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Hindley Milner Definitions

The hm-def package allows you to enforce runtime type checking for JavaScript functions using Haskell-alike Hindley Milner type signatures.

The hm-def is build on top of sanctuary-def and basically just a syntax sugar for it.

Install

$ yarn add hm-def
# or
$ npm install hm-def

Usage

First, you need to create a function definition function.

import $ from 'sanctuary-def';
import {create} from 'hm-def';

const def = create ({
  $,
  checkTypes: true,
  env: $.env,
  typeClasses: [],
});

Then instead of this:

function sum(a, b) {
  return a + b;
}

you can write:

const sum = def
  ('sum :: Number -> Number -> Number')
  (a => b => a + b);

And the calls to sum will be type checked:

sum (42) (13);
// 55

sum ('42') (13);
// TypeError: Invalid value
//
// foo :: Number -> Number -> Number
//        ^^^^^^
//          1
//
// 1)  "42" :: String
//
// The value at position 1 is not a member of ‘Number’.

Arrays

To denote an array you enclose type of its elements in square brackets:

const magnitude = def
  ('magnitude :: [Number] -> Number')
  (xs => Math.sqrt (xs.reduce ((acc, x) => acc + x * x, 0)));

magnitude ([3, 4, 0]);
// 5

magnitude (3, 4, 0);
// TypeError: Function applied to too many arguments
//
// magnitude :: Array Number -> Number
//
// ‘magnitude’ expected at most one argument but received three arguments.

Actually it’s just a shortcut to a more general:

const magnitude = def
  ('magnitude :: Array Number -> Number')
  (xs => Math.sqrt (xs.reduce ((acc, x) => acc + x * x, 0)));

Where Array is a regular unary type provided by the default environment. It takes a single type argument which describes the type of array’s elements.

Records

To denote objects with a known schema record syntax is used:

const minMax = def
  ('minMax :: [Number] -> { min :: Number, max :: Number }')
  (xs => xs.reduce (
    (acc, x) => ({
      min: Math.min (x, acc.min),
      max: Math.max (x, acc.max),
    }),
    { min: Infinity, max: -Infinity }
  ));

minMax ([1, 4, 6, 3, 4, 5, -3, 4]);
// { min: -3, max: 6 }

Maps

To describe a map of homogenous data you can use StrMap type:

const occurrences = def
  ('occurrences :: [String] -> StrMap Number')
  (xs => xs.reduce (
    (acc, x) => {
      // a bit of dirty local mutation
      acc[x] = (acc[x] || 0) + 1;
      return acc;
    },
    {}
  ));

occurrences (['foo', 'bar', 'bar', 'baz', 'bar', 'qux', 'foo']);
// {
//   foo: 2,
//   bar: 3,
//   baz: 1,
//   qux: 1,
// }

Types available

You pass type definitions with env option of HMD.create. $.env from sanctuary-def provides type info for all built-in types:

You would likely to add your own application domain types. See documentation of type constructors to learn how.

Type constraints

For most generic functions you’d like to add type constraints. Consider the function:

const concat = def
  ('concat :: a -> a -> a')
  (y => x => x.concat (y));

concat ([3, 4]) ([1, 2]);
// [1, 2, 3, 4]

concat (' world') ('Hello')
// 'Hello world'

concat (42) (13)
// TypeError: x.concat is not a function

The call to the function crashed on invalid argument types post factum. We can place a type constraint on a to fail in advance with a more clear message.

Type constraints are done with type classes. There are many type classes provided by sanctuary-type-classes and you can create your own.

To use HM definitions with type class constaints you should provide typeClasses option with classes you’d like to use later:

import $ from 'sanctuary-def';
import Z from 'sanctuary-type-classes';
import {create} from 'hm-def';

const def = create ({
  $,
  checkTypes: true,
  env: $.env,
  typeClasses: [
    // ...
    Z.Functor,
    Z.Semigroup,
    // ...
  ],
});

Then:

const concat = def
  ('concat :: Semigroup a => a -> a -> a')
  (y => x => x.concat (y));

concat ([3, 4]) ([1, 2]);
// [1, 2, 3, 4]

concat (' world') ('Hello')
// 'Hello world'

concat (42) (13)
// TypeError: Type-class constraint violation
//
// foo :: Semigroup a => a -> a -> a
//        ^^^^^^^^^^^    ^
//                       1
//
// 1)  42 :: Number
//
// ‘foo’ requires ‘a’ to satisfy the Semigroup type-class constraint; the value
// at position 1 does not.

<a name="type-constructors"></a>

Type constructors

Added in v0.3.0

If you need UnaryType or BinaryType of something you should add them into env with $.Unknown types in it. Then hm-def will recreate specific types when you will define your functions.

Assuming we have an implementation of Either a b exposed as Either.

const EitherType = $.BinaryType
  ('my-package/Either')
  ('http://example.com/my-package#Either')
  (x => x != null && x['@@type'] === 'my-package/Either')
  (either => (either.isLeft ? [either.value] : []))
  (either => (either.isRight ? [either.value] : []));
// EitherType is a function `EitherType :: Type -> Type -> Type`,

const def = HMD.create ({
  $,
  checkTypes: true,
  env: $.env.concat ([
    EitherType ($.Unknown) ($.Unknown),
  ]),
});

// Now we can just define functions as usual:
const foo = def
  ('foo :: Either Number String -> Either String String')
  ((x) => x.chain ((val) => {
    if (val >= 3) return Either.Right ('It greater than or equal 3');
    return Either.Left ('It less than 3');
  }));

foo (Either.Right (4)); // Either.Right('It greater than or equal 3')
foo (Either.Right (1)); // Either.Left('It less than 3')
foo (Either.Right ('hello')); // TypeError: The value at position 1 is not a member of ‘Number’
foo (1); // TypeError: The value at position 1 is not a member of ‘Either Number String’

Currying

Beginning with 1.0.0, functions are not automatically curried, and they are expected to be manually curried at all times:

import $ from 'sanctuary-def';
import {create} from 'hm-def';

const def = create ({
  $,
  checkTypes: true,
  env: $.env,
  typeClasses: [],
});

const foo = def
  ('foo :: a -> b -> c')
  (x => y => x + y);

foo (1) (2);
// 3

foo (1, 2);
// TypeError: ‘foo’ applied to the wrong number of arguments
//
// foo :: a -> b -> c
//        ^
//        1
//
// Expected one argument but received two arguments:
//
//   - 1
//   - 2

const bar = def
  ('bar :: a -> b -> c')
  ((x, y) => x + y);

bar (1, 2);
// TypeError: ‘bar’ applied to the wrong number of arguments
//
// bar :: a -> b -> c
//        ^
//        1
//
// Expected one argument but received two arguments:
//
//   - 1
//   - 2

This is consistent with sanctuary's way of currying, known as "familiar currying".

Changelog

1.0.0

0.3.0

0.2.1

0.2.0

Contributors

Alphabetically:

License

MIT