Add F# compatible json serializer

DOCUMENTATION.md

@@ -2935,6 +2935,9 @@ Please visit the [Giraffe.ViewEngine](https://github.com/giraffe-fsharp/Giraffe.
 
 By default Giraffe uses `System.Text.Json` for (de-)serializing JSON content. An application can modify the default serializer by registering a new dependency which implements the `Json.ISerializer` interface during application startup.
 
+It's possible to use a serializer compatible with Fsharp types: `Json.FsharpFriendlySerializer` instead of `Json.Serializer`.  
+This uses `FSharp.SystemTextJson` to customize `System.Text.Json`.
+
 #### Using a different JSON serializer
 
 You can change the entire underlying JSON serializer by creating a new class which implements the `Json.ISerializer` interface:
@@ -2968,10 +2971,6 @@ For example, one could define a `Newtonsoft.Json` serializer:
          let serializer = JsonSerializer.Create settings
          let utf8EncodingWithoutBom = UTF8Encoding(false)
 
-         new(settings : JsonSerializerSettings) = Serializer(
-             settings,
-             recyclableMemoryStreamManager.Value)
-
          static member DefaultSettings =
              JsonSerializerSettings(
                  ContractResolver = CamelCasePropertyNamesContractResolver())
@@ -3021,7 +3020,8 @@ let configureServices (services : IServiceCollection) =
     services.AddGiraffe() |> ignore
 
     // Now register your custom Json.ISerializer
-    services.AddSingleton<Json.ISerializer, NewtonsoftJson.Serializer>() |> ignore
+    services.AddSingleton<Json.ISerializer>(fun serviceProvider ->
+            NewtonsoftJson.Serializer(JsonSerializerSettings(), serviceProvider.GetService<Microsoft.IO.RecyclableMemoryStreamManager>()) :> Json.ISerializer) |> ignore
 
 [<EntryPoint>]
 let main _ =

src/Giraffe/Giraffe.fsproj

@@ -66,6 +66,7 @@
     <PackageReference Include="FSharp.Core" Version="6.0.0" />
     <PackageReference Include="Microsoft.IO.RecyclableMemoryStream" Version="3.0.*" />
     <PackageReference Include="System.Text.Json" Version="8.0.*" />
+    <PackageReference Include="FSharp.SystemTextJson" Version="1.3.*" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.*" PrivateAssets="All" />
     <PackageReference Include="Giraffe.ViewEngine" Version="1.4.*" />
   </ItemGroup>

src/Giraffe/Json.fs

@@ -6,6 +6,7 @@ module Json =
     open System.IO
     open System.Text.Json
     open System.Threading.Tasks
+    open System.Text.Json.Serialization
 
     /// <summary>
     /// Interface defining JSON serialization methods.
@@ -53,4 +54,20 @@ module Json =
                 JsonSerializer.Deserialize<'T>(Span<_>.op_Implicit(bytes.AsSpan()), options)
 
             member __.DeserializeAsync<'T> (stream : Stream) : Task<'T> =
-                JsonSerializer.DeserializeAsync<'T>(stream, options).AsTask()
+                JsonSerializer.DeserializeAsync<'T>(stream, options).AsTask()
+
+    module FsharpFriendlySerializer =
+        let DefaultOptions =
+           JsonFSharpOptions.Default()
+
+        let private appendJsonFSharpOptions (fsharpOptions: JsonFSharpOptions) (jsonOptions: JsonSerializerOptions) =
+            jsonOptions.Converters.Add(JsonFSharpConverter(fsharpOptions))
+            jsonOptions
+
+        let buildConfig (fsharpOptions: JsonFSharpOptions option) (jsonOptions: JsonSerializerOptions option) =
+            jsonOptions
+            |> Option.defaultValue (JsonSerializerOptions())
+            |> appendJsonFSharpOptions (fsharpOptions |> Option.defaultValue DefaultOptions)
+
+    type FsharpFriendlySerializer (?fsharpOptions: JsonFSharpOptions, ?jsonOptions: JsonSerializerOptions) =
+        inherit Serializer(FsharpFriendlySerializer.buildConfig fsharpOptions jsonOptions)

tests/Giraffe.Tests/HttpHandlerTests.fs

@@ -66,6 +66,76 @@ let ``GET "/json" returns json object`` () =
         | Some ctx -> Assert.Equal(expected, getBody ctx)
     }
 
+type ResponseWithFsharpType = {
+    ValueA: string option
+    ValueB: JsonUnionCaseDummy
+}
+and JsonUnionCaseDummy =
+    | JsonUnionCaseDummyA of int
+    | JsonUnionCaseDummyB
+[<Fact>]
+let ``GET "/json" returns json object with fsharp type`` () =
+    let ctx = Substitute.For<HttpContext>()
+    ctx.RequestServices
+        .GetService(typeof<Json.ISerializer>)
+        .Returns(Json.FsharpFriendlySerializer())
+    |> ignore
+
+    let app =
+        GET >=> choose [
+            route "/"     >=> text "Hello World"
+            route "/foo"  >=> text "bar"
+            route "/json" >=> json { ValueA = Some "hello"; ValueB = JsonUnionCaseDummyA 42 }
+            setStatusCode 404 >=> text "Not found" ]
+
+    ctx.Request.Method.ReturnsForAnyArgs "GET" |> ignore
+    ctx.Request.Path.ReturnsForAnyArgs (PathString("/json")) |> ignore
+    ctx.Response.Body <- new MemoryStream()
+    let expected = """{"ValueA":"hello","ValueB":{"Case":"JsonUnionCaseDummyA","Fields":[42]}}"""
+
+    task {
+        let! result = app next ctx
+
+        match result with
+        | None     -> assertFailf "Result was expected to be %s" expected
+        | Some ctx ->
+            let content = getBody ctx
+            Assert.Equal(expected, content)
+    }
+
+[<Fact>]
+let ``GET "/json" returns json object with fsharp type and use custom config`` () =
+    let ctx = Substitute.For<HttpContext>()
+    let customConfig =
+        System.Text.Json.Serialization.JsonFSharpOptions.Default()
+            .WithUnionTagNamingPolicy(System.Text.Json.JsonNamingPolicy.CamelCase)
+    ctx.RequestServices
+        .GetService(typeof<Json.ISerializer>)
+        .Returns(Json.FsharpFriendlySerializer(customConfig, Json.Serializer.DefaultOptions))
+    |> ignore
+
+    let app =
+        GET >=> choose [
+            route "/"     >=> text "Hello World"
+            route "/foo"  >=> text "bar"
+            route "/json" >=> json { ValueA = Some "hello"; ValueB = JsonUnionCaseDummyA 42 }
+            setStatusCode 404 >=> text "Not found" ]
+
+    ctx.Request.Method.ReturnsForAnyArgs "GET" |> ignore
+    ctx.Request.Path.ReturnsForAnyArgs (PathString("/json")) |> ignore
+    ctx.Response.Body <- new MemoryStream()
+    let expected = """{"valueA":"hello","valueB":{"Case":"jsonUnionCaseDummyA","Fields":[42]}}"""
+
+    task {
+        let! result = app next ctx
+
+        match result with
+        | None     -> assertFailf "Result was expected to be %s" expected
+        | Some ctx ->
+            let content = getBody ctx
+            Assert.Equal(expected, content)
+    }
+
 let DefaultMocksWithSize =
     [
         let ``powers of two`` = [ 1..10 ] |> List.map (pown 2)