Safe modelling of relational data in Haskell

Question

I find it very common to want to model relational data in my functional programs. For example, when developing a web-site I may want to have the following data structure to stor

Answer 1

I've been asked to write an answer using Opaleye. In fact there's not an awful lot to say, as the Opaleye code is fairly standard once you have a database schema. Anyway, here it is, assuming there is a user_table with columns user_id, name and birthdate, and a message_table with columns user_id, time_stamp and content.

This sort of design is explained in more detail in the Opaleye Basic Tutorial.

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE Arrows #-}

import Opaleye
import Data.Profunctor.Product (p2, p3)
import Data.Profunctor.Product.TH (makeAdaptorAndInstance)
import Control.Arrow (returnA)

data UserId a = UserId { unUserId :: a }
$(makeAdaptorAndInstance "pUserId" ''UserId)

data User' a b c = User { userId    :: a
                        , name      :: b
                        , birthDate :: c }
$(makeAdaptorAndInstance "pUser" ''User')

type User = User' (UserId (Column PGInt4))
                  (Column PGText)
                  (Column PGDate)

data Message' a b c = Message { user      :: a
                              , timestamp :: b
                              , content   :: c }
$(makeAdaptorAndInstance "pMessage" ''Message')

type Message = Message' (UserId (Column PGInt4))
                        (Column PGDate)
                        (Column PGText)


userTable :: Table User User
userTable = Table "user_table" (pUser User
  { userId    = pUserId (UserId (required "user_id"))
  , name      = required "name"
  , birthDate = required "birthdate" })

messageTable :: Table Message Message
messageTable = Table "message_table" (pMessage Message
  { user      = pUserId (UserId (required "user_id"))
  , timestamp = required "timestamp"
  , content   = required "content" })

An example query which joins the user table to the message table on the user_id field:

usersJoinMessages :: Query (User, Message)
usersJoinMessages = proc () -> do
  aUser    <- queryTable userTable    -< ()
  aMessage <- queryTable messageTable -< ()

  restrict -< unUserId (userId aUser) .== unUserId (user aMessage)

  returnA -< (aUser, aMessage)