Add "map squared" pipe operators (#586)

Author: gusty

src/FSharpPlus/Operators.fs

@@ -143,6 +143,19 @@ module Operators =
     /// <category index="1">Functor</category>
     let inline (|>>) (x: '``Functor<'T>``) (f: 'T->'U) : '``Functor<'U>`` = Map.Invoke f x
 
+    /// <summary>
+    /// Lifts a function into two Functors.
+    /// To be used in pipe-forward style expressions
+    /// </summary>
+    /// <category index="1">Functor</category>
+    let inline (|>>>) (x: '``Functor1<Functor2<'T>>``) (f: 'T -> 'U) : '``Functor1<Functor2<'U>>`` = (Map.Invoke >> Map.Invoke) f x
+
+    /// <summary>
+    /// Lifts a function into two Functors.
+    /// </summary>
+    /// <category index="1">Functor</category>
+    let inline (<<<|) (f: 'T -> 'U) (x: '``Functor1<Functor2<'T>>``) : '``Functor1<Functor2<'U>`` = (Map.Invoke >> Map.Invoke) f x
+
     /// <summary>
     /// Like map but ignoring the results.
     /// </summary>

tests/FSharpPlus.Tests/General.fs

@@ -396,6 +396,13 @@ module Functor =
         Assert.IsInstanceOf<Option<Async<int>>> (Some testVal10)
         areEqual 2 (testVal10 |> Async.RunSynchronously)
 
+    [<Test>]
+    let mapSquared () =
+        let x =
+            [Some 1; Some 2]
+            |>>> string
+        Assert.AreEqual ([Some "1"; Some "2"], x)    
+    
     [<Test>]
     let unzip () = 
         let testVal = unzip {Head = (1, 'a'); Tail = [(2, 'b');(3, 'b')]}