Use a ValueTuple instead of StateObject.

src/Narvalo.Common/Collections/Dictionary$.cs

@@ -27,8 +27,7 @@ public static partial class DictionaryExtensions
 
             if (key == null) { return Maybe<TValue>.None; }
 
-            TValue value;
-            return @this.TryGetValue(key, out value) ? Maybe.Of(value) : Maybe<TValue>.None;
+            return @this.TryGetValue(key, out TValue value) ? Maybe.Of(value) : Maybe<TValue>.None;
         }
     }
 }

src/Narvalo.Common/Internal/TryParser$.cs

@@ -12,8 +12,7 @@ internal static class TryParserExtensions
 
             if (value == null) { return null; }
 
-            T result;
-            return @this.Invoke(value, out result) ? result : (T?)null;
+            return @this.Invoke(value, out T result) ? result : (T?)null;
         }
 
         public static Maybe<T> MayInvoke<T>(this TryParser<T> @this, string value) where T : class
@@ -22,8 +21,7 @@ internal static class TryParserExtensions
 
             if (value == null) { return Maybe<T>.None; }
 
-            T result;
-            return @this.Invoke(value, out result) ? Maybe.Of(result) : Maybe<T>.None;
+            return @this.Invoke(value, out T result) ? Maybe.Of(result) : Maybe<T>.None;
         }
     }
 }

src/Narvalo.Finance/Bic.cs

@@ -181,7 +181,7 @@ public static string FromBic(string value)
                 }
                 else
                 {
-                    var retval = value.Substring(StartIndex, Length);
+                    string retval = value.Substring(StartIndex, Length);
                     return CheckContent(retval) ? retval : null;
                 }
             }
@@ -203,7 +203,7 @@ private static class CountryPart
             public static string FromBic(string value)
             {
                 Demand.True(value.Length >= StartIndex + Length);
-                var retval = value.Substring(StartIndex, Length);
+                string retval = value.Substring(StartIndex, Length);
                 return CheckContent(retval) ? retval : null;
             }
 
@@ -223,7 +223,7 @@ private static class InstitutionPart
 
             public static string FromBic(string value, BicVersion version)
             {
-                var retval = value.Substring(StartIndex, Length);
+                string retval = value.Substring(StartIndex, Length);
 
                 return CheckContent(retval, version) ? retval : null;
             }
@@ -247,7 +247,7 @@ private static class LocationPart
             public static string FromBic(string value)
             {
                 Demand.True(value.Length >= StartIndex + Length);
-                var retval = value.Substring(StartIndex, Length);
+                string retval = value.Substring(StartIndex, Length);
                 return CheckContent(retval) ? retval : null;
             }
 

src/Narvalo.Futures/Applicative/StateObject`.cs

@@ -6,13 +6,15 @@ namespace Narvalo.Applicative
     using System.Collections;
     using System.Collections.Generic;
 
+    [Obsolete]
     public static class StateObject
     {
         public static StateObject<T, TState> Create<T, TState>(T value, TState state)
             => new StateObject<T, TState>(value, state);
     }
 
     // To be replaced by ValueTuple<T, TState> when available.
+    [Obsolete]
     public partial struct StateObject<T, TState> : IEquatable<StateObject<T, TState>>, IStructuralEquatable
     {
         public StateObject(T result, TState state)

src/Narvalo.Futures/Applicative/Stateful.cs

@@ -4,8 +4,8 @@ namespace Narvalo.Applicative
 {
     using System;
 
-    // Func<TState, StateObject<T, TState>>
-    public delegate StateObject<T, TState> Stateful<T, TState>(TState state);
+    // Func<TState, (T, TState)>
+    public delegate (T result, TState state) Stateful<T, TState>(TState state);
 
     // Provides the core Monad methods.
     public static partial class Stateful
@@ -15,14 +15,14 @@ public static partial class Stateful
             Func<T, Stateful<TNext, TState>> binder)
             => state =>
             {
-                StateObject<T, TState> obj = @this(state);
+                var obj = @this(state);
 
-                return binder(obj.Result).Invoke(obj.State);
+                return binder(obj.result).Invoke(obj.state);
             };
 
         // Initialize a stateful computation from a given value.
         public static Stateful<T, TState> Of<T, TState>(T value)
-            => state => StateObject.Create(value, state);
+            => state => (value, state);
 
         public static Stateful<T, TState> Flatten<T, TState>(Stateful<Stateful<T, TState>, TState> square)
             => square.Bind(Stubs<Stateful<T, TState>>.Identity);
@@ -32,15 +32,15 @@ public static partial class Stateful
     public static partial class Stateful
     {
         public static Stateful<TState, TState> Get<TState>()
-            => state => StateObject.Create(state, state);
+            => state => (state, state);
 
         public static Stateful<Unit, TState> Put<TState>(TState newState)
-            => state => StateObject.Create(Unit.Default, newState);
+            => state => (Unit.Default, newState);
 
         public static Stateful<Unit, TState> Modify<TState>(Func<TState, TState> func)
-            => state => StateObject.Create(Unit.Default, func(state));
+            => state => (Unit.Default, func(state));
 
         public static Stateful<TResult, TState> Gets<TState, TResult>(Func<TState, TResult> func)
-            => state => StateObject.Create(func(state), state);
+            => state => (func(state), state);
     }
 }

src/Narvalo.Futures/Applicative/Stateful.g.cs

@@ -6,7 +6,7 @@
 // behavior and will be lost if the code is regenerated.
 //
 // Runtime Version: 4.0.30319.42000
-// Microsoft.VisualStudio.TextTemplating: 14.0
+// Microsoft.VisualStudio.TextTemplating: 15.0
 // </auto-generated>
 //------------------------------------------------------------------------------
 

src/Narvalo.Futures/Narvalo.Futures.csproj

@@ -11,6 +11,9 @@
     <Reference Include="System" />
     <Reference Include="System.ComponentModel.Composition" />
     <Reference Include="System.Configuration" />
+    <Reference Include="System.ValueTuple, Version=4.0.1.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\System.ValueTuple.4.3.0\lib\netstandard1.0\System.ValueTuple.dll</HintPath>
+    </Reference>
     <Reference Include="System.Xml" />
     <Reference Include="System.Xml.Linq" />
   </ItemGroup>
@@ -106,6 +109,7 @@
   </ItemGroup>
   <ItemGroup>
     <None Include="Narvalo.Futures.Version.props" />
+    <None Include="packages.config" />
   </ItemGroup>
   <ItemGroup>
     <Service Include="{508349B6-6B84-4DF5-91F0-309BEEBAD82D}" />

src/Narvalo.Futures/packages.config

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<packages>
+  <package id="System.ValueTuple" version="4.3.0" targetFramework="net461" />
+</packages>

src/Narvalo.Fx/Applicative/Either`2.cs

@@ -52,16 +52,13 @@ public abstract partial class Either<TLeft, TRight> : IStructuralEquatable, Inte
         [DebuggerTypeProxy(typeof(Either<,>.Left_.DebugView))]
         private sealed partial class Left_ : Either<TLeft, TRight>, IEquatable<Left_>
         {
-            public Left_(TLeft value)
-            {
-                Left = value;
-            }
+            public Left_(TLeft value) => Left = value;
 
             public override bool IsLeft => true;
 
             internal override TLeft Left { get; }
 
-            internal override TRight Right { get { throw new InvalidOperationException("XXX"); } }
+            internal override TRight Right => throw new InvalidOperationException("XXX");
 
             public override Maybe<TLeft> LeftOrNone() => Maybe.Of(Left);
 
@@ -97,10 +94,7 @@ private sealed class DebugView
             {
                 private readonly Either<TLeft, TRight> _inner;
 
-                public DebugView(Either<TLeft, TRight> inner)
-                {
-                    _inner = inner;
-                }
+                public DebugView(Either<TLeft, TRight> inner) => _inner = inner;
 
                 public TLeft Left => _inner.Left;
             }
@@ -112,22 +106,19 @@ public DebugView(Either<TLeft, TRight> inner)
         [DebuggerTypeProxy(typeof(Either<,>.Right_.DebugView))]
         private sealed partial class Right_ : Either<TLeft, TRight>, IEquatable<Right_>
         {
-            public Right_(TRight value)
-            {
-                Right = value;
-            }
+            public Right_(TRight value) => Right = value;
 
             public override bool IsLeft => false;
 
-            internal override TLeft Left { get { throw new InvalidOperationException("XXX"); } }
+            internal override TLeft Left => throw new InvalidOperationException("XXX");
 
             internal override TRight Right { get; }
 
             public override Maybe<TLeft> LeftOrNone() => Maybe<TLeft>.None;
 
             public override Maybe<TRight> RightOrNone() => Maybe.Of(Right);
 
-            public override Either<TRight, TLeft> Swap() { throw new InvalidOperationException("XXX"); }
+            public override Either<TRight, TLeft> Swap() => throw new InvalidOperationException("XXX");
 
             public override Either<TRight, TLeft> SwapUnchecked() => Either<TRight, TLeft>.OfLeft(Right);
 
@@ -157,10 +148,7 @@ private sealed class DebugView
             {
                 private readonly Either<TLeft, TRight> _inner;
 
-                public DebugView(Either<TLeft, TRight> inner)
-                {
-                    _inner = inner;
-                }
+                public DebugView(Either<TLeft, TRight> inner) => _inner = inner;
 
                 public TRight Right => _inner.Right;
             }
@@ -188,12 +176,12 @@ private partial class Left_
         {
             public override TLeft ToLeft() => Left;
 
-            public override TRight ToRight() { throw new InvalidCastException("XXX"); }
+            public override TRight ToRight() => throw new InvalidCastException("XXX");
         }
 
         private partial class Right_
         {
-            public override TLeft ToLeft() { throw new InvalidCastException("XXX"); }
+            public override TLeft ToLeft() => throw new InvalidCastException("XXX");
 
             public override TRight ToRight() => Right;
         }

src/Narvalo.Fx/Applicative/Fallible.cs

@@ -65,10 +65,7 @@ private sealed class DebugView
         {
             private readonly Fallible _inner;
 
-            public DebugView(Fallible inner)
-            {
-                _inner = inner;
-            }
+            public DebugView(Fallible inner) => _inner = inner;
 
             public bool IsSuccess => _inner.IsSuccess;
 

src/Narvalo.Fx/Applicative/Maybe`1.cs

@@ -115,10 +115,7 @@ private sealed class DebugView
         {
             private readonly Maybe<T> _inner;
 
-            public DebugView(Maybe<T> inner)
-            {
-                _inner = inner;
-            }
+            public DebugView(Maybe<T> inner) => _inner = inner;
 
             public bool IsSome => _inner.IsSome;
 

src/Narvalo.Fx/Applicative/Outcome.cs

@@ -49,10 +49,7 @@ private sealed class DebugView
         {
             private readonly Outcome _inner;
 
-            public DebugView(Outcome inner)
-            {
-                _inner = inner;
-            }
+            public DebugView(Outcome inner) => _inner = inner;
 
             public bool IsSuccess => _inner.IsSuccess;
 
@@ -146,8 +143,7 @@ public bool Equals(Outcome other)
             return other.IsError && Error == other.Error;
         }
 
-        public override bool Equals(object obj)
-            => (obj is Outcome) && Equals((Outcome)obj);
+        public override bool Equals(object obj) => (obj is Outcome) && Equals((Outcome)obj);
 
         public override int GetHashCode() => _error?.GetHashCode() ?? 0;
     }

src/Narvalo.Fx/Applicative/Outcome`1.cs

@@ -116,10 +116,7 @@ private sealed class DebugView
         {
             private readonly Outcome<T> _inner;
 
-            public DebugView(Outcome<T> inner)
-            {
-                _inner = inner;
-            }
+            public DebugView(Outcome<T> inner) => _inner = inner;
 
             public bool IsSuccess => _inner.IsSuccess;
 
@@ -262,14 +259,10 @@ public void OnError(Action<string> action)
         #region Publicly hidden methods.
 
         bool Internal.ISecondaryContainer<string>.Contains(string value)
-        {
-            throw new NotSupportedException();
-        }
+            => throw new NotSupportedException();
 
         bool Internal.ISecondaryContainer<string>.Contains(string value, IEqualityComparer<string> comparer)
-        {
-            throw new NotSupportedException();
-        }
+            => throw new NotSupportedException();
 
         // Alias for WhenSuccess().
         void Internal.IContainer<T>.When(Func<T, bool> predicate, Action<T> action)

src/Narvalo.Fx/Applicative/Result`2.cs

@@ -113,10 +113,7 @@ private sealed class DebugView
         {
             private readonly Result<T, TError> _inner;
 
-            public DebugView(Result<T, TError> inner)
-            {
-                _inner = inner;
-            }
+            public DebugView(Result<T, TError> inner) => _inner = inner;
 
             public bool IsSuccess => _inner.IsSuccess;
 
@@ -278,14 +275,10 @@ public void OnError(Action<TError> action)
         #region Publicly hidden methods.
 
         bool Internal.ISecondaryContainer<TError>.Contains(TError value)
-        {
-            throw new NotSupportedException();
-        }
+            => throw new NotSupportedException();
 
         bool Internal.ISecondaryContainer<TError>.Contains(TError value, IEqualityComparer<TError> comparer)
-        {
-            throw new NotSupportedException();
-        }
+            => throw new NotSupportedException();
 
         // Alias for WhenSuccess().
         void Internal.IContainer<T>.When(Func<T, bool> predicate, Action<T> action)