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