Async.bindChoice and Choice helpers

Author: eulerfx

src/FSharpx.Async/Async.fs

@@ -70,4 +70,26 @@ module AsyncExtensions =
       /// Creates an async computation which binds the result of the specified 
       /// async computation to the specified function. The computation produced 
       /// by the specified function is returned.
-      static member inline bind f a = async.Bind(a, f)
+      static member inline bind f a = async.Bind(a, f)
+
+      /// Maps over an async computation which produces a choice value
+      /// using a function which maps over Choice1Of2 and itself returns a choice. 
+      /// A value of Choice2Of2 is treated like an error and passed through.
+      static member mapChoice (f:'a -> Choice<'b, 'e>) (a:Async<Choice<'a, 'e>>) : Async<Choice<'b, 'e>> = 
+        a |> Async.map (function 
+          | Choice1Of2 a' -> f a'
+          | Choice2Of2 e -> Choice2Of2 e)
+
+      /// Binds an async computation producing a choice value to another async
+      /// computation producing a choice such that a Choice2Of2 value is passed through.
+      static member bindChoice (f:'a -> Async<Choice<'b, 'e>>) (a:Async<Choice<'a, 'e>>) : Async<Choice<'b, 'e>> = 
+        a |> Async.bind (function
+          | Choice1Of2 a' -> f a'
+          | Choice2Of2 e ->  Choice2Of2 e |> async.Return)
+
+      /// Binds an async computation producing a choice value to another async
+      /// computation producing a choice such that a Choice2Of2 value is passed through.
+      static member bindChoices (f:'a -> Async<Choice<'b, 'e2>>) (a:Async<Choice<'a, 'e1>>) : Async<Choice<'b, Choice<'e1, 'e2>>> = 
+        a |> Async.bind (function
+          | Choice1Of2 a' -> f a' |> Async.map (function Choice1Of2 b -> Choice1Of2 b | Choice2Of2 e2 -> Choice2Of2 (Choice2Of2 e2))
+          | Choice2Of2 e1 -> Choice2Of2 (Choice1Of2 e1) |> async.Return)

src/FSharpx.Async/Utils.fs

@@ -1,3 +1,15 @@
 module FSharpx.Control.Utils
 
-let inline konst a _ = a
+let inline konst a _ = a
+
+module internal Choice =
+  
+  /// Maps over the left result type.
+  let mapl (f:'a -> 'b) = function
+    | Choice1Of2 a -> f a |> Choice1Of2
+    | Choice2Of2 e -> Choice2Of2 e
+
+  /// Maps over the right result type.
+  let mapr (f:'b -> 'c) = function
+    | Choice1Of2 a -> Choice1Of2 a
+    | Choice2Of2 e -> f e |> Choice2Of2