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[BUG]: Generating Scala3 uPickle from JSON Schema is missing ReadWriter annotation for enums #2677
Description
Issue Type
Output
Context (Environment, Version, Language)
Input Format: JSON Schema
Output Language: Scala3+uPickle
CLI, npm, or app.quicktype.io: CLI
Version: 23.0.170
Description
In the generated Scala3 output there is a missing ReadWriter annotation for the generated enum Color, which leads to a build failure while the uPickle macro is trying run:
No given instance of type ReadersVersionSpecific_this.Reader[
[error] | quicktype.Color] was found.
Input Data
{
"$schema": "https://json-schema.org/draft/2019-09/schema",
"definitions": {
"Color" : {
"type" : "string",
"enum": ["Red", "Green", "Blue"]
},
"Monster" : {
"type" : "object",
"properties" : {
"name" : {
"type" : "string"
},
"color" : {
"$ref" : "#/definitions/Color"
}
},
"required" : ["name", "color"],
"additionalProperties" : false
}
},
"$ref" : "#/definitions/Monster"
}Expected Behaviour / Output
package quicktype
import upickle.default.*
case class Monster (
val color : Color,
val name : String
) derives ReadWriter
enum Color derives ReadWriter :
case Blue,Green,RedCurrent Behaviour / Output
package quicktype
import upickle.default.*
case class Monster (
val color : Color,
val name : String
) derives ReadWriter
enum Color :
case Blue,Green,Red
Steps to Reproduce
It also reproduces in the webapp, so easiest repro is:
- paste above json into webapp and JSON Schema as source
- Select Scala3 and uPickle as target
- Try to compile the resulting code with with a minimal sbt project. Something along the lines of:
val scala3Version = "3.6.2"
lazy val root = project
.in(file("."))
.settings(
name := "Quicktype Test",
version := "0.1.0-SNAPSHOT",
scalaVersion := scala3Version,
libraryDependencies += "com.lihaoyi" %%% "upickle" % "4.1.0"
)Possible Solution
Just adjust the generator to add the necessary derive for enums.
Activity
schani commentedon Jul 6, 2026
Triage sweep note (assisted by Claude): confirmed reproducible on current master (v23.3.0,
68ecb284).Root cause: confirmed exactly as reported — output is
enum Color :with noderives ReadWriter, while the corresponding case class correctly getsderives ReadWriter.UpickleRenderer(packages/quicktype-core/src/language/Scala3/UpickleRenderer.ts) only overridesemitClassDefinitionMethods; enum emission falls through to the baseScala3Renderer.emitEnumDefinition(Scala3Renderer.ts:298), which hardcodes["enum ", name, " : "]with no hook for aderivesclause. At runtime this fails withNo given instance of type Reader[Color]since uPickle has no way to derive aReader/Writerfor the enum.CirceRendereris unaffected because it fully overridesemitEnumDefinitionitself.Assessment: severity medium — only affects the non-default
--framework upickleoption, but for that framework any schema containing an enum produces code that fails to compile/run, with no workaround. Fix difficulty: easy — add an overridable hook in the base renderer (e.g. an enum-declaration-suffix method) and haveUpickleRendereremitderives ReadWriterthrough it, mirroring the class-side handling.Reproduction:
{ "$schema": "https://json-schema.org/draft/2019-09/schema", "definitions": { "Color" : { "type" : "string", "enum": ["Red", "Green", "Blue"] }, "Monster" : { "type" : "object", "properties" : { "name" : { "type" : "string" }, "color" : { "$ref" : "#/definitions/Color" } }, "required" : ["name", "color"], "additionalProperties" : false } }, "$ref" : "#/definitions/Monster" }Output:
(
enum Coloris missingderives ReadWriter, matching the issue's expected-vs-actual.)