http4s: Composing Routes, Middleware & When It Fits

Combining Routes

val allRoutes = userRoutes <+> orderRoutes <+> healthRoutes

// Combines multiple HttpRoutes[F] values into one,
// trying each in turn until one matches the request

Middleware

val loggedRoutes = Logger.httpRoutes(logHeaders = true, logBody = false)(userRoutes)

// Middleware functionally transforms one HttpRoutes[F] into
// another, wrapping it with additional behavior

Effect-Based Error Handling

Because IO has its own built-in error channel, error handling flows through that same mechanism -- keeping it consistent with the library's overall functional approach, rather than mixing in imperative try/catch.

Backend-Agnostic

http4s's core abstractions (routes, requests, responses) are defined independently of any specific server implementation, so applications can run on different underlying backends (ember, blaze) with relatively little application-level code change.

The Trade-off: Learning Curve

For developers unfamiliar with functional idioms (effect types, for-comprehensions over effects, type classes), http4s's style can mean a steeper initial learning curve than a more conventional imperative framework -- exchanged for stronger compositional and type-safety guarantees as complex request-handling logic grows.