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Type safe data migrations.

All you need to do is create a type family to index your record and provide an instance of Transform to migrate your data between versions.

Migrations are type-safe and the library uses generics to remove as much boiler-plate as possible.


data family Foo (version :: Nat)

newtype MyString = MyString { unMyString :: String }
   deriving (IsString, Show, Eq)

data instance Foo 0
  = FooV0
    { _fooId :: Int
    , _fooName :: String
  deriving (Generic, Show, Eq)

data instance Foo 1
  = FooV1
  { _fooId        :: Int
  , _fooName      :: MyString
  , _fooHonorific :: String
  deriving (Generic, Show, Eq)

instance Transform Foo 0 where
  up   v = genericUp   v (const "esquire") (const MyString)
  down v = genericDown v (const unMyString)

spjV0 :: Foo 0
spjV0 = FooV0 1 "Simon PJ"

spjV1 :: Foo 1
spjV1 = up spjV0

-- spjV1 = FooV1 1 (MyString "Simon PJ") "esquire"

Future Considerations

In the future this library will provide a high-level DSL to enable better ergonomics around type errors so that you can see which fields require specification in the migration.

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