Write safer functions using Phantom Types

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Problem
Solution
type alias LoginForm =
    { username : String
    , password : String
    , isValid : Bool
    }
​
{-| Creates a User
​
Only use this function if the LoginForm was validated.
-}
createUser : LoginForm -> User
createUser loginForm =
    if loginForm.isValid then
        User.create loginForm.username
    else
        Debug.todo "This should not happen."
type LoginForm valid =
    LoginForm
        { username : String
        , password : String
        }
​
type Valid = Valid
​
{-| Creates a User
-}
createUser : LoginForm Valid -> User
createUser (LoginForm loginForm) =
        User.create loginForm.username
        
validate : LoginForm () -> LoginForm Valid
validate =
    ...
Question
How can I  ensure that a user can only be created with a valid login form?
Answer
Use a so called Phantom Type:
type LoginForm valid = --valid is not used in the definition
    LoginForm
        { username : String
        , password : String
        }
        
type Valid = Valid
Use LoginForm () for unvalidated forms and LoginForm Valid for validated ones.
Further reading
📄Article: Advanced Types in Elm - Phantom Types by Charlie Koster

Restrict records using Opaque Types

Next - Recipes

Designing Elm package APIs

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