Decoding error for Mappings with enumerations as keys

[jcubit]

It is currently not possible to parse a Pkl module in Swift which has a Mapping property with enum as keys. Example: consider the following Pkl module

// ApplicationConfig.pkl
module ApplicationConfig

typealias Environment = "dev"|"prod"|"qa"

config: Mapping<Environment, String>

// iscConfig.pkl
amends "ApplicationConfig.pkl"

config {
  ["dev"] = "Imaginary Service Company (ISC) configuration"
}

The swift generated type is

// Code generated from Pkl module `ApplicationConfig`. DO NOT EDIT.
import PklSwift

public enum ApplicationConfig {}

extension ApplicationConfig {
    public enum Environment: String, CaseIterable, Decodable, Hashable {
        case dev = "dev"
        case prod = "prod"
        case qa = "qa"
    }

    public struct Module: PklRegisteredType, Decodable, Hashable {
        public static let registeredIdentifier: String = "ApplicationConfig"

        public var config: [Environment: String]

        public init(config: [Environment: String]) {
            self.config = config
        }
    }

    /// Load the Pkl module at the given source and evaluate it into `ApplicationConfig.Module`.
    ///
    /// - Parameter source: The source of the Pkl module.
    public static func loadFrom(source: ModuleSource) async throws -> ApplicationConfig.Module {
        try await PklSwift.withEvaluator { evaluator in
            try await loadFrom(evaluator: evaluator, source: source)
        }
    }

    /// Load the Pkl module at the given source and evaluate it with the given evaluator into
    /// `ApplicationConfig.Module`.
    ///
    /// - Parameter evaluator: The evaluator to use for evaluation.
    /// - Parameter source: The module to evaluate.
    public static func loadFrom(
        evaluator: PklSwift.Evaluator,
        source: PklSwift.ModuleSource
    ) async throws -> ApplicationConfig.Module {
        try await evaluator.evaluateModule(source: source, as: Module.self)
    }
}

Now if we try to parse this in Swift via

import PklSwift

let filepath = ...

let appConfig = try await ApplicationConfig.loadFrom(source: .path(filepath))

we get the error:

Error: dataCorrupted(Swift.DecodingError.Context(codingPath: [], debugDescription: "Expected either list, listing, or set, but got mapping", underlyingError: nil))

The exception seems to be thrown from _PklDecoder in PklUnkeyedDecodingContainer.swift.

I’m using Pkl 0.27.2 and pkl-gen-swift 0.4.1

Thank you in advance for having a look at this!