improve result structure and handling

csharp-es/Domain/Account.cs

@@ -1,4 +1,5 @@
 using CsharpEs.Library;
+using static CsharpEs.Library.ResultModule;
 
 namespace CsharpEs.Domain;
 
@@ -47,31 +48,27 @@ public static class AccountDecider
                 (state, command) switch
                 {
                     // if this is a new account, check for valid opening balance
-                    (null, AccountCommand.OpenAccount c) when c.OpeningBalance.Amount < 0m =>
+                    (null, AccountCommand.OpenAccount c) when c.OpeningBalance.Amount < 0m => Fail(
-                        Result<AccountEvent, AccountError>.Fail(
                         new AccountError.OpeningBalanceMustBeNonNegative()
                     ),
 
                     // still a new account, now we can open it
-                    (null, AccountCommand.OpenAccount c) => Result<AccountEvent, AccountError>.Ok(
+                    (null, AccountCommand.OpenAccount c) => Ok(
                         new AccountEvent.AccountOpened(c.AccountId, c.OpeningBalance)
                     ),
 
                     // if we have an account already, you can't open it
-                    (not null, AccountCommand.OpenAccount c) => Result<
+                    (not null, AccountCommand.OpenAccount c) => Fail(
-                        AccountEvent,
+                        new AccountError.AccountOpenAlready()
-                        AccountError
-                    >.Fail(new AccountError.AccountOpenAlready()),
-
-                    (null, _) => Result<AccountEvent, AccountError>.Fail(
-                        new AccountError.AccountNotFound()
                     ),
 
-                    (_, AccountCommand.WithdrawMoney c) => Result<AccountEvent, AccountError>.Ok(
+                    (null, _) => Fail(new AccountError.AccountNotFound()),
+
+                    (_, AccountCommand.WithdrawMoney c) => Ok(
                         new AccountEvent.MoneyWithdrawn(c.AccountId, c.Amount)
                     ),
 
-                    (_, AccountCommand.DepositMoney c) => Result<AccountEvent, AccountError>.Ok(
+                    (_, AccountCommand.DepositMoney c) => Ok(
                         new AccountEvent.MoneyDeposited(c.AccountId, c.Amount)
                     ),
 
@@ -86,10 +83,8 @@ public static class AccountDecider
     ) =>
         (state, @event) switch
         {
-            (null, AccountEvent.AccountOpened e) => Result<AccountState?, AccountError>.Ok(
+            (null, AccountEvent.AccountOpened e) => Ok(new AccountState(e.AccountId)),
-                new AccountState(e.AccountId)
-            ),
 
-            _ => Result<AccountState?, AccountError>.Ok(state),
+            _ => Ok(state),
         };
 }

csharp-es/Infrastructure/AccountReadModel.cs

@@ -1,6 +1,7 @@
 using System.Collections.Immutable;
 using CsharpEs.Domain;
 using CsharpEs.Library;
+using static CsharpEs.Library.ResultModule;
 
 namespace CsharpEs.Infrastructure;
 
@@ -57,9 +58,7 @@ public static class AccountBalanceReadModelModule
                             );
                         }
                         else
-                            return Result<AccountDetails, ReadModelError>.Fail(
+                            return Fail(new ReadModelError.AccountDetailsNotFound());
-                                new ReadModelError.AccountDetailsNotFound()
-                            );
                     }
                     break;
 
@@ -78,9 +77,7 @@ public static class AccountBalanceReadModelModule
                             );
                         }
                         else
-                            return Result<AccountDetails, ReadModelError>.Fail(
+                            return Fail(new ReadModelError.AccountDetailsNotFound());
-                                new ReadModelError.AccountDetailsNotFound()
-                            );
                     }
                     break;
             }
@@ -88,15 +85,13 @@ public static class AccountBalanceReadModelModule
             // Read model-side "project" may not normally return any data, but for demo
             // purposes we return the affected data directly to save us modeling a separate
             // query side.
-            return Result<AccountDetails, ReadModelError>.Ok(modelData[@event.AccountId]);
+            return Ok(modelData[@event.AccountId]);
         }
 
         Result<KeyValuePair<Guid, AccountDetails>[], ReadModelError> Query() =>
-            Result<KeyValuePair<Guid, AccountDetails>[], ReadModelError>.Ok(modelData.ToArray());
+            Ok(modelData.ToArray());
 
-        return Result<AccountBalanceReadModel, ReadModelError>.Ok(
+        return Ok(new AccountBalanceReadModel(Project, Query));
-            new AccountBalanceReadModel(Project, Query)
-        );
     }
 
     private static AccountFlags CalcAccountFlags(Money newBalance, AccountDetails oldDetails)

csharp-es/Library/Result.cs

@@ -1,68 +1,128 @@
 namespace CsharpEs.Library;
 
-public readonly record struct Result<T, E>
+// disable match exhaustion warnings - C# thinks we could have other
+// derived instances of Result<T,E> but there's no way to say
+// "don't allow creation of further derived types" while leaving
+// the type publicly visible.
+#pragma warning disable CS8509
+
+public abstract record Result<T, E>
 {
-    private readonly T _value;
+    public static implicit operator Result<T, E>(T value) => new ResultOk<T, E>(value);
 
-    private readonly E _error;
+    public static implicit operator Result<T, E>(E error) => new ResultFail<T, E>(error);
+}
 
-    public bool IsSuccess { get; }
+public sealed record ResultOk<T, E>(T Value) : Result<T, E>;
 
-    public bool IsFailure => !IsSuccess;
+public sealed record ResultFail<T, E>(E Error) : Result<T, E>;
 
-    public T Value =>
+public static class ResultModule
-        IsSuccess
+{
-            ? _value
+    public static T Ok<T>(T v) => v;
-            : throw new InvalidOperationException("No value present for failed result.");
 
-    public E Error =>
+    public static Result<T?, E> OkNone<T, E>()
-        IsFailure
+        where T : class? => new ResultOk<T?, E>(null);
-            ? _error
-            : throw new InvalidOperationException("No error present for successful result.");
 
-    private Result(T value)
+    public static E Fail<E>(E e) => e;
+}
+
+public static class ResultExtensions
+{
+    extension<TIn, E>(Result<TIn, E> result)
     {
-        _value = value;
+        public bool IsSuccess => result is ResultOk<TIn, E>;
-        _error = default!;
+        public bool IsFailure => !result.IsSuccess;
-        IsSuccess = true;
+
+        public Result<TOut, E> Bind<TOut>(Func<TIn, Result<TOut, E>> binder) =>
+            result switch
+            {
+                ResultOk<TIn, E>(var v) => binder(v),
+                ResultFail<TIn, E>(var e) => new ResultFail<TOut, E>(e),
+            };
+
+        public Result<TOut, E> Map<TOut>(Func<TIn, TOut> mapper) =>
+            result switch
+            {
+                ResultOk<TIn, E>(var v) => new ResultOk<TOut, E>(mapper(v)),
+                ResultFail<TIn, E>(var e) => new ResultFail<TOut, E>(e),
+            };
+
+        public Result<TIn, EOut> MapError<EOut>(Func<E, EOut> mapper) =>
+            result switch
+            {
+                ResultOk<TIn, E>(var v) => new ResultOk<TIn, EOut>(v),
+                ResultFail<TIn, E>(var e) => new ResultFail<TIn, EOut>(mapper(e)),
+            };
+
+        public Result<TIn, E> Tap(Action<TIn> action)
+        {
+            if (result is ResultOk<TIn, E>(var v))
+                action(v);
+            return result;
         }
 
-    private Result(E error)
+        public Result<TIn, E> TapError(Action<E> action)
         {
-        _value = default!;
+            if (result is ResultFail<TIn, E>(var e))
-        _error = error;
+                action(e);
-        IsSuccess = false;
+            return result;
         }
 
-    public static Result<T, E> Ok(T value) => new(value);
+        public Result<TIn, E> Log(string src, string msg) =>
+            Tap(result, Logging.OutputDelegate<TIn>(src, msg))
+                .TapError(m => Logging.OutputError(src, m));
 
-    public static Result<T, E> Fail(E error) => new(error);
+        public Result<TIn, E> Log(string src, Func<TIn, string> renderText) =>
+            Tap(result, x => Logging.OutputDelegate<TIn>(src, renderText(x))(x))
+                .TapError(m => Logging.OutputError(src, m));
 
-    public TResult Match<TResult>(Func<T, TResult> onSuccess, Func<E, TResult> onFailure) =>
+        public Result<TOut, E> Select<TOut>(Func<TIn, TOut> mapper) => Map(result, mapper);
-        IsSuccess ? onSuccess(_value) : onFailure(_error);
 
-    public void Switch(Action<T> onSuccess, Action<E> onFailure)
+        public Result<TOut, E> SelectMany<TIntermediate, TOut>(
+            Func<TIn, Result<TIntermediate, E>> binder,
+            Func<TIn, TIntermediate, TOut> projector
+        ) => Bind(result, x => binder(x).Map(y => projector(x, y)));
+
+        public TResult Match<TResult>(Func<TIn, TResult> onSuccess, Func<E, TResult> onFailure) =>
+            result switch
             {
-        if (IsSuccess)
+                ResultOk<TIn, E>(var v) => onSuccess(v),
-            onSuccess(_value);
+                ResultFail<TIn, E>(var e) => onFailure(e),
-        else
+            };
-            onFailure(_error);
+
+        public void Switch(Action<TIn> onSuccess, Action<E> onFailure)
+        {
+            switch (result)
+            {
+                case ResultOk<TIn, E>(var v):
+                    onSuccess(v);
+                    break;
+                case ResultFail<TIn, E>(var e):
+                    onFailure(e);
+                    break;
+            }
+        }
     }
 
-    public static Result<T, E> Catch(Func<T> f, Func<Exception, E> exceptionMapper)
+    extension<TIn, E>(Result<TIn, E>)
+    {
+        public static Result<TIn, E> Catch(Func<TIn> f, Func<Exception, E> exceptionMapper)
         {
             try
             {
-            var result = f();
+                return new ResultOk<TIn, E>(f());
-            return Result<T, E>.Ok(result);
             }
             catch (Exception e)
             {
-            return Result<T, E>.Fail(exceptionMapper(e));
+                return new ResultFail<TIn, E>(exceptionMapper(e));
             }
         }
 
-    public static Result<T, E> Catch(Func<Result<T, E>> f, Func<Exception, E> exceptionMapper)
+        public static Result<TIn, E> Catch(
+            Func<Result<TIn, E>> f,
+            Func<Exception, E> exceptionMapper
+        )
         {
             try
             {
@@ -70,69 +130,8 @@ public readonly record struct Result<T, E>
             }
             catch (Exception e)
             {
-            return Result<T, E>.Fail(exceptionMapper(e));
+                return new ResultFail<TIn, E>(exceptionMapper(e));
+            }
         }
     }
 }
-
-public static class ResultExtensions
-{
-    public static Result<TOut, E> Bind<TIn, TOut, E>(
-        this Result<TIn, E> result,
-        Func<TIn, Result<TOut, E>> binder
-    ) => result.IsSuccess ? binder(result.Value) : Result<TOut, E>.Fail(result.Error);
-
-    public static Result<TOut, E> Map<TIn, TOut, E>(
-        this Result<TIn, E> result,
-        Func<TIn, TOut> mapper
-    ) =>
-        result.IsSuccess
-            ? Result<TOut, E>.Ok(mapper(result.Value))
-            : Result<TOut, E>.Fail(result.Error);
-
-    public static Result<T, E> Tap<T, E>(this Result<T, E> result, Action<T> action)
-    {
-        if (result.IsSuccess)
-            action(result.Value);
-
-        return result;
-    }
-
-    public static Result<T, E> TapError<T, E>(this Result<T, E> result, Action<E> action)
-    {
-        if (result.IsFailure)
-            action(result.Error);
-
-        return result;
-    }
-
-    public static Result<T, EOut> MapError<T, EIn, EOut>(
-        this Result<T, EIn> result,
-        Func<EIn, EOut> map
-    ) =>
-        result.IsSuccess
-            ? Result<T, EOut>.Ok(result.Value)
-            : Result<T, EOut>.Fail(map(result.Error));
-
-    public static Result<T, E> Log<T, E>(this Result<T, E> result, string src, string msg) =>
-        Tap(result, Logging.OutputDelegate<T>(src, msg)).TapError(m => Logging.OutputError(src, m));
-
-    public static Result<T, E> Log<T, E>(
-        this Result<T, E> result,
-        string src,
-        Func<T, string> renderText
-    ) =>
-        Tap(result, x => Logging.OutputDelegate<T>(src, renderText(x))(x))
-            .TapError(m => Logging.OutputError(src, m));
-
-    public static Result<TOut, E> Select<TIn, TOut, E>(
-        this Result<TIn, E> result,
-        Func<TIn, TOut> mapper
-    ) => result.Map(mapper);
-
-    public static Result<TOut, E> SelectMany<TIn, TIntermediate, TOut, E>(
-        this Result<TIn, E> result,
-        Func<TIn, Result<TIntermediate, E>> binder,
-        Func<TIn, TIntermediate, TOut> projector
-    ) => result.Bind(x => binder(x).Map(y => projector(x, y)));
-}

csharp-es/Program.cs

@@ -2,6 +2,7 @@
 using CsharpEs.Domain;
 using CsharpEs.Infrastructure;
 using CsharpEs.Library;
+using static CsharpEs.Library.ResultModule;
 
 public abstract record DemoError
 {
@@ -66,7 +67,7 @@ public class Program
             .Bind(readModel =>
                 demoCommands
                     .Aggregate(
-                        Result<AccountState?, DemoError>.Ok(null),
+                        OkNone<AccountState?, DemoError>(),
                         (stateResult, command) =>
                             stateResult
                                 .Bind(state =>

csharp-fp3.Tests/AccountTests.cs

@@ -1,4 +1,5 @@
 using CsharpFp3.Domain;
+using CsharpFp3.Library;
 
 namespace CsharpFp3.Tests;
 
@@ -11,46 +12,52 @@ public class AccountTests
     {
         var result = AccountDomain.Open(Guid.NewGuid(), new Money(-1m));
 
-        Assert.IsType<AccountError.OpeningBalanceMustBeNonNegative>(result.Error);
+        Assert.True(result.Match(_ => false, e => e is AccountError.OpeningBalanceMustBeNonNegative));
     }
 
     [Fact]
     public void Withdrawing_a_zero_amount_returns_AmountMustBePositive()
     {
-        var account = AccountDomain.Open(Guid.NewGuid(), new Money(100m)).Value;
+        var accountResult = AccountDomain.Open(Guid.NewGuid(), new Money(100m));
+        var account = accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success"));
 
         var result = AccountDomain.Withdraw(account, new Money(0m));
 
-        Assert.IsType<AccountError.AmountMustBePositive>(result.Error);
+        Assert.True(result.Match(_ => false, e => e is AccountError.AmountMustBePositive));
     }
 
     [Fact]
     public void Withdrawing_a_negative_amount_returns_AmountMustBePositive()
     {
-        var account = AccountDomain.Open(Guid.NewGuid(), new Money(100m)).Value;
+        var accountResult = AccountDomain.Open(Guid.NewGuid(), new Money(100m));
+        var account = accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success"));
 
         var result = AccountDomain.Withdraw(account, new Money(-10m));
 
-        Assert.IsType<AccountError.AmountMustBePositive>(result.Error);
+        Assert.True(result.Match(_ => false, e => e is AccountError.AmountMustBePositive));
     }
 
     [Fact]
     public void Withdrawing_more_than_the_balance_returns_InsufficientBalance()
     {
-        var account = AccountDomain.Open(Guid.NewGuid(), new Money(100m)).Value;
+        var accountResult = AccountDomain.Open(Guid.NewGuid(), new Money(100m));
+        var account = accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success"));
 
         var result = AccountDomain.Withdraw(account, new Money(101m));
 
-        Assert.IsType<AccountError.InsufficientBalance>(result.Error);
+        Assert.True(result.Match(_ => false, e => e is AccountError.InsufficientBalance));
     }
 
     [Fact]
     public void InsufficientBalance_error_reports_the_current_balance_and_attempted_amount()
     {
-        var account = AccountDomain.Open(Guid.NewGuid(), new Money(100m)).Value;
+        var accountResult = AccountDomain.Open(Guid.NewGuid(), new Money(100m));
+        var account = accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success"));
 
-        var error = Assert.IsType<AccountError.InsufficientBalance>(
+        var result = AccountDomain.Withdraw(account, new Money(150m));
-            AccountDomain.Withdraw(account, new Money(150m)).Error
+        var error = result.Match(
+            _ => throw new InvalidOperationException("Expected failure"),
+            e => e is AccountError.InsufficientBalance ins ? ins : throw new InvalidOperationException("Expected InsufficientBalance")
         );
 
         Assert.Equal(100m, error.Balance.Amount);
@@ -60,13 +67,14 @@ public class AccountTests
     [Fact]
     public void Successive_withdrawals_are_each_applied_to_the_running_balance()
     {
-        var account = AccountDomain.Open(Guid.NewGuid(), new Money(300m)).Value;
+        var accountResult = AccountDomain.Open(Guid.NewGuid(), new Money(300m));
+        var account = accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success"));
 
         var result1 = AccountDomain.Withdraw(account, new Money(100m));
-        var updatedAccount1 = result1.Value;
+        var updatedAccount1 = result1.Match(v => v, _ => throw new InvalidOperationException("Expected success"));
 
         var result2 = AccountDomain.Withdraw(updatedAccount1, new Money(100m));
-        var updatedAccount2 = result2.Value;
+        var updatedAccount2 = result2.Match(v => v, _ => throw new InvalidOperationException("Expected success"));
 
         Assert.Equal(100m, updatedAccount2.Balance.Amount);
     }

csharp-fp3.Tests/ResultTests.cs

@@ -1,4 +1,5 @@
 using CsharpFp3.Library;
+using static CsharpFp3.Library.ResultModule;
 
 namespace CsharpFp3.Tests;
 
@@ -10,7 +11,7 @@ public class ResultTests
     [Fact]
     public void Ok_is_success()
     {
-        var result = Result<int, string>.Ok(42);
+        Result<int, string> result = new ResultOk<int, string>(42);
 
         Assert.True(result.IsSuccess);
         Assert.False(result.IsFailure);
@@ -19,42 +20,26 @@ public class ResultTests
     [Fact]
     public void Fail_is_failure()
     {
-        var result = Result<int, string>.Fail("oops");
+        Result<int, string> result = new ResultFail<int, string>("oops");
 
         Assert.True(result.IsFailure);
         Assert.False(result.IsSuccess);
     }
 
     [Fact]
-    public void Ok_exposes_value()
+    public void Ok_exposes_value_via_Match()
     {
-        var result = Result<int, string>.Ok(42);
+        Result<int, string> result = new ResultOk<int, string>(42);
 
-        Assert.Equal(42, result.Value);
+        Assert.Equal(42, result.Match(v => v, _ => 0));
     }
 
     [Fact]
-    public void Fail_exposes_error()
+    public void Fail_exposes_error_via_Match()
     {
-        var result = Result<int, string>.Fail("oops");
+        Result<int, string> result = new ResultFail<int, string>("oops");
 
-        Assert.Equal("oops", result.Error);
+        Assert.Equal("oops", result.Match(_ => "", e => e));
-    }
-
-    [Fact]
-    public void Accessing_Value_on_a_failed_result_throws()
-    {
-        var result = Result<int, string>.Fail("oops");
-
-        Assert.Throws<InvalidOperationException>(() => result.Value);
-    }
-
-    [Fact]
-    public void Accessing_Error_on_a_successful_result_throws()
-    {
-        var result = Result<int, string>.Ok(42);
-
-        Assert.Throws<InvalidOperationException>(() => result.Error);
     }
 
     // --- Match ---
@@ -62,7 +47,7 @@ public class ResultTests
     [Fact]
     public void Match_calls_onSuccess_for_Ok()
     {
-        var result = Result<int, string>.Ok(10);
+        Result<int, string> result = new ResultOk<int, string>(10);
 
         var output = result.Match(v => $"value:{v}", e => $"error:{e}");
 
@@ -72,7 +57,7 @@ public class ResultTests
     [Fact]
     public void Match_calls_onFailure_for_Fail()
     {
-        var result = Result<int, string>.Fail("bad");
+        Result<int, string> result = new ResultFail<int, string>("bad");
 
         var output = result.Match(v => $"value:{v}", e => $"error:{e}");
 
@@ -84,7 +69,7 @@ public class ResultTests
     [Fact]
     public void Switch_calls_onSuccess_for_Ok()
     {
-        var result = Result<int, string>.Ok(7);
+        Result<int, string> result = new ResultOk<int, string>(7);
         var called = false;
 
         result.Switch(_ => called = true, _ => { });
@@ -95,7 +80,7 @@ public class ResultTests
     [Fact]
     public void Switch_calls_onFailure_for_Fail()
     {
-        var result = Result<int, string>.Fail("err");
+        Result<int, string> result = new ResultFail<int, string>("err");
         var called = false;
 
         result.Switch(_ => { }, _ => called = true);
@@ -111,7 +96,7 @@ public class ResultTests
         var result = Result<int, string>.Catch(() => 99, ex => ex.Message);
 
         Assert.True(result.IsSuccess);
-        Assert.Equal(99, result.Value);
+        Assert.Equal(99, result.Match(v => v, _ => 0));
     }
 
     [Fact]
@@ -123,7 +108,7 @@ public class ResultTests
         );
 
         Assert.True(result.IsFailure);
-        Assert.Equal("boom", result.Error);
+        Assert.Equal("boom", result.Match(_ => "", e => e));
     }
 
     // --- Bind ---
@@ -131,33 +116,34 @@ public class ResultTests
     [Fact]
     public void Bind_chains_to_the_next_result_on_success()
     {
-        var result = Result<int, string>.Ok(5).Bind(v => Result<string, string>.Ok($"got {v}"));
+        Result<int, string> result = new ResultOk<int, string>(5);
+        var chained = result.Bind(v => new ResultOk<string, string>($"got {v}"));
 
-        Assert.Equal("got 5", result.Value);
+        Assert.Equal("got 5", chained.Match(v => v, _ => ""));
     }
 
     [Fact]
     public void Bind_short_circuits_on_failure_without_calling_the_binder()
     {
         var binderCalled = false;
-        var result = Result<int, string>
+        Result<int, string> result = new ResultFail<int, string>("nope");
-            .Fail("nope")
+        var chained = result.Bind(v =>
-            .Bind(v =>
         {
             binderCalled = true;
-                return Result<string, string>.Ok($"got {v}");
+            return new ResultOk<string, string>($"got {v}");
         });
 
         Assert.False(binderCalled);
-        Assert.Equal("nope", result.Error);
+        Assert.Equal("nope", chained.Match(_ => "", e => e));
     }
 
     [Fact]
     public void Bind_propagates_the_inner_failure_when_the_binder_returns_Fail()
     {
-        var result = Result<int, string>.Ok(5).Bind(_ => Result<string, string>.Fail("inner fail"));
+        Result<int, string> result = new ResultOk<int, string>(5);
+        var chained = result.Bind(_ => new ResultFail<string, string>("inner fail"));
 
-        Assert.Equal("inner fail", result.Error);
+        Assert.Equal("inner fail", chained.Match(_ => "", e => e));
     }
 
     // --- Map ---
@@ -165,23 +151,25 @@ public class ResultTests
     [Fact]
     public void Map_transforms_the_value_on_success()
     {
-        var result = Result<int, string>.Ok(3).Map(v => v * 2);
+        Result<int, string> result = new ResultOk<int, string>(3);
+        var mapped = result.Map(v => v * 2);
 
-        Assert.Equal(6, result.Value);
+        Assert.Equal(6, mapped.Match(v => v, _ => 0));
     }
 
     [Fact]
     public void Map_does_not_call_the_mapper_on_failure()
     {
         var mapperCalled = false;
-        var result = Result<int, string>.Fail("err").Map(v =>
+        Result<int, string> result = new ResultFail<int, string>("err");
+        var mapped = result.Map(v =>
         {
             mapperCalled = true;
             return v * 2;
         });
 
         Assert.False(mapperCalled);
-        Assert.Equal("err", result.Error);
+        Assert.Equal("err", mapped.Match(_ => "", e => e));
     }
 
     // --- Tap ---
@@ -190,17 +178,19 @@ public class ResultTests
     public void Tap_calls_the_action_and_returns_the_original_result_on_success()
     {
         var seen = -1;
-        var result = Result<int, string>.Ok(8).Tap(v => seen = v);
+        Result<int, string> result = new ResultOk<int, string>(8);
+        var tapped = result.Tap(v => seen = v);
 
         Assert.Equal(8, seen);
-        Assert.Equal(8, result.Value);
+        Assert.Equal(8, tapped.Match(v => v, _ => 0));
     }
 
     [Fact]
     public void Tap_does_not_call_the_action_on_failure()
     {
         var called = false;
-        Result<int, string>.Fail("err").Tap(_ => called = true);
+        Result<int, string> result = new ResultFail<int, string>("err");
+        result.Tap(_ => called = true);
 
         Assert.False(called);
     }
@@ -211,17 +201,19 @@ public class ResultTests
     public void TapError_calls_the_action_and_returns_the_original_result_on_failure()
     {
         var seen = "";
-        var result = Result<int, string>.Fail("bad").TapError(e => seen = e);
+        Result<int, string> result = new ResultFail<int, string>("bad");
+        var tapped = result.TapError(e => seen = e);
 
         Assert.Equal("bad", seen);
-        Assert.Equal("bad", result.Error);
+        Assert.Equal("bad", tapped.Match(_ => "", e => e));
     }
 
     [Fact]
     public void TapError_does_not_call_the_action_on_success()
     {
         var called = false;
-        Result<int, string>.Ok(1).TapError(_ => called = true);
+        Result<int, string> result = new ResultOk<int, string>(1);
+        result.TapError(_ => called = true);
 
         Assert.False(called);
     }
@@ -231,23 +223,25 @@ public class ResultTests
     [Fact]
     public void MapError_transforms_the_error_on_failure()
     {
-        var result = Result<int, string>.Fail("oops").MapError(e => e.Length);
+        Result<int, string> result = new ResultFail<int, string>("oops");
+        var mapped = result.MapError(e => e.Length);
 
-        Assert.Equal(4, result.Error);
+        Assert.Equal(4, mapped.Match(_ => 0, e => e));
     }
 
     [Fact]
     public void MapError_does_not_call_the_mapper_on_success()
     {
         var mapperCalled = false;
-        var result = Result<int, string>.Ok(1).MapError(e =>
+        Result<int, string> result = new ResultOk<int, string>(1);
+        var mapped = result.MapError(e =>
         {
             mapperCalled = true;
             return e.Length;
         });
 
         Assert.False(mapperCalled);
-        Assert.Equal(1, result.Value);
+        Assert.Equal(1, mapped.Match(v => v, _ => 0));
     }
 
     // --- Select / SelectMany (LINQ support) ---
@@ -255,41 +249,48 @@ public class ResultTests
     [Fact]
     public void Select_is_equivalent_to_Map()
     {
-        var result = Result<int, string>.Ok(4).Select(v => v + 1);
+        Result<int, string> result = new ResultOk<int, string>(4);
+        var selected = result.Select(v => v + 1);
 
-        Assert.Equal(5, result.Value);
+        Assert.Equal(5, selected.Match(v => v, _ => 0));
     }
 
     [Fact]
     public void SelectMany_sequences_two_successful_results()
     {
+        Result<int, string> aResult = new ResultOk<int, string>(3);
+        Result<int, string> bResult = new ResultOk<int, string>(4);
         var result =
-            from a in Result<int, string>.Ok(3)
+            from a in aResult
-            from b in Result<int, string>.Ok(4)
+            from b in bResult
             select a + b;
 
-        Assert.Equal(7, result.Value);
+        Assert.Equal(7, result.Match(v => v, _ => 0));
     }
 
     [Fact]
     public void SelectMany_short_circuits_when_the_first_result_fails()
     {
+        Result<int, string> aResult = new ResultFail<int, string>("first fail");
+        Result<int, string> bResult = new ResultOk<int, string>(4);
         var result =
-            from a in Result<int, string>.Fail("first fail")
+            from a in aResult
-            from b in Result<int, string>.Ok(4)
+            from b in bResult
             select a + b;
 
-        Assert.Equal("first fail", result.Error);
+        Assert.Equal("first fail", result.Match(_ => "", e => e));
     }
 
     [Fact]
     public void SelectMany_short_circuits_when_the_second_result_fails()
     {
+        Result<int, string> aResult = new ResultOk<int, string>(3);
+        Result<int, string> bResult = new ResultFail<int, string>("second fail");
         var result =
-            from a in Result<int, string>.Ok(3)
+            from a in aResult
-            from b in Result<int, string>.Fail("second fail")
+            from b in bResult
             select a + b;
 
-        Assert.Equal("second fail", result.Error);
+        Assert.Equal("second fail", result.Match(_ => "", e => e));
     }
 }

csharp-fp3.Tests/WithdrawMoneyTests.cs

@@ -2,6 +2,7 @@ using CsharpFp3.Application;
 using CsharpFp3.Domain;
 using CsharpFp3.Infrastructure;
 using CsharpFp3.Library;
+using static CsharpFp3.Library.ResultModule;
 
 namespace CsharpFp3.Tests;
 
@@ -15,7 +16,8 @@ public class WithdrawMoneyTests
     ) BuildHandler()
     {
         var repo = InMemoryAccountRepository.Create();
-        var withdraw = AccountApplication.CreateWithdrawMoney(repo).Value;
+        var result = AccountApplication.CreateWithdrawMoney(repo);
+        var withdraw = result.Match(v => v, _ => throw new InvalidOperationException("Expected success"));
         return (withdraw, repo);
     }
 
@@ -24,11 +26,12 @@ public class WithdrawMoneyTests
     {
         var (withdraw, repo) = BuildHandler();
         var accountId = Guid.NewGuid();
-        repo.SaveAccount(AccountDomain.Open(accountId, new Money(200m)).Value);
+        var accountResult = AccountDomain.Open(accountId, new Money(200m));
+        repo.SaveAccount(accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success")));
 
         withdraw(accountId, 75m);
 
-        var account = repo.LoadAccount(accountId).Value;
+        var account = repo.LoadAccount(accountId).Match(v => v, _ => throw new InvalidOperationException("Expected success"));
         Assert.Equal(125m, account.Balance.Amount);
     }
 
@@ -37,11 +40,12 @@ public class WithdrawMoneyTests
     {
         var (withdraw, repo) = BuildHandler();
         var accountId = Guid.NewGuid();
-        repo.SaveAccount(AccountDomain.Open(accountId, new Money(100m)).Value);
+        var accountResult = AccountDomain.Open(accountId, new Money(100m));
+        repo.SaveAccount(accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success")));
 
         withdraw(accountId, 100m);
 
-        var account = repo.LoadAccount(accountId).Value;
+        var account = repo.LoadAccount(accountId).Match(v => v, _ => throw new InvalidOperationException("Expected success"));
         Assert.Equal(0m, account.Balance.Amount);
     }
 
@@ -52,7 +56,7 @@ public class WithdrawMoneyTests
 
         var result = withdraw(Guid.NewGuid(), 50m);
 
-        Assert.IsType<AccountError.AccountNotFound>(result.Error);
+        Assert.True(result.Match(_ => false, e => e is AccountError.AccountNotFound));
     }
 
     [Fact]
@@ -60,11 +64,12 @@ public class WithdrawMoneyTests
     {
         var (withdraw, repo) = BuildHandler();
         var accountId = Guid.NewGuid();
-        repo.SaveAccount(AccountDomain.Open(accountId, new Money(50m)).Value);
+        var accountResult = AccountDomain.Open(accountId, new Money(50m));
+        repo.SaveAccount(accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success")));
 
         var result = withdraw(accountId, 100m);
 
-        Assert.IsType<AccountError.InsufficientBalance>(result.Error);
+        Assert.True(result.Match(_ => false, e => e is AccountError.InsufficientBalance));
     }
 
     [Fact]
@@ -73,8 +78,9 @@ public class WithdrawMoneyTests
         var (withdraw, _) = BuildHandler();
         var accountId = Guid.NewGuid();
 
-        var error = Assert.IsType<AccountError.AccountNotFound>(
+        var error = withdraw(accountId, 50m).Match(
-            withdraw(accountId, 50m).Error
+            _ => throw new InvalidOperationException("Expected failure"),
+            e => e is AccountError.AccountNotFound notFound ? notFound : throw new InvalidOperationException("Expected AccountNotFound")
         );
 
         Assert.Equal(accountId, error.AccountId);
@@ -85,10 +91,12 @@ public class WithdrawMoneyTests
     {
         var (withdraw, repo) = BuildHandler();
         var accountId = Guid.NewGuid();
-        repo.SaveAccount(AccountDomain.Open(accountId, new Money(50m)).Value);
+        var accountResult = AccountDomain.Open(accountId, new Money(50m));
+        repo.SaveAccount(accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success")));
 
-        var error = Assert.IsType<AccountError.InsufficientBalance>(
+        var error = withdraw(accountId, 120m).Match(
-            withdraw(accountId, 120m).Error
+            _ => throw new InvalidOperationException("Expected failure"),
+            e => e is AccountError.InsufficientBalance ins ? ins : throw new InvalidOperationException("Expected InsufficientBalance")
         );
 
         Assert.Equal(50m, error.Balance.Amount);
@@ -100,11 +108,12 @@ public class WithdrawMoneyTests
     {
         var (withdraw, repo) = BuildHandler();
         var accountId = Guid.NewGuid();
-        repo.SaveAccount(AccountDomain.Open(accountId, new Money(50m)).Value);
+        var accountResult = AccountDomain.Open(accountId, new Money(50m));
+        repo.SaveAccount(accountResult.Match(v => v, _ => throw new InvalidOperationException("Expected success")));
 
         withdraw(accountId, 999m);
 
-        var account = repo.LoadAccount(accountId).Value;
+        var account = repo.LoadAccount(accountId).Match(v => v, _ => throw new InvalidOperationException("Expected success"));
         Assert.Equal(50m, account.Balance.Amount);
     }
 }

csharp-fp3/Application/AccountApplication.cs

@@ -1,6 +1,7 @@
 using CsharpFp3.Domain;
 using CsharpFp3.Infrastructure;
 using CsharpFp3.Library;
+using static CsharpFp3.Library.ResultModule;
 
 namespace CsharpFp3.Application;
 
@@ -16,8 +17,9 @@ public abstract record AppError
 public static class AccountApplication
 {
     public static CreateResultType CreateWithdrawMoney(Repository repo) =>
-        CreateResultType.Ok(
+        Ok(
-            (accountId, amount) =>
+            // types needed in this lambda, otherwise c# doesn't love us
+            (Guid accountId, decimal amount) =>
                 repo.LoadAccount(accountId)
                     .Log("App load", "Account loaded")
                     .Log("App exec wdrwl", $"[App] Executing withdrawal of {amount:0.00}...")

csharp-fp3/Domain/Account.cs

@@ -1,4 +1,5 @@
 using CsharpFp3.Library;
+using static CsharpFp3.Library.ResultModule;
 
 namespace CsharpFp3.Domain;
 
@@ -19,22 +20,19 @@ public static class AccountDomain
 {
     public static Result<Account, AccountError> Open(Guid id, Money openingBalance) =>
         openingBalance.Amount < 0m
-            ? Result<Account, AccountError>.Fail(new AccountError.OpeningBalanceMustBeNonNegative())
+            ? Fail(new AccountError.OpeningBalanceMustBeNonNegative())
-            : Result<Account, AccountError>.Ok(new Account(id, openingBalance));
+            : Ok(new Account(id, openingBalance));
 
     public static Result<Account, AccountError> Withdraw(Account account, Money amount) =>
         (account.Balance.Amount, amount.Amount) switch
         {
-            (_, <= 0m) => Result<Account, AccountError>.Fail(
+            (_, <= 0m) => Fail(new AccountError.AmountMustBePositive()),
-                new AccountError.AmountMustBePositive()
+
+            var (balance, transaction) when balance < transaction => Fail(
+                new AccountError.InsufficientBalance(account.Balance, amount)
             ),
 
-            var (balance, transaction) when balance < transaction => Result<
+            var (balance, transaction) => Ok(
-                Account,
-                AccountError
-            >.Fail(new AccountError.InsufficientBalance(account.Balance, amount)),
-
-            var (balance, transaction) => Result<Account, AccountError>.Ok(
                 account with
                 {
                     Balance = new Money(balance - transaction),

csharp-fp3/Infrastructure/InMemoryAccountRepository.cs

@@ -1,6 +1,7 @@
 using CsharpFp3.Domain;
 using CsharpFp3.Library;
 using static CsharpFp3.Library.Logging;
+using static CsharpFp3.Library.ResultModule;
 
 namespace CsharpFp3.Infrastructure;
 
@@ -17,13 +18,10 @@ public static class InMemoryAccountRepository
 
         Result<Account, AccountError> GetById(Guid id) =>
             store.TryGetValue(id, out var account)
-                ? LogReturn(
+                ? LogReturn($"[Repo] Loaded account {id}", Ok(account))
-                    $"[Repo] Loaded account {id}",
-                    Result<Account, AccountError>.Ok(account)
-                )
                 : LogReturn(
                     $"[Repo] Account {id} not found",
-                    Result<Account, AccountError>.Fail(new AccountError.AccountNotFound(id))
+                    Fail(new AccountError.AccountNotFound(id))
                 );
 
         Result<Account, AccountError> Save(Account account)
@@ -33,7 +31,7 @@ public static class InMemoryAccountRepository
             // no failure modes here
             return LogReturn(
                 $"[Repo] Saved account {account.Id} with balance {account.Balance.Amount:0.00}",
-                Result<Account, AccountError>.Ok(account)
+                Ok(account)
             );
         }
 

csharp-fp3/Library/Logging.cs

@@ -8,14 +8,38 @@ public static class Logging
         return r;
     }
 
-    public static Action<T> Output<T>(string src, string message) =>
+    static string Indent(string text, int indent = 4)
+    {
+        var prefix = new string(' ', indent);
+
+        return Environment.NewLine
+            + string.Join(
+                Environment.NewLine,
+                text.Split(Environment.NewLine).Select(line => prefix + line)
+            );
+    }
+
+    public static string Format(Object? o) =>
+        Indent(ObjectDumper.Dump(o, new DumpOptions() { DumpStyle = DumpStyle.CSharp }));
+
+    public static void Output(string src, string message, Object? x)
+    {
+        Console.WriteLine($"[{src}] {message} | {Format(x)}");
+    }
+
+    public static void Output(string src, string message)
+    {
+        Console.WriteLine($"[{src}] {message}");
+    }
+
+    public static Action<T> OutputDelegate<T>(string src, string message) =>
         x =>
         {
-            Console.WriteLine($"[{src}] {message} | {x}");
+            Output(src, message, x);
         };
 
     public static void OutputError<T>(string src, T error)
     {
-        Console.Error.WriteLine($"\e[1;31m[{src} ERROR]\e[0m {error}");
+        Console.Error.WriteLine($"\e[1;31m[{src} ERROR]\e[0m {Format(error)}");
     }
 }

csharp-fp3/Library/Result.cs

@@ -1,126 +1,137 @@
 namespace CsharpFp3.Library;
 
-public readonly record struct Result<T, E>
+// disable match exhaustion warnings - C# thinks we could have other
+// derived instances of Result<T,E> but there's no way to say
+// "don't allow creation of further derived types" while leaving
+// the type publicly visible.
+#pragma warning disable CS8509
+
+public abstract record Result<T, E>
 {
-    private readonly T _value;
+    public static implicit operator Result<T, E>(T value) => new ResultOk<T, E>(value);
 
-    private readonly E _error;
+    public static implicit operator Result<T, E>(E error) => new ResultFail<T, E>(error);
+}
 
-    public bool IsSuccess { get; }
+public sealed record ResultOk<T, E>(T Value) : Result<T, E>;
 
-    public bool IsFailure => !IsSuccess;
+public sealed record ResultFail<T, E>(E Error) : Result<T, E>;
 
-    public T Value =>
+public static class ResultModule
-        IsSuccess
+{
-            ? _value
+    public static T Ok<T>(T v) => v;
-            : throw new InvalidOperationException("No value present for failed result.");
 
-    public E Error =>
+    public static Result<T?, E> OkNone<T, E>()
-        IsFailure
+        where T : class? => new ResultOk<T?, E>(null);
-            ? _error
-            : throw new InvalidOperationException("No error present for successful result.");
 
-    private Result(T value)
+    public static E Fail<E>(E e) => e;
-    {
-        _value = value;
-        _error = default!;
-        IsSuccess = true;
-    }
-
-    private Result(E error)
-    {
-        _value = default!;
-        _error = error;
-        IsSuccess = false;
-    }
-
-    public static Result<T, E> Ok(T value) => new(value);
-
-    public static Result<T, E> Fail(E error) => new(error);
-
-    public TResult Match<TResult>(Func<T, TResult> onSuccess, Func<E, TResult> onFailure) =>
-        IsSuccess ? onSuccess(_value) : onFailure(_error);
-
-    public void Switch(Action<T> onSuccess, Action<E> onFailure)
-    {
-        if (IsSuccess)
-            onSuccess(_value);
-        else
-            onFailure(_error);
-    }
-
-    public static Result<T, E> Catch(Func<T> f, Func<Exception, E> exceptionMapper)
-    {
-        try
-        {
-            var result = f();
-            return Result<T, E>.Ok(result);
-        }
-        catch (Exception e)
-        {
-            return Result<T, E>.Fail(exceptionMapper(e));
-        }
-    }
 }
 
 public static class ResultExtensions
 {
-    public static Result<TOut, E> Bind<TIn, TOut, E>(
+    extension<TIn, E>(Result<TIn, E> result)
-        this Result<TIn, E> result,
-        Func<TIn, Result<TOut, E>> binder
-    ) => result.IsSuccess ? binder(result.Value) : Result<TOut, E>.Fail(result.Error);
-
-    public static Result<TOut, E> Map<TIn, TOut, E>(
-        this Result<TIn, E> result,
-        Func<TIn, TOut> mapper
-    ) =>
-        result.IsSuccess
-            ? Result<TOut, E>.Ok(mapper(result.Value))
-            : Result<TOut, E>.Fail(result.Error);
-
-    public static Result<T, E> Tap<T, E>(this Result<T, E> result, Action<T> action)
     {
-        if (result.IsSuccess)
+        public bool IsSuccess => result is ResultOk<TIn, E>;
-            action(result.Value);
+        public bool IsFailure => !result.IsSuccess;
 
+        public Result<TOut, E> Bind<TOut>(Func<TIn, Result<TOut, E>> binder) =>
+            result switch
+            {
+                ResultOk<TIn, E>(var v) => binder(v),
+                ResultFail<TIn, E>(var e) => new ResultFail<TOut, E>(e),
+            };
+
+        public Result<TOut, E> Map<TOut>(Func<TIn, TOut> mapper) =>
+            result switch
+            {
+                ResultOk<TIn, E>(var v) => new ResultOk<TOut, E>(mapper(v)),
+                ResultFail<TIn, E>(var e) => new ResultFail<TOut, E>(e),
+            };
+
+        public Result<TIn, EOut> MapError<EOut>(Func<E, EOut> mapper) =>
+            result switch
+            {
+                ResultOk<TIn, E>(var v) => new ResultOk<TIn, EOut>(v),
+                ResultFail<TIn, E>(var e) => new ResultFail<TIn, EOut>(mapper(e)),
+            };
+
+        public Result<TIn, E> Tap(Action<TIn> action)
+        {
+            if (result is ResultOk<TIn, E>(var v))
+                action(v);
             return result;
         }
 
-    public static Result<T, E> TapError<T, E>(this Result<T, E> result, Action<E> action)
+        public Result<TIn, E> TapError(Action<E> action)
         {
-        if (result.IsFailure)
+            if (result is ResultFail<TIn, E>(var e))
-            action(result.Error);
+                action(e);
-
             return result;
         }
 
-    public static Result<T, EOut> MapError<T, EIn, EOut>(
+        public Result<TIn, E> Log(string src, string msg) =>
-        this Result<T, EIn> result,
+            Tap(result, Logging.OutputDelegate<TIn>(src, msg))
-        Func<EIn, EOut> map
-    ) =>
-        result.IsSuccess
-            ? Result<T, EOut>.Ok(result.Value)
-            : Result<T, EOut>.Fail(map(result.Error));
-
-    public static Result<T, E> Log<T, E>(this Result<T, E> result, string src, string msg) =>
-        Tap(result, Logging.Output<T>(src, msg)).TapError(m => Logging.OutputError(src, m));
-
-    public static Result<T, E> Log<T, E>(
-        this Result<T, E> result,
-        string src,
-        Func<T, string> renderText
-    ) =>
-        Tap(result, x => Logging.Output<T>(src, renderText(x))(x))
                 .TapError(m => Logging.OutputError(src, m));
 
-    public static Result<TOut, E> Select<TIn, TOut, E>(
+        public Result<TIn, E> Log(string src, Func<TIn, string> renderText) =>
-        this Result<TIn, E> result,
+            Tap(result, x => Logging.OutputDelegate<TIn>(src, renderText(x))(x))
-        Func<TIn, TOut> mapper
+                .TapError(m => Logging.OutputError(src, m));
-    ) => result.Map(mapper);
 
-    public static Result<TOut, E> SelectMany<TIn, TIntermediate, TOut, E>(
+        public Result<TOut, E> Select<TOut>(Func<TIn, TOut> mapper) => Map(result, mapper);
-        this Result<TIn, E> result,
+
+        public Result<TOut, E> SelectMany<TIntermediate, TOut>(
             Func<TIn, Result<TIntermediate, E>> binder,
             Func<TIn, TIntermediate, TOut> projector
-    ) => result.Bind(x => binder(x).Map(y => projector(x, y)));
+        ) => Bind(result, x => binder(x).Map(y => projector(x, y)));
+
+        public TResult Match<TResult>(Func<TIn, TResult> onSuccess, Func<E, TResult> onFailure) =>
+            result switch
+            {
+                ResultOk<TIn, E>(var v) => onSuccess(v),
+                ResultFail<TIn, E>(var e) => onFailure(e),
+            };
+
+        public void Switch(Action<TIn> onSuccess, Action<E> onFailure)
+        {
+            switch (result)
+            {
+                case ResultOk<TIn, E>(var v):
+                    onSuccess(v);
+                    break;
+                case ResultFail<TIn, E>(var e):
+                    onFailure(e);
+                    break;
+            }
+        }
+    }
+
+    extension<TIn, E>(Result<TIn, E>)
+    {
+        public static Result<TIn, E> Catch(Func<TIn> f, Func<Exception, E> exceptionMapper)
+        {
+            try
+            {
+                return new ResultOk<TIn, E>(f());
+            }
+            catch (Exception e)
+            {
+                return new ResultFail<TIn, E>(exceptionMapper(e));
+            }
+        }
+
+        public static Result<TIn, E> Catch(
+            Func<Result<TIn, E>> f,
+            Func<Exception, E> exceptionMapper
+        )
+        {
+            try
+            {
+                return f();
+            }
+            catch (Exception e)
+            {
+                return new ResultFail<TIn, E>(exceptionMapper(e));
+            }
+        }
+    }
 }

csharp-fp3/Program.cs

@@ -31,9 +31,7 @@ Result<Repository, AppError>
                         "csharp-fp3 exec",
                         $"Executing withdrawal {withdrawalAmount:0.00} from account {accountId}"
                     )
-                    .Bind<Account, Account, AccountError>(account =>
+                    .Bind(account => withdrawMoney(account.Id, withdrawalAmount))
-                        withdrawMoney(account.Id, withdrawalAmount)
-                    )
                     .Log("csharp-fp3 new balance", account => $"New balance is {account.Balance}")
                     .MapError(ae => (AppError)new AppError.InnerAccountError(ae))
             )

csharp-fp3/csharp-fp3.csproj

@@ -6,4 +6,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
   </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="ObjectDumper.NET" Version="4.3.2" />
+  </ItemGroup>
 </Project>