Write safer functions using Phantom Types

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

Last modified 2yr ago

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