ListCollector

src/Fable.Transforms/Replacements.Util.fs

@@ -322,7 +322,6 @@ let (|BuiltinDefinition|_|) =
     | Types.dateOnly -> Some BclDateOnly
     | Types.timeOnly -> Some BclTimeOnly
     | "System.Timers.Timer" -> Some BclTimer
-    | Types.decimal
     | Types.fsharpSet -> Some(FSharpSet(Any))
     | Types.fsharpMap -> Some(FSharpMap(Any, Any))
     | Types.hashset -> Some(BclHashSet(Any))

src/Fable.Transforms/Replacements.fs

@@ -1148,6 +1148,8 @@ let tryEntityIdent (com: Compiler) entFullName =
         makeImportLib com Any "AsyncReplyChannel" "AsyncBuilder" |> Some
     | "Microsoft.FSharp.Control.FSharpEvent`1" -> makeImportLib com Any "Event" "Event" |> Some
     | "Microsoft.FSharp.Control.FSharpEvent`2" -> makeImportLib com Any "Event$2" "Event" |> Some
+    | "Microsoft.FSharp.Core.CompilerServices.ListCollector`1" ->
+        makeImportLib com Any "ListCollector$1" "FSharp.Core.CompilerServices" |> Some
     | _ -> None
 
 let tryConstructor com (ent: Entity) =
@@ -4566,6 +4568,7 @@ let private replacedModules =
             "Microsoft.FSharp.Collections.ListModule", listModule
             "Microsoft.FSharp.Collections.HashIdentity", fsharpModule
             "Microsoft.FSharp.Collections.ComparisonIdentity", fsharpModule
+            "Microsoft.FSharp.Core.CompilerServices.ListCollector`1", bclType
             "Microsoft.FSharp.Core.CompilerServices.RuntimeHelpers", seqModule
             "Microsoft.FSharp.Collections.SeqModule", seqModule
             Types.keyValuePair, keyValuePairs

src/fable-library/FSharp.Core.CompilerServices.fs

@@ -0,0 +1,17 @@
+namespace Microsoft.FSharp.Core.CompilerServices
+
+[<NoEquality; NoComparison>]
+type ListCollector<'T>() =
+    let collector = ResizeArray<'T>()
+
+    member this.Add(value: 'T) = collector.Add(value)
+
+    member this.AddMany(values: seq<'T>) = collector.AddRange(values)
+
+    // In the particular case of closing with a final add of an F# list
+    // we can simply stitch the list into the end of the resulting list
+    member this.AddManyAndClose(values: seq<'T>) =
+        collector.AddRange(values)
+        Seq.toList collector
+
+    member this.Close() = Seq.toList collector

src/fable-library/Fable.Library.fsproj

@@ -33,6 +33,7 @@
     <Compile Include="Random.fs" />
     <Compile Include="Set.fs" />
     <Compile Include="Map.fs" />
+    <Compile Include="FSharp.Core.CompilerServices.fs" />
     <TypeScriptCompile Include="Async.ts" />
     <TypeScriptCompile Include="AsyncBuilder.ts" />
     <TypeScriptCompile Include="BitConverter.ts" />

tests/Js/Main/Fable.Tests.fsproj

@@ -78,6 +78,7 @@
     <Compile Include="UriTests.fs" />
     <Compile Include="TimeOnlyTests.fs" />
     <Compile Include="TimeSpanTests.fs" />
+    <Compile Include="ListCollectorTests.fs" />
     <Compile Include="Main.fs" />
   </ItemGroup>
 

tests/Js/Main/ListCollectorTests.fs

@@ -0,0 +1,26 @@
+module Fable.Tests.ListCollector
+
+open Util.Testing
+open Fable.Tests.Util
+open Microsoft.FSharp.Core.CompilerServices
+
+let cutOffLast list =
+    let mutable headList = ListCollector<'a>()
+
+    let rec visit list =
+        match list with
+        | []
+        | [ _ ] -> ()
+        | head :: tail ->
+            headList.Add(head)
+            visit tail
+
+    visit list
+    headList.Close()
+
+let tests =
+  testList "ListCollector" [
+    testCase "ListCollector.Add and .Close" <| fun () ->
+        let result = cutOffLast [ 1; 2; 3 ]
+        result |> equal [ 1; 2 ]
+  ]

tests/Js/Main/Main.fs

@@ -50,6 +50,7 @@ let allTests =
     TypeTests.tests
     UnionTypes.tests
     Uri.tests
+    ListCollector.tests
   |]
 
 #if FABLE_COMPILER