Support for multiproject builds with nix

- Closes #28

common/src/Configuration.hs

@@ -0,0 +1,23 @@
+{-# Language NoImplicitPrelude #-}
+{-# Language DeriveGeneric #-}
+{-# Language DuplicateRecordFields #-}
+module Configuration where
+
+import ClassyPrelude
+import Dhall (Interpret)
+
+data Pg = Pg { username :: Text
+             , password :: Text
+             , host :: Text
+             , database :: Text }
+        deriving (Show, Generic)
+
+newtype Store = Store { path :: Text } deriving (Show, Generic)
+
+data Config = Config { database :: Pg
+                     , store :: Store }
+            deriving (Show, Generic)
+
+instance Interpret Pg
+instance Interpret Store
+instance Interpret Config

common/src/Data/Versioned.hs

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+{-# Language KindSignatures #-}
+{-# Language DataKinds #-}
+{-# Language DefaultSignatures #-}
+{-# Language MultiParamTypeClasses #-}
+{-# Language FunctionalDependencies #-}
+module Data.Versioned where
+
+import GHC.TypeLits
+import ClassyPrelude
+import Data.Generics.Product
+
+newtype Versioned (v :: Nat) a = Versioned a deriving (Show)
+
+instance Functor (Versioned v) where
+  fmap f (Versioned a) = Versioned (f a)
+
+instance Applicative (Versioned v) where
+  pure = Versioned
+  (Versioned f) <*> (Versioned a) = Versioned (f a)
+
+class Migrate a b | b -> a where
+  migrate :: a -> b
+  default migrate :: (Subtype b a) => a -> b
+  migrate = upcast