (String.Format ("{0}: {1}", prefix, MessageUtil.MessageFromStatus (s)));
        }
    }

    public class APIRepository
    {
        public Result<String, LoginStatus>Login (string url, APIStatus s)
        {
            if (s == APIStatus.ok) {
                return Result<String,LoginStatus>.Success (new LoginStatus (){ Token = "dummy" });
            } else {
                return MessageUtil.MessageFromStatus<LoginStatus> (s, "login");
            }
        }

        public Result<String, UserInfo>GetUserInfo (string url, APIStatus s, LoginStatus status)
        {
            if (s == APIStatus.ok) {
                return Result<String,UserInfo>.Success (new UserInfo (){ Name = "dummy name" });
            } else {
                return MessageUtil.MessageFromStatus<UserInfo> (s, "status");
            }
        }
    }

    class MainClass
    {
        public static void Main (string[] args)
        {
            Console.WriteLine ("Hello World!");
            var m = new MainClass ();
            var repo = new APIRepository ();
            Console.WriteLine ("ok,ok             => {0}", m.GetThis0 (repo));
            Console.WriteLine ("ng,ok             => {0}", m.GetThis1 (repo));
            Console.WriteLine ("ok,timeout        => {0}", m.GetThis2 (repo));
            Console.WriteLine ("notfound,notfound => {0}", m.GetThis3 (repo));
        }

        public Result<string, UserInfo>GetThis0 (APIRepository repo)
        {
            return repo.Login ("*login*", APIStatus.ok).Merge<UserInfo> (
                (LoginStatus s) => repo.GetUserInfo ("*status*", APIStatus.ok, s)
            );
        }

        public Result<string, UserInfo>GetThis1 (APIRepository repo)
        {
            return repo.Login ("*login*", APIStatus.ng).Merge<UserInfo> (
                (LoginStatus s) => repo.GetUserInfo ("*status*", APIStatus.ok, s)
            );
        }

        public Result<string, UserInfo>GetThis2 (APIRepository repo)
        {
            return repo.Login ("*login*", APIStatus.ok).Merge<UserInfo> (
                (LoginStatus s) => repo.GetUserInfo ("*status*", APIStatus.timeout, s)
            );
        }

        public Result<string, UserInfo>GetThis3 (APIRepository repo)
        {
            return repo.Login ("*login*", APIStatus.notfound).Merge<UserInfo> (
                (LoginStatus s) => repo.GetUserInfo ("*status*", APIStatus.notfound, s)
            );
        }
    }
}

ok,ok             => Success[TT.UserInfo]
ng,ok             => Failure[login: arya- ng.]
ok,timeout        => Failure[status: oo timeout.]
notfound,notfound => Failure[login: oops not found.]