The effects pattern

In a usual Elm application the update function returns (Model, Cmd Msg). Commands in Elm are an opaque types so testing update functions is not easy. We cannot easily inspect the commands and see if they are doing the right thing, we cannot also simulate these commands as we don't know what they are doing.

Pattern

What pattern to deal with this is returning an Effect type instead of Cmd msg.

type Effect
	= SaveUser User
	| LogoutUser
	| LoadData
	| ...

update : Msg -> Model -> (Model, List Effect)

At the last moment possible we will have a function that converts the Effect into actual commands:

runEffects : List Effect -> Cmd Msg

This makes a Elm application a lot more testable. This is approach taken by elm-program-test.

Here is a more detailed blog post about this pattern.