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Last active Oct 4, 2019
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Yet Another Nix Type System (aka YANTS)


Note: Yants now has its own repository.

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


  • 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))!


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

Primitives & simple polymorphism




Nested structs!

nested structs





# 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 "${n}<${}>" (c t); };
poly2 = n: c: {
"${n}" = t1: t2: typedef "${n}<${},${}>" (c t1 t2);
typeSet = foldl' (s: t: s // (if t ? "name" then { "${}" = t; } else t)) {};
# Struct implementation. Checks that all fields match their declared
# types, no optional fields are missing and no unexpected fields
# occur in the struct.
# Anonymous structs are supported (e.g. for nesting) by omitting the
# name.
checkField = def: value: current: field:
let fieldVal = if hasAttr field value then value."${field}" else null;
type = def."${field}";
checked = type.check fieldVal;
in if checked then (current && true)
else if isNull fieldVal then (throw "Missing required ${} field '${field}'")
else (throw "Field ${field} is of type ${typeOf fieldVal}, but expected ${}");
checkExtraneous = name: def: present:
if (length present) == 0 then true
else if (hasAttr (head present) def)
then checkExtraneous name def (tail present)
else (throw "Found unexpected field '${head present}' in struct '${name}'");
struct' = name: def: {
inherit name def;
check = value:
let fieldMatch = foldl' (checkField def value) true (attrNames def);
noExtras = checkExtraneous name def (attrNames value);
in (isAttrs value && fieldMatch && noExtras);
__functor = self: value: if self.check value then value
else (throw "Expected '${}'-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 '${}'");
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} -> ${}")
"λ :: ${(head self.sig).name}" (tail self.sig);
__functor = _: f;
defun' = sig: func: if length sig > 2
then mkFunc sig (x: defun' (tail sig) (func ((head sig) x)))
else mkFunc sig (x: ((head (tail sig)) (func ((head sig) x))));
defun = sig: func: if length sig < 2
then (throw "Signature must at least have two types (a -> b)")
else defun' sig func;
in (typeSet [
# Primitive types
(typedef "any" (_: true))
(typedef "int" isInt)
(typedef "bool" isBool)
(typedef "float" isFloat)
(typedef "string" isString)
(typedef "derivation" (x: isAttrs x && x ? "type" && x.type == "derivation"))
(typedef "function" (x: isFunction x || (isAttrs x && x ? "__functor"
&& isFunction x.__functor)))
# Polymorphic types
(poly "option" (t: v: (isNull v) || t.check v))
(poly "list" (t: v: isList v && (foldl' (s: e: s && (
if t.check e then true
else throw "Expected list element of type '${}', 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 '${}', but '${toPretty e}' is of type '${typeOf e}'"
)) true (attrValues v))))
(poly2 "either" (t1: t2: v: t1.check v || t2.check v))
]) // { inherit struct enum defun; }
with (import ./yants.nix {});
with builtins;
# Note: Derivations are not included in the tests below as they cause
# issues with deepSeq.
deepSeq rec {
# Test that all primitive types match
primitives = [
(int 15)
(bool false)
(float 13.37)
(string "Hello!")
(function (x: x * 2))
# Test that polymorphic types work as intended
poly = [
(option int null)
(list string [ "foo" "bar" ])
(either int float 42)
# Test that structures work as planned.
person = struct "person" {
name = string;
age = int;
contact = option (struct {
email = string;
phone = option string;
testPerson = person {
name = "Brynhjulf";
age = 42; = "brynhjulf@yants.nix";
# Test enum definitions & matching
colour = enum "colour" [ "red" "blue" "green" ];
testMatch = colour.match "red" {
red = "It is in fact red!";
blue = throw "It should not be blue!";
green = throw "It should not be green!";
# Test curried function definitions
func = defun [ string int string ]
(name: age: "${name} is ${toString age} years old");
testFunc = func "Brynhjulf" 42;
} "All tests passed!\n"
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