some-dict-of

Have you ever needed an existential wrapper that just guaranteed class membership, but you didn’t need to know the specifics of the type in question?

Well, you probably don’t - it’s not idiomatic Haskell for the most part. But sometimes this comes up, and I wanted a nice packaging of the technique. I wrote this mostly to support the discover-instances library.

SomeDictOf

Let’s consider a type that wraps anything that can be shown, along with a value of that type. We can make one by hand like this:

data SomeShowable where
    SomeShowable :: forall a. Show a => a -> SomeShowable

showList :: [SomeShowable] -> [String]
showList = map (\(SomeShowable a) -> show a)

With SomeDictOf, we can generalize this pattern to other classes and even other containers.

someShowable :: SomeDictOf Identity Show
someShowable =
    SomeDictOf (Identity (3 :: Int))

We’re carrying an Int, but the type does not reveal this. We can create a list of values like this, and we can call show on them.

showValues
    :: [SomeDictOf Identity Show]
    -> [String]
showValues =
    map (\(SomeDictOf (Identity showable)) -> show showable)

We can also carry around evidence of a type class instance, and then write generic stuff based on it. Consider the PersistEntity from the persistent database library.

tables :: [SomeDictOf Proxy PersistEntity]
tables = [ SomeDictOf (Proxy @User), SomeDictOf (Proxy @Organization) ]

Now we can iterate over these types and, say, load all the rows out of the database and verify they parse.

checkRows :: SqlPersistT [[PersistValue]]
checkRows = do
    forM tables $ \(SomeDictOf (Proxy :: Proxy table)) -> do
        results <- selectList [] [] :: SqlPersistT m [Entity table]
        pure (map toPersistValue results)

Or we can do some metaprogramming based on their EntityDefs, since PersistEntity has a method Proxy a -> EntityDef.

getDefinitions :: [EntityDef]
getDefinitions =
    map (\(SomeDictOf p) -> entityDef p) tables