Adapt bind for non-sealed types

Author: gusty

src/FSharpPlus/List.fs

@@ -58,13 +58,14 @@ module ListT =
         loop l1 l2 : ListT<'mt>
 
     let inline bind f (source: ListT<'mt>) : ListT<'mu> =
-        let rec loop f input =
-            wrap (    
-                (unwrap input : 'mit) >>= function
+        let _mnil _  = (result Unchecked.defaultof<'t> : 'mt) >>= fun (_: 't) -> (result Unchecked.defaultof<'u>) : 'mu
+        let rec loop f (ListT input) =
+            ListT (
+                (unbox input : 'mit) >>= function
                         | Nil -> result <| (Nil : ListTNode<'mu,'u>) : 'miu
                         | Cons (h:'t, t: ListT<'mt>) ->
-                            let ( res) = concat (f h: ListT<'mu>) (loop f t )
-                            unwrap res  : 'miu)
+                            let res = concat (f h: ListT<'mu>) (loop f t)
+                            unwrap res  : 'miu) 
         loop f source : ListT<'mu>
 
     let inline unfold (f:'State -> '``M<('T * 'State) option>``) (s:'State) : ListT<'MT> =

tests/FSharpPlus.Tests/ListT.fs

@@ -7,6 +7,7 @@ open NUnit.Framework
 open FsCheck
 open Helpers
 open System.Collections.Generic
+open System.Threading.Tasks
 
 module BasicTests =
     [<Test>]
@@ -25,8 +26,12 @@ module BasicTests =
 
     // Compile tests
     let binds () =
-        let res = listT ( [| [1..4] |]) >>= fun x -> listT ( [| [x * 2] |])
-        () // but for some reason it doesn't work for Task, ResizeArray, Lazy and seq
+        let res1 = listT [| [1..4] |] >>= fun x -> listT [| [x * 2] |]
+        let res2 = listT (Task.FromResult [1..4]) |> ListT.bind (fun x -> listT (Task.FromResult  [x * 2]))
+        let res3 = listT (ResizeArray [ [1..4] ]) |> ListT.bind (fun x -> listT (ResizeArray [ [x * 2] ]))
+        let res4 = listT (lazy [1..4]) |> ListT.bind (fun x -> listT (lazy ( [x * 2])))
+        let (res5: ListT<_ seq>) = listT (seq [ [1..4] ]) |> ListT.bind (fun x -> listT (seq [ [x * 2] ]))
+        () // Note: seq needs type annotation, the non-sealead types don't work with generic >>= (internal error, unsolved type var)
 
     let bind_for_ideantity () =
         let res = listT (Identity [1..4]) >>= fun x -> listT (Identity [x * 2])