Parse don't validate

When we have external data (e.g. user input or remote data) it is a common pattern to validate this data before using it.

Anti-pattern

A common approach is to ask if the data is valid and then use it e.g.

type alias UserInput =
    { name: Maybe String
    , age: Maybe Int
    }

isValidUser : UserInput -> Bool

The problem with this approach is that after doing this you still have a UserInput with can still hold invalid values.

Pattern

A better approach is to "parse" your input and return a known valid type. e.g.

 type alias UserInput =
    { name: Maybe String
    , age: Maybe Int
    }

type alias ValidUser =
    { name: String
    , age: Int
    }

validateUser : UserInput -> Result String ValidUser

In this way you ensure that you have a valid type to work with later on. Some examples where this is useful:

• Validating user input
• Parsing JSON from external sources