Add Done/Loop support directly to Monad

Author: louthy

LanguageExt.Core/Traits/Monads/Monad/Monad.Module.cs

@@ -33,6 +33,22 @@ public static MB bind<M, MB, A, B>(K<M, A> ma, Func<A, MB> f)
         where M : Monad<M> =>
         (MB)bind(ma, x => f(x));
 
+    /// <summary>
+    /// Lift a Next.Done value into the monad
+    /// </summary>
+    [Pure]
+    public static K<M, Next<A, B>> done<M, A, B>(B value) 
+        where M : Monad<M> =>
+        M.Pure(Next.Done<A, B>(value));
+    
+    /// <summary>
+    /// Lift a Next.Done value into the monad
+    /// </summary>
+    [Pure]
+    public static K<M, Next<A, B>> loop<M, A, B>(A value) 
+        where M : Monad<M> =>
+        M.Pure(Next.Loop<A, B>(value));
+    
     /// <summary>
     /// Allow for tail-recursion by using a trampoline function that returns a monad with the bound value
     /// wrapped by `Next`, which enables decision-making about whether to keep the computation going or not.  

LanguageExt.Core/Traits/Monads/Monad/Monad.Trait.cs

@@ -35,6 +35,18 @@ public interface Monad<M> :
     //
     //  Default implementations
     //
+    
+    /// <summary>
+    /// Lift a Next.Done value into the monad
+    /// </summary>
+    public static virtual K<M, Next<A, B>> Done<A, B>(B value) =>
+        M.Pure(Next.Done<A, B>(value));
+    
+    /// <summary>
+    /// Lift a Next.Done value into the monad
+    /// </summary>
+    public static virtual K<M, Next<A, B>> Loop<A, B>(A value) =>
+        M.Pure(Next.Loop<A, B>(value));
 
     public static virtual K<M, C> SelectMany<A, B, C>(K<M, A> ma, Func<A, K<M, B>> bind, Func<A, B, C> project) =>
         ma.Bind(x => bind(x).Map(y => project(x, y)));

Samples/TestBed/RecurTests.cs

@@ -30,7 +30,7 @@ public static Seq<int> insertSort(int value, Seq<int> sorted) =>
     public static void recurIsSameAsBind<M>(Func<K<M, int>, K<M, int>, bool>? equals = null)
         where M : Monad<M>
     {
-        var example = toSeq(Range(1, 40000)).Strict();
+        var example = toSeq(Range(1, 20000)).Strict();
 
         Console.ForegroundColor = ConsoleColor.Yellow;
         Console.WriteLine("------------------------------------------------------------------------------------------");
@@ -72,15 +72,24 @@ public static void recurIsSameAsBind<M>(Func<K<M, int>, K<M, int>, bool>? equals
         K<M, Next<(int Total, Seq<int> Values), int>> sumTail((int Total, Seq<int> Values) pair) =>
             pair.Values switch
             {
-                []          => M.Pure(Next.Done<(int, Seq<int>), int>(pair.Total)),
-                var (x, xs) => M.Pure(Next.Loop<(int, Seq<int>), int>((pair.Total + x, xs))) 
+                []          => M.Done<(int, Seq<int>), int>(pair.Total),
+                var (x, xs) => M.Loop<(int, Seq<int>), int>((pair.Total + x, xs)) 
             };
 
+        
+        K<M, Next<(int Total, Seq<int> Values), int>> sumTail2((int Total, Seq<int> Values) pair) =>
+            pair.Values switch
+            {
+                []          => M.Done<(int, Seq<int>), int>(pair.Total),
+                var (x, xs) => M.Loop<(int, Seq<int>), int>((pair.Total + x, xs)) 
+            };
+        
         K<M, int> sumNoTail(Seq<int> values) =>
             values switch
             {
-                [var x]     => M.Pure(x),
-                var (x, xs) => sumNoTail(xs) * (t => x + t)
+                []          => M.Pure(0),
+                var (x, xs) => from t in sumNoTail(xs)
+                               select t + x
             };
     }
 }