Add List.chunkBy (#561)

src/FSharpPlus/Control/Collection.fs

@@ -289,7 +289,11 @@ type GroupBy =
 type ChunkBy =
     static member ChunkBy (x: Id<'T>  , f: 'T->'Key, _: Id<'Key*Id<'T>>    , [<Optional>]_impl: ChunkBy) = let a = Id.run x in Id.create (f a, x)
     static member ChunkBy (x: seq<'T> , f: 'T->'Key, _: seq<'Key*seq<'T>>  , [<Optional>]_impl: ChunkBy) = Seq.chunkBy f x |> Seq.map (fun (x,y) -> x, y :> _ seq)
-    static member ChunkBy (x: list<'T>, f: 'T->'Key, _: list<'Key*list<'T>>, [<Optional>]_impl: ChunkBy) = Seq.chunkBy f x |> Seq.map (fun (x,y) -> x, Seq.toList  y) |> Seq.toList
+    static member ChunkBy (x: list<'T>, f: 'T->'Key, _: list<'Key*list<'T>>, [<Optional>]_impl: ChunkBy) =
+        #if TEST_TRACE
+        Traces.add "ChunkBy, list<'T>"
+        #endif
+        List.chunkBy f x
     static member ChunkBy (x: 'T []   , f: 'T->'Key, _: ('Key*('T [])) []  , [<Optional>]_impl: ChunkBy) = Seq.chunkBy f x |> Seq.map (fun (x,y) -> x, Seq.toArray y) |> Seq.toArray
 
     static member inline Invoke (projection: 'T->'Key) (source: '``Collection<'T>``) : '``Collection<'Key * 'Collection<'T>>`` = 

src/FSharpPlus/Extensions/List.fs

@@ -340,6 +340,48 @@ module List =
                 loop (ls,rs)
         loop (list1, list2)
     #endif
+
+    /// <summary>
+    /// Chunks the list up into groups with the same projected key by applying
+    /// the key-generating projection function to each element and yielding a list of 
+    /// keys tupled with values.
+    /// </summary>
+    ///
+    /// <remarks>
+    /// Each key is tupled with an array of all adjacent elements that match 
+    /// to the key, therefore keys are not unique but can't be adjacent
+    /// as each time the key changes a new group is yield.
+    /// 
+    /// The ordering of the original list is respected.
+    /// </remarks>
+    ///
+    /// <param name="projection">A function that transforms an element of the list into a comparable key.</param>
+    /// <param name="source">The input list.</param>
+    ///
+    /// <returns>The resulting list of keys tupled with a list of matching values</returns>
+    let chunkBy (projection: 'T -> 'Key) (source: _ list) =
+        #if FABLE_COMPILER
+        Seq.chunkBy projection source |> Seq.map (fun (x, y) -> x, Seq.toList y) |> Seq.toList
+        #else
+        match source with
+        | [] -> []
+        | x::xs ->
+            let mutable acc = new ListCollector<_> ()
+            let mutable members = new ListCollector<_> ()
+            let rec loop source g =
+                match source with
+                | [] -> acc.Add (g, members.Close ())
+                | x::xs ->
+                    let key = projection x
+                    if g <> key then
+                        acc.Add (g, members.Close ())
+                        members <- new ListCollector<_> ()
+                    members.Add x
+                    loop xs key
+            members.Add x
+            loop xs (projection x)
+            acc.Close ()
+        #endif
 
     /// <summary>Same as choose but with access to the index.</summary>
     /// <param name="mapping">The mapping function, taking index and element as parameters.</param>

tests/FSharpPlus.Tests/Collections.fs

@@ -0,0 +1,24 @@
+namespace FSharpPlus.Tests
+
+open FSharpPlus
+open FSharpPlus.Data
+open NUnit.Framework
+
+#if TEST_TRACE
+open FSharpPlus.Internals
+#endif
+
+module Collections =
+
+    [<Test>]    
+    let chunkBy () =
+        #if TEST_TRACE
+        Traces.reset()
+        #endif
+        let source = [1; 2; 3; 5; 7; 9]
+        let expected = [(1, [1]); (0, [2]); (1, [3; 5; 7; 9])]
+        let actual = chunkBy (flip (%) 2) source
+        CollectionAssert.AreEqual(expected, actual)
+        #if TEST_TRACE
+        CollectionAssert.AreEqual (["ChunkBy, list<'T>"], Traces.get())
+        #endif

tests/FSharpPlus.Tests/FSharpPlus.Tests.fsproj

@@ -23,6 +23,7 @@
     <Compile Include="Parsing.fs" />
     <Compile Include="Traversals.fs" />
     <Compile Include="Indexables.fs" />
+    <Compile Include="Collections.fs" />
     <Compile Include="Validations.fs" />
     <Compile Include="Task.fs" />
     <Compile Include="ValueTask.fs" />

tests/FSharpPlus.Tests/General.fs

@@ -472,7 +472,7 @@ module Functor =
         let nel = zip (NonEmptyList.ofList [1; 2]) (NonEmptyList.ofList ["a"; "b"; "c"])
         CollectionAssert.AreEqual (NonEmptyList.ofList [1,"a"; 2,"b"], nel)
 
-module Collections =
+module Collections2 =
 
     open System.Collections.Concurrent