What is nova-nix?

A pure Haskell implementation of Nix that treats Windows as a first-class target:

• Parser — Hand-rolled recursive descent parser for the full Nix expression language. 13 precedence levels, 17 AST constructors, all syntax forms including search paths (<nixpkgs>) and dynamic attribute keys ({ ${expr} = val; }). Direct Text consumption for maximum throughput.
• Lazy Evaluator — Thunk-based evaluation with environment closures, knot-tying for recursive bindings via Haskell laziness. All 17 AST constructors handled: literals, strings with interpolation, attribute sets (recursive and non-recursive), let bindings, lambdas with formal parameters, if/then/else, with, assert, unary/binary operators, function application, list construction, attribute selection, has-attribute checks, and search path resolution.
• 106 Built-in Functions — Type checks, arithmetic (min, max, mod), bitwise, strings, lists, attribute sets, higher-order (map, filter, foldl', sort, genList, concatMap, mapAttrs), JSON (toJSON/fromJSON), hashing (SHA-256/SHA-512/SHA-1/MD5), base64 (encode/decode), version parsing, replaceStrings, tryEval, deepSeq, genericClosure, setFunctionArgs/functionArgs, string context introspection (hasContext, getContext, appendContext), IO builtins (import, readFile, pathExists, readDir, getEnv, toPath, toFile, findFile, scopedImport, fetchurl, fetchTarball, fetchGit), derivation, placeholder, storePath, and more. 16 builtins available at top level without builtins. prefix (toString, map, throw, import, derivation, abort, baseNameOf, dirOf, isNull, removeAttrs, placeholder, scopedImport, fetchTarball, fetchGit, fetchurl, toFile) — matching the real Nix language spec.
• Search Path Resolution — <nixpkgs> desugars to builtins.findFile builtins.nixPath "nixpkgs" — matching real Nix semantics. NIX_PATH environment variable is parsed at startup, and --nix-path CLI flags merge with it. Directory imports (import ./dir) resolve to dir/default.nix automatically.
• Dynamic Attribute Keys — { ${expr} = val; } fully supported in all contexts: non-recursive attrs, recursive attrs, let bindings, attribute selection, and has-attribute checks. Key resolution is cleanly separated from value thunk construction to preserve knot-tying in recursive bindings.
• String Context Tracking — Every string carries invisible metadata tracking which store paths it references. Context propagates through interpolation, concatenation, replaceStrings, and all string operations. The derivation builtin collects contexts into drvInputDrvs and drvInputSrcs — matching real Nix semantics.
• Content-Addressed Store — /nix/store on Unix, C:\nix\store on Windows, with real SQLite metadata tracking (ValidPaths + Refs tables, WAL mode)
• Derivation Builder — Full build loop with recursive dependency resolution: topological sort via Kahn's algorithm, binary cache substitution before local builds, input validation, reference scanning, output registration
• Binary Substituter — HTTP binary cache protocol: narinfo fetch + parse, Ed25519 signature verification, NAR download/decompress/unpack, store registration. Priority-ordered multi-cache support. Built on nova-cache.
• ATerm Serialization — Full round-trip .drv serialization and parsing with string escape handling

Every module is pure by default. IO lives at the boundaries only.

Try It

git clone https://github.com/Novavero-AI/nova-nix.git
cd nova-nix
cabal run nova-nix -- --strict eval test.nix

Output:

{ count = 6; greeting = "Hello, nova-nix!"; items = [ 2 4 6 8 10 ]; nested = { a = 1; b = 2; c = 4; }; types = { attrs = "set"; int = "int"; list = "list"; string = "string"; }; }

That's a Nix expression with let bindings, rec attrs, lambdas, builtins.map, builtins.typeOf, and arithmetic — parsed, lazily evaluated, and pretty-printed. On Windows, macOS, or Linux.

CLI

nova-nix eval FILE.nix                        # Evaluate a .nix file, print result
nova-nix eval --expr 'EXPR'                   # Evaluate an inline expression
nova-nix build FILE.nix                       # Build a derivation from a .nix file
nova-nix --nix-path nixpkgs=/path eval FILE   # Add search paths (repeatable)

Evaluate

$ nova-nix --strict eval test.nix
{ count = 6; greeting = "Hello, nova-nix!"; items = [ 2 4 6 8 10 ]; nested = { a = 1; b = 2; c = 4; }; types = { attrs = "set"; int = "int"; list = "list"; string = "string"; }; }

Inline expressions:

$ nova-nix eval --expr '1 + 2'
3

$ nova-nix eval --expr 'builtins.map (x: x * x) [1 2 3 4 5]'
[ 1 4 9 16 25 ]

$ nova-nix eval --expr '{ x = 1; y = 2; }.x + { x = 1; y = 2; }.y'
3

Search paths:

$ nova-nix --nix-path nixpkgs=/path/to/nixpkgs eval --expr 'import <nixpkgs> {}'

$ NIX_PATH=nixpkgs=/path/to/nixpkgs nova-nix eval --expr 'import <nixpkgs> {}'

Build

$ cat > hello.nix <<'EOF'
derivation {
  name = "hello";
  system = builtins.currentSystem;
  builder = "/bin/sh";
  args = [ "-c" "mkdir -p $out && echo 'Hello from nova-nix!' > $out/greeting.txt" ];
}
EOF

$ nova-nix build hello.nix
/nix/store/abc...-hello

The build command evaluates the .nix file, extracts the derivation, builds the full dependency graph, topologically sorts it, checks binary caches for substitutes, builds anything missing locally, and registers all outputs in the store DB.

Quick Start

Add to your .cabal file:

build-depends: nova-nix

Parse a Nix Expression

import Nix.Parser (parseNix)
import Nix.Expr.Types

main :: IO ()
main = do
  case parseNix "<stdin>" "let x = 1 + 2; in x" of
    Left err   -> print err
    Right expr -> print expr
    -- ELet [NamedBinding [StaticKey "x"]
    --        (EBinary OpAdd (ELit (NixInt 1)) (ELit (NixInt 2)))]
    --      (EVar "x")

Evaluate an Expression

import Nix.Parser (parseNix)
import Nix.Eval (eval, PureEval(..), NixValue(..))
import Nix.Builtins (builtinEnv)

main :: IO ()
main = do
  case parseNix "<stdin>" "let x = 5; y = x * 2; in y + 1" of
    Left err -> print err
    Right expr -> case runPureEval (eval (builtinEnv 0 []) expr) of
      Left err  -> putStrLn ("Error: " ++ show err)
      Right val -> print val  -- VInt 11

The evaluator is polymorphic via MonadEval — PureEval runs without IO, while EvalIO can access the filesystem for import, readFile, etc.

Lazy Evaluation in Action

-- Nix is lazy: unused bindings are never evaluated
-- runPureEval (eval (builtinEnv 0 []) expr) where expr parses:
--   "let unused = builtins.throw \"boom\"; x = 42; in x"
-- Right (VInt 42)  —  "boom" is never triggered

-- Recursive attribute sets with self-reference
--   "rec { a = 1; b = a + 1; c = b * 2; }.c"
-- Right (VInt 4)

-- Lambda closures, set patterns with defaults
--   "({ name, greeting ? \"Hello\" }: \"${greeting}, ${name}!\") { name = \"Nix\"; }"
-- Right (VStr "Hello, Nix!")

Modules

Parser

Evaluator

Store + Builder

Architecture

                     Pure Core (no IO)
  +-------------------------------------------------+
  |                                                 |
  |  Parser --> Expr.Types --> Eval --> Builtins     |
  |                 |           |                    |
  |          Parser.Lexer    Eval.Types              |
  |          Parser.Expr     Eval.Operator           |
  |          Parser.Internal Eval.StringInterp       |
  |          ParseError      Eval.Context            |
  |                             |                    |
  |                        Derivation --> Hash        |
  |                             |                    |
  |                    Store.Path  DependencyGraph    |
  |                                                 |
  +-------------------------------------------------+
                        |
               IO Boundary (thin)
  +-------------------------------------------------+
  |  Eval.IO   Store.DB   Store   Builder   Substituter|
  +-------------------------------------------------+

Evaluator design:

• MonadEval typeclass — The evaluator is eval :: (MonadEval m) => Env -> Expr -> m NixValue, polymorphic in its effect monad. PureEval (newtype over Either Text) runs all pure tests with no IO. EvalIO provides readFileText, doesPathExist, listDirectory, getEnvVar, getCurrentTime, writeToStore, scopedImportFile, runProcess for IO builtins.
• Thunk-based lazy evaluation with memoization — List elements and attribute set values are stored as unevaluated thunks (Thunk Expr Env). Only forced when a value is demanded. (x: 1) (throw "boom") returns 1 because x is never referenced. In EvalIO, each thunk carries a per-thunk IORef memo cell — forced once, then cached in place (matching real Nix's in-place mutation). Dead thunks are reclaimed by GC naturally.
• Knot-tying via Haskell laziness — Recursive let and rec { } create self-referential environments. The Thunk type has a lazy Env field so thunks can capture environments that include themselves. Haskell's own laziness resolves the recursion. Dynamic attribute keys are resolved monadi­cally before knot-tying — the two-phase design (resolveBindingKeys then buildResolvedBindingsMap) cleanly separates key evaluation from value thunk construction.
• Search path desugaring — <nixpkgs> is its own AST constructor (ESearchPath), desugared at eval time to builtins.findFile builtins.nixPath "nixpkgs" — exactly how real Nix handles it. builtins.nixPath is populated from NIX_PATH and --nix-path flags.
• With-scope chain — Env has lexical bindings (always win) plus a stack of with-scopes walked innermost-first. let a = 1; in with { a = 2; }; a correctly returns 1 because lexical scope takes priority.
• Short-circuit operators — &&, ||, and -> are handled directly in eval (not delegated to Operator) because they must not evaluate both operands.
• String context propagation — Every VStr carries a StringContext tracking store path references (SCPlain, SCDrvOutput, SCAllOutputs). Context merges through interpolation, concatenation, and string builtins. The derivation builtin collects all context into drvInputDrvs/drvInputSrcs.

Build pipeline:

1. Evaluate .nix file to extract derivation
2. Build dependency graph by reading .drv files from the store (BFS traversal)
3. Topologically sort via Kahn's algorithm — leaves first, cycle detection
4. For each dependency in build order: check store cache, try binary substitution, build locally
5. Build execution: validate inputs, set up environment, run builder process, scan references, register outputs in SQLite DB

Key numbers:

• 22 modules — all implemented
• 511 tests — hand-rolled harness, no framework dependencies
• Zero partial functions — total by construction, T.uncons over T.head/T.tail
• Strict by default — bang patterns on all data fields (except Thunk's Env, which is lazy for knot-tying)

The Hard Problems

Building Nix on Windows means solving real platform differences:

The biggest challenge isn't any single feature — it's nixpkgs compatibility. nixpkgs is 80,000+ packages defined as one massive recursive attrset. It exercises every builtin, every edge case in string context tracking, and every lazy evaluation pattern. The evaluator must handle all of this correctly and fast enough (~2-5 seconds for full nixpkgs eval).

Roadmap

Next

• Full import <nixpkgs> {} performance — nixpkgs lib layer evaluates correctly; stdenv bootstrap runs but needs memory optimization for the full 80,000+ package set (currently OOM at 2GB, investigating space leak)
• nova-nix shell — Enter a development shell (like nix shell)
• nova-nix repl — Interactive evaluator

Long-Term

• Nix daemon protocol compatibility
• XZ decompression — Enable nova-cache compression flag for real binary cache downloads
• Store bootstrap — Ship prebuilt bash + coreutils for Windows builds

Build & Test

cabal build                              # Build library + CLI
cabal test                               # Run all 511 tests
cabal build --ghc-options="-Werror"      # Warnings as errors (CI default)
cabal haddock                            # Generate API docs

Requires GHC 9.6 and cabal-install 3.10+.

_{MIT License · Novavero AI}