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 =
    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​

Reusable views

Non empty lists using Zippers

Last modified 3yr ago