Step

A Step s msg o describes one step of an interaction. Here's a few things about the interactions we're talking about her

• Interactions can involve the end user of your app, external servers, javascript: anything you might need to coordinate with in an Elm app
• Interactions can be in one of a finite set of states. The states are represented by the type s
• Interactions change state in response to messages, actions from the outside world. The messages are represented by the type msg
• When responding to a message, the state of the interaction might change, and Cmds may be fired, potentially producing more messages.
• Interactions might go on indefinitely, or end. If they end, they result in a final value of type o. This might seem a little weird, but it has some nice properties

Step s msg o is inteneded to be the return value of an update function. It encodes a bunch of common update function patterns, uniting them under a simple mental model, that simultaneously reduces boilerplate and eases composition.

Apply a function to the state inside a step, if it's there

Most useful in building a bigger step out of a sub-step you happen to have lying around

Keep the interaction in the state it was in.

NOTE: This will prevent any commands from being returned

Step.stay == (Step.stay |> Step.withCmd myHttpCall)

If you want to stay in the same state, but run some commands, use Step.to explicitly

Step.to MySameState |> Step.withCmd myHttpCall

Run the first suceeding step, with priority given to the second argument

Intended to be used pipeline style

Step.to { loading = True } |> Step.orElse (Step.to { loading = False }) == Step.to { loading = True }


Step.noop |> Step.orElse Step.to { loading = True } == Step.to { loading = True }

Turn a Step into the usual TEA update tuple

It must be a step in an interaction that continues forever. We know it is if the type variable o isn't a specifc type, and can thus be chosen to be Never

Step to a new state in the interaction

If we're stepping to a new state, add an action to fire off

Apply a function to any msgs conteined in the step

Also used for building larger interaction steps out of smaller ones

End the interaction by returning a value of type o

Map over the output of an interaction, if we've reached the end

Choose a step based on the result of another interaction

You can use this in combination with map and mapMsg to glue the end of one interaction to the beginning of another.

turn an update function that returns a Step to a normal Elm Architecture update function

uses run internally to default with the provided model in case of a stay

starting from an initial state, fold an update function over a list of messages

A helper for building steps out of tasks

Build a step from a normal elm update tuple

Step to the state denoted by the a in the Just case, and stay otherwise