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Introduction to GHC's Generics

Gautier DI FOLCO March 20, 2024 [dev] #haskell

In my previous log, I have focused on an annoying aeson, I have mentioned that it was involving some GHC's Generics code which were too complex for this log.

I had some feedbacks suggesting that it might be interesting to dive deeper in this subject, so here I am!

In brief, GHC's Generics is a mechanism allowing to write code depending on data-types type-level structure.

It is defined as this:

class Generic a where
  -- | Generic representation type
  type Rep a :: Type -> Type
  -- | Convert from the datatype to its representation
  from  :: a -> (Rep a) x
  -- | Convert from the representation to the datatype
  to    :: (Rep a) x -> a

Rep a gives a representation based on very specialized type (e.g. M1, K1, :+:, etc.), I have never made sense of it (even after 15-20 derivation mechanisms implementation), so, instead I rely on examples.

Usually you let the GHC deriving it:

data T
  = A0
  | A1 ()
  | A2 { a2f0 :: () }
  | A3 { a3f0 :: (), a3f1 :: () }
  deriving stock (Generic)

Let's have a look at Rep T in ghci (GHC's repl)

> :kind! Rep T
Rep T :: Type -> Type
= M1
    (MetaData "T" "Ghci3" "interactive" False)
    ((M1 C (MetaCons "A0" PrefixI False) U1
      :+: M1
            (MetaCons "A1" PrefixI False)
                  Nothing NoSourceUnpackedness NoSourceStrictness DecidedLazy)
               (K1 R ())))
     :+: (M1
            (MetaCons "A2" PrefixI True)
                  (Just "a2f0") NoSourceUnpackedness NoSourceStrictness DecidedLazy)
               (K1 R ()))
          :+: M1
                (MetaCons "A3" PrefixI True)
                      (Just "a3f0") NoSourceUnpackedness NoSourceStrictness DecidedLazy)
                   (K1 R ())
                 :*: M1
                          (Just "a3f1") NoSourceUnpackedness NoSourceStrictness DecidedLazy)
                       (K1 R ()))))

Overwhelming, I know, let's break this down.

I have the following heuristics: datatype are described by layers of SomeType metadataPhantomTypes actualTypes proxy.

Let's peel the onion:

It's quite verbose, but each type is simple by it-self.

Then we have to process them, let's illustrate that with an extract of the library I have described in my previous log:

class GToSumText f where
  gToSumText :: f a -> T.Text

It will be our type class to work on Generic types.

It will be used to define generic instances (based on from output):

  (Generic a, GToSumText (Rep a)) =>
  ToSumText (PureSum a)
  toSumText = gToSumText . from . unPureSum

We can start with the top-level case which is not interesting, so we only extract its value:

instance (GToSumText a) => GToSumText (M1 D meta a) where -- base type
  gToSumText (M1 x) = gToSumText x

Then we arrive at constructor-level, and since we only need constructor name, we can stop here and extract it through symbolVal which convert a type-level literal (string) to a value-level String:

instance (KnownSymbol cntr, EnsureEmpty a) => GToSumText (M1 C ('MetaCons cntr p b) a) where -- constructor
  gToSumText (M1 _) = T.pack $ symbolVal $ Proxy @cntr

Then, the last interesting case is the sum type:

instance (GToSumText a, GToSumText b) => GToSumText (a :+: b) where -- sum type
  gToSumText (R1 x) = gToSumText x
  gToSumText (L1 x) = gToSumText x

Now, what will happen ate runtime, let's take A0, which will be converted via from to:

M1 -- Datatype
  (L1 -- Left part of the sum type (:+:)
     (M1 -- "A0" constructor
        U1 -- No value

The heavy-lifting being done at instance selection.

It works well when you have the value, but when you only have the type, it's a bit more complex.

For instance, when we had to define FromJSON (PureSum a), we need type's name for withText :: String (Value -> Parser a) -> Parser a

instance (FromSumText a, Generic a, GConstructorName (Rep a)) => FromJSON (PureSumWith transformation a) where
  parseJSON = withText (getConst $ (to @a) <$> gConstructorName) pureSumWithParser

To do so, we can rely on Const, which as a non-operant Functor instance.

Not, it is not operant because it is a functor over a phantom type

newtype Const a b = Const { getConst :: a }

instance Functor (Const m) where
    fmap _ (Const v) = Const v

Finally we can simply fetch the literal:

class GConstructorName f where
  gConstructorName :: Const String (f a)

instance (KnownSymbol typeName) => GConstructorName (M1 D ('MetaData typeName c i b) a) where -- base type
  gConstructorName = Const $ symbolVal (Proxy @typeName)

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