merge(yants): Integrate yants into depot at //depot/nix/yants

This commit is contained in:
Vincent Ambo 2019-12-20 21:47:38 +00:00
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Yants subtree contains no further derivations.

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yants
=====
[![Build Status](https://travis-ci.org/tazjin/yants.svg?branch=master)](https://travis-ci.org/tazjin/yants)
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 & matching sum types
* Defining function signatures (including curried functions)
* Types are composable! `option string`! `list (either int (option float))`!
* Type errors also compose!
Currently lacking:
* Any kind of inference
* Convenient syntax for attribute-set function signatures
## Primitives & simple polymorphism
![simple](screenshots/simple.png)
## Structs
![structs](screenshots/structs.png)
## Nested structs!
![nested structs](screenshots/nested-structs.png)
## Enums!
![enums](screenshots/enums.png)
## Functions!
![functions](screenshots/functions.png)
# Usage
Yants can be imported from its `default.nix`. A single attribute (`lib`) can be
passed, which will otherwise be imported from `<nixpkgs>`.
Examples for the most common import methods would be:
1. Import into scope with `with`:
```nix
with (import ./default.nix {});
# ... Nix code that uses yants ...
```
2. Import as a named variable:
```nix
let yants = import ./default.nix {};
in yants.string "foo" # or other uses ...
````
3. Overlay into `pkgs.lib`:
```nix
# wherever you import your package set (e.g. from <nixpkgs>):
import <nixpkgs> {
overlays = [
(self: super: {
lib = super.lib // { yants = import ./default.nix { inherit (super) lib; }; };
})
];
}
# yants now lives at lib.yants, besides the other library functions!
```
Please see my [Nix one-pager](https://github.com/tazjin/nix-1p) for more generic
information about the Nix language and what the above constructs mean.
# Stability
The current API of Yants is **not yet** considered stable, but it works fine and
should continue to do so even if used at an older version.
Yants' tests use Nix versions above 2.2 - compatibility with older versions is
not guaranteed.

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# 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.
{ lib ? (import <nixpkgs> {}).lib }:
with builtins; let
prettyPrint = lib.generators.toPretty {};
# typedef' :: struct {
# name = string;
# checkType = function; (a -> result)
# checkToBool = option function; (result -> bool)
# toError = option function; (a -> result -> string)
# def = option any;
# match = option function;
# } -> type
# -> (a -> b)
# -> (b -> bool)
# -> (a -> b -> string)
# -> type
#
# This function creates an attribute set that acts as a type.
#
# It receives a type name, a function that is used to perform a
# check on an arbitrary value, a function that can translate the
# return of that check to a boolean that informs whether the value
# is type-conformant, and a function that can construct error
# messages from the check result.
#
# This function is the low-level primitive used to create types. For
# many cases the higher-level 'typedef' function is more appropriate.
typedef' = { name, checkType
, checkToBool ? (result: result.ok)
, toError ? (_: result: result.err)
, def ? null
, match ? null }: {
inherit name checkToBool toError;
# check :: a -> bool
#
# This function is used to determine whether a given type is
# conformant.
check = value: checkToBool (checkType value);
# checkType :: a -> struct { ok = bool; err = option string; }
#
# This function checks whether the passed value is type conformant
# and returns an optional type error string otherwise.
inherit checkType;
# __functor :: a -> a
#
# This function checks whether the passed value is type conformant
# and throws an error if it is not.
#
# The name of this function is a special attribute in Nix that
# makes it possible to execute a type attribute set like a normal
# function.
__functor = self: value:
let result = self.checkType value;
in if checkToBool result then value
else throw (toError value result);
};
typeError = type: val:
"expected type '${type}', but value '${prettyPrint val}' is of type '${typeOf val}'";
# typedef :: string -> (a -> bool) -> type
#
# typedef is the simplified version of typedef' which uses a default
# error message constructor.
typedef = name: check: typedef' {
inherit name;
checkType = check;
checkToBool = r: r;
toError = value: _result: typeError name value;
};
checkEach = name: t: l: foldl' (acc: e:
let res = t.checkType e;
isT = t.checkToBool res;
in {
ok = acc.ok && isT;
err = if isT
then acc.err
else acc.err + "${prettyPrint e}: ${t.toError e res}\n";
}) { ok = true; err = "expected type ${name}, but found:\n"; } l;
in lib.fix (self: {
# Primitive types
any = typedef "any" (_: true);
int = typedef "int" isInt;
bool = typedef "bool" isBool;
float = typedef "float" isFloat;
string = typedef "string" isString;
path = typedef "path" (x: typeOf x == "path");
drv = typedef "derivation" (x: isAttrs x && x ? "type" && x.type == "derivation");
function = typedef "function" (x: isFunction x || (isAttrs x && x ? "__functor"
&& isFunction x.__functor));
# Type for types themselves. Useful when defining polymorphic types.
type = typedef "type" (x:
isAttrs x
&& hasAttr "name" x && self.string.check x.name
&& hasAttr "checkType" x && self.function.check x.checkType
&& hasAttr "checkToBool" x && self.function.check x.checkToBool
&& hasAttr "toError" x && self.function.check x.toError
);
# Polymorphic types
option = t: typedef' rec {
name = "option<${t.name}>";
checkType = v:
let res = t.checkType v;
in {
ok = isNull v || (self.type t).checkToBool res;
err = "expected type ${name}, but value does not conform to '${t.name}': "
+ t.toError v res;
};
};
eitherN = tn: typedef "either<${concatStringsSep ", " (map (x: x.name) tn)}>"
(x: any (t: (self.type t).check x) tn);
either = t1: t2: self.eitherN [ t1 t2 ];
list = t: typedef' rec {
name = "list<${t.name}>";
checkType = v: if isList v
then checkEach name (self.type t) v
else {
ok = false;
err = typeError name v;
};
};
attrs = t: typedef' rec {
name = "attrs<${t.name}>";
checkType = v: if isAttrs v
then checkEach name (self.type t) (attrValues v)
else {
ok = false;
err = typeError name v;
};
};
# Structs / record types
#
# 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.
#
# TODO: Support open records?
struct =
# Struct checking is more involved than the simpler types above.
# To make the actual type definition more readable, several
# helpers are defined below.
let
# checkField checks an individual field of the struct against
# its definition and creates a typecheck result. These results
# are aggregated during the actual checking.
checkField = def: name: value: let result = def.checkType value; in rec {
ok = def.checkToBool result;
err = if !ok && isNull value
then "missing required ${def.name} field '${name}'\n"
else "field '${name}': ${def.toError value result}\n";
};
# checkExtraneous determines whether a (closed) struct contains
# any fields that are not part of the definition.
checkExtraneous = def: has: acc:
if (length has) == 0 then acc
else if (hasAttr (head has) def)
then checkExtraneous def (tail has) acc
else checkExtraneous def (tail has) {
ok = false;
err = acc.err + "unexpected struct field '${head has}'\n";
};
# checkStruct combines all structure checks and creates one
# typecheck result from them
checkStruct = def: value:
let
init = { ok = true; err = ""; };
extraneous = checkExtraneous def (attrNames value) init;
checkedFields = map (n:
let v = if hasAttr n value then value."${n}" else null;
in checkField def."${n}" n v) (attrNames def);
combined = foldl' (acc: res: {
ok = acc.ok && res.ok;
err = if !res.ok then acc.err + res.err else acc.err;
}) init checkedFields;
in {
ok = combined.ok && extraneous.ok;
err = combined.err + extraneous.err;
};
struct' = name: def: typedef' {
inherit name def;
checkType = value: if isAttrs value
then (checkStruct (self.attrs self.type def) value)
else { ok = false; err = typeError name value; };
toError = _: result: "expected '${name}'-struct, but found:\n" + result.err;
};
in arg: if isString arg then (struct' arg) else (struct' "anon" arg);
# Enums & pattern matching
enum =
let
plain = name: def: typedef' {
inherit name def;
checkType = (x: isString x && elem x def);
checkToBool = x: x;
toError = value: _: "'${prettyPrint value} is not a member of enum ${name}";
};
enum' = name: def: lib.fix (e: (plain name def) // {
match = x: actions: deepSeq (map e (attrNames actions)) (
let
actionKeys = attrNames actions;
missing = foldl' (m: k: if (elem k actionKeys) then m else m ++ [ k ]) [] def;
in if (length missing) > 0
then throw "Missing match action for members: ${prettyPrint missing}"
else actions."${e x}");
});
in arg: if isString arg then (enum' arg) else (enum' "anon" arg);
# Sum types
#
# The representation of a sum type is an attribute set with only one
# value, where the key of the value denotes the variant of the type.
sum =
let
plain = name: def: typedef' {
inherit name def;
checkType = (x:
let variant = elemAt (attrNames x) 0;
in if isAttrs x && length (attrNames x) == 1 && hasAttr variant def
then let t = def."${variant}";
v = x."${variant}";
res = t.checkType v;
in if t.checkToBool res
then { ok = true; }
else {
ok = false;
err = "while checking '${name}' variant '${variant}': "
+ t.toError v res;
}
else { ok = false; err = typeError name x; }
);
};
sum' = name: def: lib.fix (s: (plain name def) // {
match = x: actions:
let variant = deepSeq (s x) (elemAt (attrNames x) 0);
actionKeys = attrNames actions;
defKeys = attrNames def;
missing = foldl' (m: k: if (elem k actionKeys) then m else m ++ [ k ]) [] defKeys;
in if (length missing) > 0
then throw "Missing match action for variants: ${prettyPrint missing}"
else actions."${variant}" x."${variant}";
});
in arg: if isString arg then (sum' arg) else (sum' "anon" arg);
# Typed function definitions
#
# These definitions wrap the supplied function in type-checking
# forms that are evaluated when the function is called.
#
# Note that typed functions themselves are not types and can not be
# used to check values for conformity.
defun =
let
mkFunc = sig: f: {
inherit sig;
__toString = self: foldl' (s: t: "${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))));
in sig: func: if length sig < 2
then (throw "Signature must at least have two types (a -> b)")
else defun' sig func;
})

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with builtins;
with (import ./default.nix {});
# 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))
(path /nix)
];
# 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 = "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 sum type definitions
creature = sum "creature" {
human = struct {
name = string;
age = option int;
};
pet = enum "pet" [ "dog" "lizard" "cat" ];
};
testSum = creature {
human = {
name = "Brynhjulf";
age = 42;
};
};
testSumMatch = creature.match testSum {
human = v: "It's a human named ${v.name}";
pet = v: throw "It's not supposed to be a pet!";
};
# Test curried function definitions
func = defun [ string int string ]
(name: age: "${name} is ${toString age} years old");
testFunc = func "Brynhjulf" 42;
# Test that all types are types.
testTypes = map type [
any bool drv float int string path
(attrs int)
(eitherN [ int string bool ])
(either int string)
(enum [ "foo" "bar" ])
(list string)
(option int)
(option (list string))
(struct { a = int; b = option string; })
(sum { a = int; b = option string; })
];
} "All tests passed!\n"