How do I create several related data types in haskell?

Question

I have a User type that represents a user saved in the database. However, when displaying users, I only want to return a subset of these fields so I made a different type without the hash. When creating a user, a password will be provided instead of a hash, so I made another type for that.

This is clearly the worst, because there is tons of duplication between my types. Is there a better way to create several related types that all share some fields, but add some fields and remove others?

{-# LANGUAGE DeriveGeneric #}

data User = User {
  id :: String,
  email :: String,
  hash :: String,
  institutionId :: String
} deriving (Show, Generic)

data UserPrintable = UserPrintable {
  email :: String,
  id :: String,
  institutionId :: String
} deriving (Generic)

data UserCreatable = UserCreatable {
  email :: String,
  hash :: String,
  institutionId :: String
} deriving (Generic)

data UserFromRequest = UserFromRequest {
  email :: String,
  institutionId :: String,
  password :: String
} deriving (Generic)

-- UGHHHHHHHHHHH

Why do you need UserPrintable at all? Just create a showPrintable ::User->String function. — jamshidh
– jamshidh, Commented Dec 18, 2014 at 18:21
I suspect you are trying to solve the wrong problem. What are you trying to achieve? If you are just interested in a subset of fields under different circumstances then just pass the whole type. Maybe you need to replace UserFromRequest with a function that computes the hash from the password. BTW, have you considered where salts will go? — Paul Johnson
– Paul Johnson, Commented Dec 18, 2014 at 18:40

Tikhon Jelvis · Accepted Answer · 2014-12-18 18:41:01Z

In this case, I think you can replace your various User types with functions. So instead of UserFromRequest, have:

userFromRequest :: Email -> InstitutionId -> String -> User

Note how you can also make separate types for Email and InstitutionId, which will help you avoid a bunch of annoying mistakes. This serves the same purpose as taking a record with labelled fields as an argument, while also adding a bit of extra static safety. You can just implement these as newtypes:

newtype Email = Email String deriving (Show, Eq)

Similarly, we can replace UserPrintable with showUser.

UserCreatable might be a bit awkard however, depending on how you need to use it. If all you ever do with it is take it as an argument and create a database row, then you can refactor it into a function the same way. But if you actually need the type for a bunch of things, this isn't a good solution.

In this second case, you have a couple of decent options. One would be to just make id a Maybe and check it each time. A better one would be to create a generic type WithId a which just adds an id field to anything:

data WithId a = { id :: DatabaseId, content :: a }

Then have a User type with no id and have your database functions work with a WithId User.

Collectives™ on Stack Overflow

How do I create several related data types in haskell?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related