Fix Free.map3

src/FSharpPlus/Data/Free.fs

@@ -62,11 +62,11 @@ module Free =
         loop y x
 
     let inline map3 (f: 'T->'U->'V->'W) (x: Free<'``Functor<'T>``,'T>) (y: Free<'``Functor<'U>``,'U>) (z: Free<'``Functor<'V>``,'V>) : Free<'``Functor<'W>``,'W> =
-        let rec loop (y: Free<_,_>) (x: Free<_,_>) (z: Free<_,_>) =
+        let rec loop (y: Free<_,_>) (z: Free<_,_>) (x: Free<_,_>) =
             match run x with
-            | Pure x -> map2<'U,'V,'W,'``Functor<'U>``,'``Functor<Free<'Functor<'U>,'U>>``,'``Functor<Free<'Functor<'V>,'V>>``,'``Functor<Free<'Functor<'W>,'W>>``,'``Functor<'V>``,'``Functor<'W>``> (f x) y z : Free<'``Functor<'W>``,'W>
-            | Roll (x: ^``Functor<Free<'Functor<'T>,'T>>``) -> Roll (Map.Invoke (loop y: Free<'``Functor<'T>``,'T> -> _) x: '``Functor<Free<'Functor<'W>,'W>>``)
-        loop y x z
+            | Pure x -> map2<'U,'V,'W,'``Functor<'U>``,'``Functor<Free<'Functor<'U>,'U>>``,'``Functor<Free<'Functor<'W>,'W>>``,'``Functor<Free<'Functor<'V>,'V>>``,'``Functor<'V>``,'``Functor<'W>``> (f x) y z : Free<'``Functor<'W>``,'W>
+            | Roll (x: ^``Functor<Free<'Functor<'T>,'T>>``) -> Roll (Map.Invoke (loop y z: Free<'``Functor<'T>``,'T> -> _) x: '``Functor<Free<'Functor<'W>,'W>>``)
+        loop y z x
 
     /// Folds the Free structure into a Monad
     let inline fold (f: '``Functor<'T>`` -> '``Monad<'T>``) (x: Free<'``Functor<'U>``,'U>) : '``Monad<'U>`` =
@@ -105,4 +105,4 @@ type Free<'``functor<'t>``,'t> with
 
     static member        Delay (x: unit -> Free<'``Functor<'T>``,'T>) = x ()
 
-#endif
+#endif

tests/FSharpPlus.Tests/Free.fs

@@ -316,3 +316,39 @@ module Fold =
             |> Identity.run
 
         areStEqual (Ok { Id = FooId "1"; Name = "test" }) response
+
+module Lift3 =
+
+    type Instruction<'next> =
+        | Read of int * (string -> 'next)
+        static member Map(i, f) =
+            match i with
+            | Read (x, next) -> Read(x, next >> f)
+
+    let read x = Read(x, id) |> Free.liftF
+
+    type ApplicativeBuilder<'a>() =
+        inherit MonadFxStrictBuilder<'a>()
+
+        member inline _.BindReturn(x, f) = map f x
+
+    let applicative<'a> = ApplicativeBuilder<'a>()
+
+    [<Test>]
+    let ``should be able to use applicative CE which requires Lift3`` () =
+        let program = 
+            applicative {
+                let! a = read 1
+                and! b = read 2
+                and! c = read 3
+                return a, b, c
+            }
+
+        let result = 
+            program 
+            |> Free.fold 
+                (function 
+                | Read (i, next) -> i |> string |> next |> result) 
+            |> Identity.run
+
+        areStEqual result ("1", "2", "3")