Write safer functions using Phantom Types
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 =
...
How can I ensure that a user can only be created with a valid login form?
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.Last modified 3yr ago