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Typesafe HTTP requests with lenses

Example showing how to create and apply lenses to requests and responses to both extract and inject typesafe values out of and into HTTP messages. Note that since the http4k Request/Response objects are immutable, all injection occurs via copy.

Gradle setup

    compile group: "org.http4k", name: "http4k-core", version: "3.241.0"

Standard (exception based) approach

Errors in extracting Lenses are propagated as exceptions which are caught and handled by the CatchLensFailure Filter.

Using custom "Result" ADTs

An alternative approach to using Exceptions to automatically produce BadRequests is to use an Either-type structure, and this would be easy to implement - but the lack of an in-built Result/Either type in the standard Kotlin library means that we don't have a single representation to use without shackling ourselves to another Either-containing library such as Arrow or Result4k.

Additionally, the lack of Higher Kinded Types in Kotlin means that we are unable to provide a generic method for converting standard lenses. However, it is easy to implement an extension method to use in specific use cases.

Below is an example which uses a custom Result ADT - this will work for all extraction Lenses that you define: