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https://github.com/xodio/hm-def

Runtime type checking for JS with Hindley Milner signatures
https://github.com/xodio/hm-def

functional-programming hindley-milner invariant javascript type-checking

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Runtime type checking for JS with Hindley Milner signatures

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README 

        
# Hindley Milner Definitions



The `hm-def` package allows you to enforce runtime type checking for JavaScript

functions using Haskell-alike [Hindley

Milner](https://github.com/ramda/ramda/wiki/Type-Signatures) type signatures.



The `hm-def` is build on top of

[sanctuary-def](https://github.com/sanctuary-js/sanctuary-def)

and basically just a syntax sugar for it.



## Install



```bash

$ yarn add hm-def

# or

$ npm install hm-def

```



## Usage



First, you need to create a function definition function.



```javascript

import $ from 'sanctuary-def';

import {create} from 'hm-def';



const def = create ({

  $,

  checkTypes: true,

  env: $.env,

  typeClasses: [],

});

```



Then instead of this:



```javascript

function sum(a, b) {

  return a + b;

}

```



you can write:



```javascript

const sum = def

  ('sum :: Number -> Number -> Number')

  (a => b => a + b);

```



And the calls to `sum` will be type checked:



```javascript

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:



```javascript

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:



```javascript

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:



```javascript

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:



```javascript

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:



- AnyFunction

- Arguments

- Array

- Boolean

- Date

- Error

- HtmlElement

- Null

- Number

- Object

- RegExp

- StrMap

- String

- Symbol

- Undefined



You would likely to add your own application domain types. See [documentation

of type

constructors](https://github.com/sanctuary-js/sanctuary-def#type-constructors)

to learn how.



### Type constraints



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

function:



```javascript

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](https://github.com/sanctuary-js/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:



```javascript

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:



```javascript

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.

```






### 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`.



```javascript

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:



```javascript

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"](https://github.com/sanctuary-js/sanctuary/issues/438).



## Changelog



### 1.0.0



* Update `sanctuary-*`, building, and testing dependencies.

* Breaking :exclamation: functions are no longer curried automatically. See the

  [currying section](#currying).



### 0.3.0



* Update `sanctuary-def` dependency to version 0.14.0

* BREAKING :exclamation: All Unary/Binary Types with variable types inside should be

  specified in `env` with `$.Unknown` types. Then, when you define functions, `hm-def`

  will recreate specific types for these functions. (See more)[#type-constructors]



  Since version 0.10.0 of `sanctuary-def` environments must be of type `Array Type`.

  So it must not contain type constructors anymore.

  ([sanctuary-js/sanctuary-def#124](https://github.com/sanctuary-js/sanctuary-def/pull/124))



### 0.2.1



* Update `ramda` dependency to version 0.24.1



### 0.2.0



* Add `def.curried`

* Fix errors when using some non-nullary types like built-in `Array` or `StrMap`



## Contributors



Alphabetically:



* [davidchambers](https://github.com/davidchambers)

* [evgenykochetkov](https://github.com/evgenykochetkov)

* [Gipphe](https://github.com/Gipphe)

* [nkrkv](https://github.com/nkrkv)



## License



MIT