Merge pull request #333 from daniel-chambers/fix-loops-in-maybe

Bugfix for loops in the maybe computation expression

src/FSharpx.Extras/ComputationExpressions/Monad.fs

@@ -116,8 +116,9 @@ module Option =
             this.TryFinally(body res, fun () -> match res with null -> () | disp -> disp.Dispose())
 
         member this.While(guard, f) =
-            if not (guard()) then this.Zero() else
-            this.Bind(f(), fun _ -> this.While(guard, f))
+            if not (guard()) then Some () else
+            do f() |> ignore
+            this.While(guard, f)
 
         member this.For(sequence:seq<_>, body) =
             this.Using(sequence.GetEnumerator(),

tests/FSharpx.Tests/MaybeTest.fs

@@ -80,3 +80,27 @@ let ``monadplus laws``() =
         fun a b -> mplus (ret a) b = ret a
 //    fsCheck "left distribution" <|
 //        fun a b f -> (mplus a b >>= f) = (mplus (a >>= f) (b >>= f))
+
+[<Test>]
+let ``for loops enumerate entire sequence and subsequent expressions also run``() =
+    let count = ref 0
+    let result = maybe {
+        for i in [1;2;3] do
+            incr count
+        return true
+    }
+
+    !count |> should equal 3
+    result |> should equal (Some true)
+
+[<Test>]
+let ``while loops execute until guard is false and subsequent expressions also run``() =
+    let count = ref 0
+    let result = maybe {
+        while !count < 3 do
+            incr count
+        return true
+    }
+
+    !count |> should equal 3
+    result |> should equal (Some true)