Missing DU Support for OpenAPI Discriminator Schemas

[houstonhaynes]

Hawaii doesn't natively generate F# discriminated unions from OpenAPI discriminator schemas (oneOf with mapping). These are semantically discriminated unions but Hawaii generates them as separate types without a union type.

OpenAPI Source

# From Cloudflare Workers OpenAPI spec
workersbindingitem:
  oneOf:
    - $ref: "#/components/schemas/workersbindingkindassets"
    - $ref: "#/components/schemas/workersbindingkindbrowser"
    - $ref: "#/components/schemas/workersbindingkindd1"
    - $ref: "#/components/schemas/workersbindingkindr2bucket"
    # ... 27 total binding types
  discriminator:
    propertyName: type
    mapping:
      assets: "#/components/schemas/workersbindingkindassets"
      browser: "#/components/schemas/workersbindingkindbrowser"
      # ... etc

Current Hawaii Output

Hawaii generates individual types but no discriminated union:

type workersbindingkindassets = { ... }
type workersbindingkindbrowser = { ... }
type workersbindingkindd1 = { ... }
// ... 27 more types

// But no:
// type workersbindingitem =
//     | Assets of workersbindingkindassets
//     | Browser of workersbindingkindbrowser
//     | D1 of workersbindingkindd1

CloudflareFS Workaround

We created a post-processing script that analyzes the generated code and automatically creates discriminated unions:

File: generators/hawaii/post-process-discriminators.fsx

// Auto-detects binding kind types and generates DU
let getBindingKindTypes (typesContent: string) =
    let pattern = @"type (workersbindingkind[a-z0-9]+)\s*="
    Regex.Matches(typesContent, pattern)
    |> Seq.cast<Match>
    |> Seq.map (fun m -> m.Groups.[1].Value)
    |> Seq.toList

let generateDiscriminatedUnion (typeName: string) (cases: (string * string) list) =
    let typeDecl = sprintf "type %s =" typeName
    let unionCases =
        cases
        |> List.map (fun (caseName, schemaType) ->
            sprintf "    | %s of %s" caseName schemaType)
        |> String.concat "\n"
    sprintf "%s\n%s\n" typeDecl unionCases

Generated Output:

// Discriminated union (auto-generated by post-processor)
type workersbindingitem =
    | Assets of workersbindingkindassets
    | Browser of workersbindingkindbrowser
    | D1 of workersbindingkindd1
    | R2bucket of workersbindingkindr2bucket
    // ... 27 cases total

Integration: generators/hawaii/generate-workers.ps1

# Step 1: Run Hawaii
dotnet run --project D:\repos\Hawaii\src\Hawaii.fsproj -- --config workers-hawaii.json

# Step 2: Post-process discriminated unions
dotnet fsi post-process-discriminators.fsx Generated-Workers\Types.fs

Proposed Enhancement

Hawaii should detect OpenAPI discriminator schemas and automatically generate F# discriminated unions:

1. Detect oneOf with discriminator in OpenAPI schema
2. Generate individual case types (current behavior)
3. Additionally generate a discriminated union type
4. Use the discriminator.propertyName for JSON serialization
5. Handle discriminator.mapping for case name mapping

Benefits

• Type safety: Single union type instead of loose collection of types
• Pattern matching: F# exhaustiveness checking works
• Semantic correctness: OpenAPI discriminators ARE discriminated unions
• No post-processing: Works out of the box

Reference Implementation

CloudflareFS has a working post-processor that demonstrates the pattern. We're happy to contribute this logic to Hawaii if it would be helpful.