Falco.UnionRoutes

Define your routes as F# discriminated unions. Get exhaustive pattern matching, type-safe links, and automatic parameter extraction. Inspired by Haskell's Servant library.

type PostRoute =
    | List of page: QueryParam<int> option           // GET /posts?page=1
    | Detail of id: PostId                           // GET /posts/{id}
    | Create of JsonBody<PostInput> * PreCondition<UserId>  // POST /posts (JSON body + auth)

let handlePost route : HttpHandler =
    match route with
    | List page -> Response.ofJson (getPosts page)
    | Detail postId -> Response.ofJson (getPost postId)
    | Create (JsonBody input, PreCondition userId) -> Response.ofJson (createPost userId input)

What you get: - automatic extraction, parsing of query params. Handlers don't need to parse query params or check if user is logged in, etc. - Route.link (Detail postId) -> "/posts/abc-123" (type-checked) - Route/query params and auth automatically extracted based on field types

Installation

dotnet add package Falco.UnionRoutes

*API Reference*

How It Works

Routes are discriminated unions. Field names become URL parameters:

type PostRoute =
    | List                                  // GET /posts
    | Detail of id: Guid                    // GET /posts/{id}
    | Create                                // POST /posts (convention: Create -> POST)
    | Delete of id: Guid                    // DELETE /posts/{id} (convention: Delete -> DELETE)

Special marker types change where values come from:

| Search of query: QueryParam<string>               // GET /posts/search?query=hello
| Search of q: QueryParam<string> option            // optional query param
| Create of PreCondition<UserId>                    // UserId from auth extractor, not URL
| Edit of PreCondition<UserId> * id: Guid           // auth + route param
| Admin of OverridablePreCondition<AdminId> * data  // skippable precondition (child routes can opt out)
| Create of JsonBody<PostInput> * PreCondition<UserId>  // JSON body + auth
| Submit of FormBody<LoginInput>                    // form-encoded body

Single-case wrapper DUs are auto-unwrapped:

type PostId = PostId of Guid
| Detail of id: PostId                      // extracts Guid from URL, wraps in PostId

Basic Usage

// 1. Define routes
type Route =
    | Home                                    // GET /home
    | Posts of PostRoute                      // /posts/...
    | [<Route(Path = "")>] Admin of AdminRoute

type PostInput = { Title: string; Body: string }

type PostRoute =
    | List of page: QueryParam<int> option              // GET /posts?page=1
    | Detail of id: PostId                              // GET /posts/{id}
    | Create of JsonBody<PostInput> * PreCondition<UserId>  // POST /posts (JSON body + auth)

type AdminRoute =
    | Dashboard of PreCondition<AdminId>      // GET /dashboard

// 2. Configure extraction — extractors are async (HttpContext -> Task<Result<'T, 'E>>)
let requireAuth : Extractor<UserId, AppError> = fun ctx ->
    Task.FromResult(
        match ctx.User.FindFirst(ClaimTypes.NameIdentifier) with
        | null -> Error NotAuthenticated
        | claim -> Ok (UserId (Guid.Parse claim.Value)))

let requireAdmin : Extractor<AdminId, AppError> = fun ctx ->
    Task.FromResult(
        if ctx.User.IsInRole("Admin") then
            match ctx.User.FindFirst(ClaimTypes.NameIdentifier) with
            | null -> Error NotAuthenticated
            | claim -> Ok (AdminId (Guid.Parse claim.Value))
        else Error (Forbidden "Admin role required"))

let config: EndpointConfig<AppError> = {
    Preconditions =
        [ yield! Extractor.precondition<UserId, AppError> requireAuth
          yield! Extractor.precondition<AdminId, AppError> requireAdmin ]
    Parsers = []
    MakeError = fun msg -> BadRequest msg
    CombineErrors = fun errors -> errors |> List.head
    ToErrorResponse = fun e -> Response.withStatusCode 400 >> Response.ofPlainText (string e)
}

// 3. Handle routes (compiler ensures exhaustive, routes already hydrated)
let handlePost route : HttpHandler =
    match route with
    | List page -> Response.ofJson (getPosts page)
    | Detail postId -> Response.ofJson (getPost postId)
    | Create (JsonBody input, PreCondition userId) -> Response.ofJson (createPost userId input)

let handleRoute route : HttpHandler =
    match route with
    | Home -> Response.ofPlainText "home"
    | Posts p -> handlePost p
    | Admin (Dashboard (PreCondition adminId)) -> Response.ofPlainText "admin"

// 4. Generate endpoints - extraction happens automatically
let endpoints = Route.endpoints config handleRoute

Reference

See the Example App for a complete working example. Run it with mise run example.

Route Conventions

Routing behavior:

RESTful case names (no case name prefix in path):

Override with attributes:

[<Route(RouteMethod.Put)>]                           // just method
[<Route(Path = "custom/{id}")>]                      // just path
[<Route(RouteMethod.Put, Path = "custom/{id}")>]     // both
[<Route(Constraints = [| RouteConstraint.Alpha |], MinLength = 3, MaxLength = 50)>]  // constraints
[<Route(MinValue = 1, MaxValue = 100)>]              // integer range
[<Route(Pattern = @"^\d{3}-\d{4}$")>]               // regex pattern

Implicit type constraints — applied automatically based on field types:

Implicit and explicit constraints combine: a Guid field with [<Route(Constraints = [| Required |])>] produces {id:guid:required}.

Parser constraints — applied by Route.endpoints when custom parsers declare constraints:

Extractor.constrainedParser<Slug> [| RouteConstraint.Alpha |] parseFn  // adds :alpha
Extractor.typedParser<bool, ToggleState> parseFn                       // adds :bool (from input type)

Parser constraints are applied at endpoint registration time. Route.info and Route.link reflect only type-based constraints since they don't require EndpointConfig.

Marker Types

Preconditions

OverridablePreCondition<'T> lets child routes opt out with attributes:

type ItemRoute =
    | List                                             // inherits preconditions
    | [<SkipAllPreconditions>] Public                  // skips all overridable preconditions
    | [<SkipPrecondition(typeof<UserId>)>] Limited     // skips specific one

type Route =
    | Items of userId: UserId * OverridablePreCondition<UserId> * ItemRoute

• PreCondition<'T> — strict, always runs, cannot be skipped
• OverridablePreCondition<'T> — skippable via [<SkipAllPreconditions>] or [<SkipPrecondition(typeof<T>)>]

Nested Routes

type PostDetailRoute =
    | Show                                         // GET    /posts/{id}
    | Edit                                         // GET    /posts/{id}/edit
    | Delete                                       // DELETE /posts/{id}
    | Patch                                        // PATCH  /posts/{id}

type PostRoute =
    | List of page: QueryParam<int> option                 // GET    /posts?page=1
    | Create of JsonBody<PostInput> * PreCondition<UserId> // POST   /posts (JSON body + auth)
    | Search of query: QueryParam<string>                  // GET    /posts/search?query=hello
    | Member of id: Guid * PostDetailRoute                 //        /posts/{id}/...

Member produces a param-only path (no case-name prefix). Show/Delete/Patch collapse to the same path with different methods. Edit produces /edit.

Route Validation

Route.endpoints automatically validates at startup and will fail fast with descriptive errors. Route.validate combines all checks for use in tests.

Structure errors (Route.validateStructure):

Uniqueness errors (checked by Route.validate and Route.endpoints):

Precondition errors (Route.validatePreconditions):

Routes with overlapping patterns are automatically sorted by specificity (/posts/new before /posts/{id}).

[<Fact>]
let ``all routes are valid`` () =
    let result = Route.validate<Route, AppError> config.Preconditions
    Assert.Equal(Ok (), result)

Key Functions

// Route module
Route.endpoints config handler       // Generate endpoints with extraction (main entry point)
Route.respond returns value          // Type-safe JSON response via Returns<'T>
Route.link route                     // Type-safe URL: "/posts/abc-123"
Route.info route                     // RouteInfo with Method and Path
Route.allRoutes<Route>()             // Enumerate all routes
Route.validateStructure<Route>()     // Validate path structure only
Route.validatePreconditions<Route, Error> preconditions  // Check precondition coverage
Route.validate<Route, Error> preconditions               // Full validation (for tests)

// EndpointConfig record (passed to Route.endpoints)
{ Preconditions = [...]              // Auth/validation extractors
  Parsers = [...]                    // Custom type parsers
  MakeError = fun msg -> ...         // String -> error type
  CombineErrors = fun errors -> ...  // Combine multiple errors
  ToErrorResponse = fun e -> ... }   // Error -> HTTP response

// Extractor module - create extractors for EndpointConfig
// Extractors are async: Extractor<'T,'E> = HttpContext -> Task<Result<'T,'E>>
Extractor.precondition<UserId, Error> extractFn              // Both PreCondition + OverridablePreCondition
Extractor.preconditionSync<UserId, Error> syncExtractFn      // Sync convenience wrapper
Extractor.parser<Slug> parseFn                               // For custom types (string input)
Extractor.constrainedParser<Slug> [| Alpha |] parseFn        // String parser + route constraints
Extractor.typedParser<bool, Toggle> parseFn                  // Typed parser (pre-parsed input)

OpenAPI Spec Generation

Generate OpenAPI 3.0 JSON from your route types — useful for documentation, client generation, or API gateways.

Programmatic (library):

let spec = Spec.generate<Route> { Title = "My API"; Version = "1.0.0"; Description = None }
printfn "%s" spec

CLI tool:

# Install as a global tool
dotnet tool install Falco.UnionRoutes.Cli

# Generate to stdout (auto-detects route type)
falco-routes MyApp/MyApp.fsproj --title "My API"

# Specify route type and output file
falco-routes MyApp/MyApp.fsproj MyApp.Route --output openapi.json

# Skip build if already compiled
falco-routes MyApp/MyApp.fsproj --no-build --title "My API" --version "2.0.0"

The CLI builds the project, loads the output assembly, and auto-detects the root route type. If multiple root route types exist, specify which one as the second argument.

License

MIT