Requêtes et réponses

In http4s a Request[F] is an immutable value carrying the method, URI, headers, HTTP version, and a streaming body. You inspect it with pure accessors and never mutate it in place.

To change a request you produce a copy via withHeaders, withUri, or similar combinators.

The body of a request or response is an fs2.Stream[F, Byte]. It is consumed lazily and effectfully, so large payloads never have to sit fully in memory.

You usually do not touch raw bytes; an EntityDecoder handles decoding for you.

An EntityDecoder[F, A] knows how to turn the body stream into an A. Call req.as[A] to decode, which returns F[A] and fails the effect on a malformed body.

A built-in decoder exists for String; richer ones come from JSON modules.

To access the request object inside a case, bind it with @. Here req names the whole request while the pattern still matches method and path.

Without the binder you only have the destructured parts and cannot read the body or headers.

Headers live in request.headers. Use headers.get[H] with a typed header model, or headers.get(CIString("X-Custom")) for arbitrary names.

Typed headers like Authorization and Content-Type parse and validate their values for you.

The DSL response builders such as Ok, Created, and NotFound are functions returning F[Response[F]]. Passing an argument sets the body via an EntityEncoder.

Each builder corresponds to a status code, so Created(...) yields 201.

For a status without a convenience builder, or to customize, construct a Response[F] with an explicit Status and chain withEntity.

This gives full control over status, headers, and body in one expression.

An EntityEncoder[F, A] serializes a value into the response body and sets the Content-Type. String and byte encoders are built in; JSON encoders come from circe.

When you pass a value to Ok(value), http4s looks up the encoder implicitly.

Add headers by passing them as extra arguments to a builder, or chain .map(_.putHeaders(...)) on the response effect.

Typed header models ensure correctly formatted values, for example a Location header on a 201 Created.

req.as[A] fails the F effect when the body cannot be decoded. Use req.attemptAs[A].value to get an F[Either[DecodeFailure, A]] and respond with 400 on the left.

This keeps malformed-input handling explicit and total.

Set cookies with response.addCookie(ResponseCookie(name, value)). For redirects, use a 3xx builder and a Location header.

SeeOther(Location(uri)) performs a 303, the idiomatic post-redirect-get response.

Think about safe body decoding.

Requests and responses are immutable values with streaming fs2 bodies. You decode with as/attemptAs via EntityDecoder and encode with EntityEncoder.

Builders set status codes, typed headers stay safe, and cookies and redirects are just response transformations.