Making impossible states Impossible

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 =
    Response String
    Failed Error

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

Further reading

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