A good coding practice in Elm is to define the Model such that impossible states can not occur.

Instead of writing

type alias ServerRequest =
    { Response (Maybe String)
    , Failed (Maybe Error)
    }
    -- There can not be a Response and an Error at the same time.

one can use

type ServerRequest =
    Waiting
    Response String
    Failed Error

In this section we will look at common ways how to avoid impossible states.

Further reading

• Video: Making impossible states impossible by Richard Feldman

• Example: Tic Tac Toe​

• Example: Subset of {a,b}​

• Example: Up to 2 choices of a set​