Derix

Managing packages with class.

Typeclass instance resolution as package dependency resolution.

Derix explores a structural correspondence: package resolution and typeclass resolution are the same problem. Both are proof search in a lazy evidence environment. A resolved package set (nixpkgs) and a resolved dictionary environment (GHC) are both lazy fixed points of a self-referential record of evidence thunks.

The name is a triple pun: Nix derivation (build recipe), logical derivation (proof tree), Haskell deriving (automatic instance generation) — all the same object, in the Nix ecosystem.

The current design is in doc/encoding.md.

callPackage is a class method. pkgs is a phantom type. Dependencies are constraints. Instance resolution IS dependency resolution.

class Package (name :: Symbol) where
 type Deps name (pkgs :: Type) :: Constraint
 type Output name :: Type
 type Output name = Derivation name
 callPackage :: forall pkgs -> Deps name pkgs => Output name

Core types:

• Drv — the raw .drv store derivation (what the Nix daemon builds)
• Derivation name — existential + phantom-tagged wrapper (carries extra info, prevents accidental swaps at compile time)
• Has name pkgs — package set membership witness
• HasDrv — class bridging rich types to raw Drv

Nix Haskell
──────────────────────────────── ────────────────────────────────
{ stdenv, zlib }: type Deps "openssl" pkgs =
 stdenv.mkDerivation { ... } (Has "stdenv" pkgs, Has "zlib" pkgs)
callPackage ./openssl.nix {} callPackage @"openssl" @Nixpkgs
pkgs.stdenv.mkDerivation callPackage @"stdenv" @Nixpkgs
 (a function in pkgs) :: MkDerivationArgs -> Drv
missing argument error No instance for Has "foo" Nixpkgs
 (at eval time) (at compile time)

• doc/calculus.md — formal resolution calculus (syntax, inference rules, call-by-need semantics, lazy fixed points, overlays, coherence, metatheory)
• doc/design-space.md — survey of GHC features mapped to package management concepts

Actively exploring the design space.

MIT