stock

Constraint solving plugin that enables extensible, composable deriving without GHC.Generics.

It enables deriving through a Stock(1,2) newtype that the plugin generates instances for at compile-time: Cls (Stock A). This synthesises class instances directly in GHC Core, same as hand-written, with no Generic deriving.

The classes each wrapper synthesises:

• Stock: Eq, Ord, Show, Read, Semigroup, Monoid, Enum, Bounded, Ix, Generic (that's right)
• Stock1: Functor / Contravariant, Foldable, Applicative, Generic1, Eq1, Ord1, Show1, Read1, Traversable†
• Stock2: Bifunctor, Bifoldable, Eq2, Ord2, Show2, Read2, Category, Bitraversable†

(unpack / unboxed-strict fields are rejected, with a clear error)

† Traversable / Bitraversable are synthesized at the wrapper (Stock1 F / Stock2 P) and usable directly, or put on your own type with a one-liner:

instance Traversable F where
  traverse g = fmap unStock1 . traverse g . Stock1

A bare deriving via Stock1 F can't reach them: traverse's result f (t b) puts the wrapper under an abstract applicative f, and coercing under an abstract f (nominal role) is unsound — the same wall that stops GeneralizedNewtypeDeriving. The instance itself is perfectly ordinary (it's what GHC's own DeriveTraversable builds), so the one-liner — which re-wraps with a real fmap, not a coercion — gives you the instance, and it honours Override1 / Override2 (which deriving stock Traversable can't).

{-# options_ghc -fplugin Stock #-}

{-# language DerivingVia #-}

import Stock

data Colour = Red | Green | Blue
  deriving (Eq, Ord, Show, Read, Enum, Bounded, Ix) via 
    Stock Colour

data Tree a = Leaf | Node (Tree a) a (Tree a)
  deriving (Eq, Ord, Show) via 
    Stock (Tree a)

data Trio a = Trio Int a [a]
  deriving (Functor, Foldable) via 
    Stock1 Trio

The plugin must be enabled (-fplugin Stock, in ghc-options or a per-file options_ghc).

How it works

For a wanted Cls (Stock T) the plugin unwraps the newtype with its coercion, matches T's constructors and builds the Cls-class dictionary directly as Core, requesting each field's own instance as a fresh wanted (GHC solves Eq Int etc. itself). It's direct synthesis of the wrapped constraint, not delegation.

Per-field modifiers

newtype Override a config = Override a

Override reshapes individual fields during synthesis by specifying their behaviour (zero-cost).

data One = One { x :: Int, y :: Int }
  deriving (Semigroup, Monoid)
  via Overriding One
    [ x via Sum, y via Product ]

combines x additively and y multiplicatively.

There are a few methods of addressing a field, the plugin allows minor notational conveniences.

• x via F ("x" := F), modifies the field with the F wrapper
• Int via F (Int := F), modifies every Int field
• Con at 0 via F (At Con 0 := F), modifies field 0 of constructor
• 'Con --> F, a path: modifies every field of Con; 'Con --> 0 --> F only its field 0. Each non-terminal hop is a constructor, a position (Nat) or a label (Symbol); the terminal hop is the modifier
• '[ [F, _] ] ('[ [F, Keep] ]), modifies field 0 only, _ keeps field

Each modifier is either particular via field F Int, a constructor to modify F or a blank _ (or Keep) which leaves a field untouched. A function-typed modifier needs no parentheses: x via a -> f b reads as x := (a -> f b), since via binds looser than ->.

The surface plugin. The lowercase, quote-free surface (x via Sum, Con at 0, the _ blank) is lowered to the honest marker form the solver reads ("x" := Sum, At Con 0, Keep) by the same -fplugin Stock at parse time (Stock.Surface). The generated markers (:=, At, Keep) are qualified to match however you imported Stock.Override, so import Stock.Override qualified as O with O.Override … '[ … via Sum, _ ] resolves too.

Higher-order

Override1 / Override2 lift the same idea over type constructors instead of types.

data Zip a = Zip [a]
  deriving 
    (Functor, Applicative, Foldable) 
  via
  Overriding1 Zip '[ '[ZipList] ]

Adding a class

A companion package introduces a new class Cls for synthesis by writing a DeriveStock Cls instance.

The -fplugin Stock plugin discovers it: looks up the instance, loads the deriver with GHC's own plugin loader, and runs it for deriving Cls via Stock T.

instance DeriveStock Semigroup where 
  deriveStock :: Deriver
  deriveStock = Deriver \cls datatype -> do
    let (<+>) = head (classMethods cls)
    a <- fresh (dtVia datatype) "a"
    b <- fresh (dtVia datatype) "b"
    body <- fromProduct datatype (dtVia datatype) (Var a) \xs ->    -- (match)  a = C x..
            fromProduct datatype (dtVia datatype) (Var b) \ys ->    -- (match)  b = C y..
            toProduct datatype <$> czipFields cls                   -- (build)  C (x <> y)..
              (\ft d x y -> mkApps (Var (<+>)) [Type ft, d, x, y]) (productCon datatype) xs ys
    EvExpr <$> classDictWith cls (dtVia datatype) [] [(0, mkLams [a, b] body)]

The deriver must live in a different module from where it's used. The plugin loads it as compiled code, same-module instances won't work. A normal dependency (separate package, or just a separate module built with -dynamic-too) works.

Performance

cabal bench bench runs identical workloads against a type defined three ways: via Stock, GHC's stock deriving, and hand-written. All three give matching checksums and run within noise of each other — verified by rebuilding and re-running on GHC 9.8 – 9.14:

Ord: sort 100000 3-field records      via Stock 0.075s  stock 0.074s  hand 0.073s
Functor: fmap (+1) x50 over 100000    via Stock 0.136s  stock 0.136s  hand 0.137s

Conclusions / realizations

Using inspection-testing shows that the code we generate is byte-identical to stock (==-). Eq, Ord, Enum, Functor optimise to the same Core as GHC's own deriving on a twin type.

Read (and Read1) build readPrec exactly as GHC's derived Read does — the same ReadPrec combinators — and let readsPrec come from the class default, so the result is byte-faithful including the order of ambiguous infix parses. A parity harness (test/Twin.hs) checks the full readsPrec output against GHC's own derived Read on a name-identical twin, over valid / whitespace / parenthesised / negative / garbage inputs at several precedences.

Acknowledgments

Developed with substantial assistance from Claude (Anthropic).

License

BSD-3-Clause.