Transform the Model the Cmd will be left untouched

Combine 2 Returns with a function

map2
  (\modelA modelB -> { modelA | foo = modelB.foo })
  retA
  retB

Map an Return into a Return containing a Model function

Transform the Model of and add a new Cmd to the queue

Map on the Cmd.

Create a Return from a given Model

Chain together expressions from Model to Return.

-- arbitrary function to demonstrate
foo : Model -> Return Msg Model
foo ({bar} as model) =
  -- forking logic
  if bar < 10
  -- that side effects may be added
  then (model, getAjaxThing)
  -- that the model may be updated
  else ({model | bar = model.bar - 2 }, Cmd.none)

They are now chainable with andThen...

resulting : Return msg { model | bar : Int }
resulting =
  myReturn `andThen` foo
           `andThen` foo
           `andThen` foo

Here we changed up foo three times, but we can use any function of type (a -> Return msg b).

Commands will be accumulated automatically as is the case with all functions in this library.

Go point free with andThen chaining. Looking at the example from andThen

resulting : Return msg { model | bar : Int }
resulting =
  myReturn `andThen` foo
           `andThen` foo
           `andThen` foo

this code roughly becomes:

doFoo3Times : { model | bar : Int } -> Return msg { model | bar : Int }
doFoo3Times =
  foo >>> foo >>> foo

Construct a new Return from parts

Add a Cmd to a Return, the Model is uneffected

Add a Cmd to a Return based on its Model, the Model will not be effected