HODI

Higher Order Dependency Injection

HODI is a set of functions designed to be simple and consistent with a minimal implementation.

Why HODI?

• Testable by default: pass dependencies as arguments, not service locators
• Type-safe: compiler catches missing deps before runtime
• Works with Giraffe/web handlers

Install

PM> Install-Package HODI

or...

dotnet add package HODI

Defining Handlers

Instead of...

let oldWayHandler : HttpHandler =
    fun (next: HttpFunc) (ctx: HttpContext) ->
        task {
            let dep = ctx.GetService<Dependency>()
            // ...
        }

this can be done...

let anotherWayHandler : HttpHandler =
    fun (dep: Dependency) (next: HttpFunc) (ctx: HttpContext) ->
        task {
            // ...
        }

Usage

The functions can be used when defining routes.

let app =
    choose [
        route "/a" >=> exampleHandler |> inject
        route "/b" >=> handlerWith2Dependencies |> inject2
        routef "/c/%s" >=> handlerWithArgumentAndDependency |> injectPlus
    ]

Unit Testing

When unit testing the HTTPHandler functions, you could simply pass the dependencies without having to set up a test server. In other words, the tests can focus on the handlers behavior instead of caring about dependency injection.