Yet Another Nix Type System (aka YANTS)

yants.md

yants

Note: Yants now has its own repository.

---

This is a tiny type-checker for data in Nix, written in Nix.

Features:

• Checking of primitive types (int, string etc.)
• Checking polymorphic types (option, list, either)
• Defining & checking struct/record types
• Defining & matching enum types
• Defining function signatures (including curried functions)
• Types are composable! option string! list (either int (option float))!

Lacking:

• Any kind of inference (Nix's reflection ability is not strong enough)
• Convenient syntax for attribute-set function signatures

Primitives & simple polymorphism

Structs

Nested structs!

Enums!

Functions!

yants.nix

# Copyright 2019 Google LLC
# SPDX-License-Identifier: Apache-2.0
#
# Provides a "type-system" for Nix that provides various primitive &
# polymorphic types as well as the ability to define & check records.
#
# All types (should) compose as expected.

{ toPretty ? ((import <nixpkgs> {}).lib.generators.toPretty {}) }:

with builtins; let
  typeError = type: val:
  throw "Expected type '${type}', but value '${toPretty val}' is of type '${typeOf val}'";

  typedef = name: check: {
    inherit name check;
    __functor = self: value:
      if check value then value
      else typeError name value;
  };

  poly = n: c: { "${n}" = t: typedef "${n}<${t.name}>" (c t); };

  poly2 = n: c: {
    "${n}" = t1: t2: typedef "${n}<${t1.name},${t2.name}>" (c t1 t2);
  };

  typeSet = foldl' (s: t: s // (if t ? "name" then { "${t.name}" = t; } else t)) {};

  # Struct implementation. Checks that all fields match their declared
  # types, no optional fields are missing and no unexpected fields
  # occur in the struct.
  #
  # Anonymous structs are supported (e.g. for nesting) by omitting the
  # name.
  checkField = def: value: current: field:
  let fieldVal = if hasAttr field value then value."${field}" else null;
      type = def."${field}";
      checked = type.check fieldVal;
  in if checked then (current && true)
     else if isNull fieldVal then (throw "Missing required ${type.name} field '${field}'")
          else  (throw "Field ${field} is of type ${typeOf fieldVal}, but expected ${type.name}");

  checkExtraneous = name: def: present:
  if (length present) == 0 then true
  else if (hasAttr (head present) def)
    then checkExtraneous name def (tail present)
    else (throw "Found unexpected field '${head present}' in struct '${name}'");

  struct' = name: def: {
    inherit name def;
    check = value:
      let fieldMatch = foldl' (checkField def value) true (attrNames def);
          noExtras = checkExtraneous name def (attrNames value);
      in (isAttrs value && fieldMatch && noExtras);

    __functor = self: value: if self.check value then value
      else (throw "Expected '${self.name}'-struct, but ${toPretty value} is of type ${typeOf value}");
  };

  struct = arg: if isString arg then (struct' arg)
                else (struct' "anonymous" arg);

  enum = name: values: rec {
    inherit name values;
    check = (x: elem x values);
    __functor = self: x: if self.check x then x
    else (throw "'${x}' is not a member of enum '${self.name}'");
    match = x: actions: let
      actionKeys = map (__functor { inherit name check; }) (attrNames actions);
      missing = foldl' (m: k: if (elem k actionKeys) then m else m ++ [ k ]) [] values;
    in if (length missing) > 0
       then throw "Missing match action for members: ${toPretty missing}"
       else actions."${__functor { inherit name check; } x}";
  };

  mkFunc = sig: f: {
    inherit sig;
    __toString = self: foldl' (s: t: "${s} -> ${t.name}")
                              "λ :: ${(head self.sig).name}" (tail self.sig);
    __functor = _: f;
  };
  defun' = sig: func: if length sig > 2
    then mkFunc sig (x: defun' (tail sig) (func ((head sig) x)))
    else mkFunc sig (x: ((head (tail sig)) (func ((head sig) x))));

  defun = sig: func: if length sig < 2
    then (throw "Signature must at least have two types (a -> b)")
    else defun' sig func;
in (typeSet [
  # Primitive types
  (typedef "any" (_: true))
  (typedef "int" isInt)
  (typedef "bool" isBool)
  (typedef "float" isFloat)
  (typedef "string" isString)
  (typedef "derivation" (x: isAttrs x && x ? "type" && x.type == "derivation"))
  (typedef "function" (x: isFunction x || (isAttrs x && x ? "__functor"
                                           && isFunction x.__functor)))
  # Polymorphic types
  (poly "option" (t: v: (isNull v) || t.check v))

  (poly "list" (t: v: isList v && (foldl' (s: e: s && (
    if t.check e then true
    else throw "Expected list element of type '${t.name}', but '${toPretty e}' is of type '${typeOf e}'"
  )) true v)))

  (poly "attrs" (t: v: isAttrs v && (foldl' (s: e: s && (
    if t.check e then true
    else throw "Expected attribute set element of type '${t.name}', but '${toPretty e}' is of type '${typeOf e}'"
  )) true (attrValues v))))

  (poly2 "either" (t1: t2: v: t1.check v || t2.check v))
]) // { inherit struct enum defun; }

z-enums.png

z-functions.png

z-nested-structs.png

z-simple.png

z-structs.png

z-yants-tests.nix

with (import ./yants.nix {});
with builtins;

# Note: Derivations are not included in the tests below as they cause
# issues with deepSeq.

deepSeq rec {
  # Test that all primitive types match
  primitives = [
    (int 15)
    (bool false)
    (float 13.37)
    (string "Hello!")
    (function (x: x * 2))
  ];

  # Test that polymorphic types work as intended
  poly = [
    (option int null)
    (list string [ "foo" "bar" ])
    (either int float 42)
  ];

  # Test that structures work as planned.
  person = struct "person" {
    name = string;
    age  = int;

    contact = option (struct {
      email = string;
      phone = option string;
    });
  };

  testPerson = person {
    name = "Brynhjulf";
    age  = 42;
    contact.email = "[email protected]";
  };

  # Test enum definitions & matching
  colour = enum "colour" [ "red" "blue" "green" ];
  testMatch = colour.match "red" {
    red = "It is in fact red!";
    blue = throw "It should not be blue!";
    green = throw "It should not be green!";
  };

  # Test curried function definitions
  func = defun [ string int string ]
  (name: age: "${name} is ${toString age} years old");

  testFunc = func "Brynhjulf" 42;
} "All tests passed!\n"