patroon

1.5.3 • Public • Published

Patroon

Pattern matching in JavaScript without additional syntax.

NPM NPM Downloads 100% Code Coverage Standard Code Style License

Installation

Patroon is hosted on the NPM repository.

npm install patroon

Usage

const {

  // Match Helpers
  patroon,
  matches,

  // Pattern Helpers
  every,
  some,
  multi,
  reference,
  instanceOf,
  _,

  // Errors
  NoMatchError,
  UnevenArgumentCountError,
  PatroonError,
} = require('patroon')

Let's see what valid and less valid uses of patroon are.

You can try out patroon over at RunKit.

Primitive

The simplest thing one can do is to match on a primitive.

Numbers:

patroon(
  2, 3,
  1, 2
)(1)
2

Strings:

patroon(
  'a', 'b',
  'c', 'd'
)('c')
d

Booleans:

patroon(
  true, false,
  false, true
)(true)
false

Symbols:

const a = Symbol('a')
const b = Symbol('b')
const c = Symbol('c')

patroon(
  a, b,
  b, c
)(b)
Symbol(c)

Nil values:

patroon(
  null, undefined,
  undefined, null,
)(undefined)
null

Regular Expression

Will check if a Regex matches the passed string using the string's .test method.

patroon(
  /^bunion/, 'string starts with bunion',
  /^banana/, 'string starts with banana'
)('banana tree')
string starts with banana

Placeholder

The _ is a placeholder/wildcard value that is useful to implement a default case.

patroon(
  1, 'value is 1',
  'a', 'value is a',
  _, 'value is something else'
)(true)
value is something else

We can combine the _ with other patroon features.

Object

Patroon can help you match objects that follow a certain spec.

patroon(
  {b: _}, 'has a "b" property',
  {a: _}, 'has an "a" property'
)({b: 2})
has a "b" property

Next we also match on the key's value.

patroon(
  {a: 1}, 'a is 1',
  {a: 2}, 'a is 2',
  {a: 3}, 'a is 3'
)({a: 2})
a is 2

What about nested objects?

patroon(
  {a: {a: 1}}, 'a.a is 1',
  {a: {a: 2}}, 'a.a is 2',
  {a: {a: 3}}, 'a.a is 3'
)({a: {a: 2}})
a.a is 2

Instance

Sometimes it's nice to know if the value is of a certain type. We'll use the builtin node error constructors in this example.

patroon(
  instanceOf(TypeError), 'is a type error',
  instanceOf(Error), 'is an error'
)(new Error())
is an error

Patroon uses instanceof to match on types.

new TypeError() instanceof Error
true

Because of this you can match a TypeError with an Error.

patroon(
  instanceOf(Error), 'matches on error',
  instanceOf(TypeError), 'matches on type error'
)(new TypeError())
matches on error

An object of a certain type might also have values we would want to match on. Here you should use the every helper.

patroon(
  every(instanceOf(TypeError), { value: 20 }), 'type error where value is 20',
  every(instanceOf(Error), { value: 30 }), 'error where value is 30',
  every(instanceOf(Error), { value: 20 }), 'error where value is 20'
)(Object.assign(new TypeError(), { value: 20 }))
type error where value is 20

Matching on an object type can be written in several ways.

patroon({}, 'is object')({})
patroon(Object, 'is object')({})

These are all equivalent.

Arrays can also be matched in a similar way.

patroon([], 'is array')([])
patroon(Array, 'is array')([])

A less intuitive case:

patroon({}, 'is object')([])
patroon([], 'is array')({})

Patroon allows this because Arrays can have properties defined.

const array = []
array.prop = 42

patroon({prop: _}, 'has prop')(array)

The other way around is also allowed even if it seems weird.

const object = {0: 42}
patroon([42], 'has 0th')(object)

If you do not desire this loose behavior you can use a predicate to make sure something is an array or object.

patroon(Array.isArray, 'is array')([])

Reference

If you wish to match on the reference of a constructor you can use the ref helper.

patroon(
  instanceOf(Error), 'is an instance of Error',
  reference(Error), 'is the Error constructor'
)(Error)
is the Error constructor

Array

patroon(
  [], 'is an array',
)([1, 2, 3])
is an array
patroon(
  [1], 'is an array that starts with 1',
  [1,2], 'is an array that starts with 1 and 2',
  [], 'is an array',
)([1, 2])
is an array that starts with 1

Think of patterns as a subset of the value you are trying to match. In the case of arrays. [1,2] is a subset of [1,2,3]. [2,3] is not a subset of [1,2,3] because arrays also care about the order of elements.

We can also use an object pattern to match certain indexes. The same can be written using an array pattern and the _. The array pattern can become a bit verbose when wanting to match on a bigger index.

These two patterns are equivalent:

patroon(
  {6: 7}, 'Index 6 has value 7',
  [_, _, _, _, _, _, 7], 'Index 6 has value 7'
)([1, 2, 3, 4, 5, 6, 7])
Index 6 has value 7

A function that returns the lenght of an array:

const count = patroon(
  [_], ([, ...xs]) => 1 + count(xs),
  [], 0
)

count([0,1,2,3])
4

A function that looks for a certain pattern in an array:

const containsPattern = patroon(
  [0, 0], true,
  [_, _], ([, ...rest]) => containsPattern(rest),
  [], false
)

containsPattern([1,0,1,0,0])
true

A toPairs function:

const toPairs = patroon(
  [_, _], ([a, b, ...c], p = []) => toPairs(c, [...p, [a, b]]),
  _, (_, p = []) => p
)

toPairs([1, 2, 3, 4])
[ [ 1, 2 ], [ 3, 4 ] ]

An exercise would be to change toPairs to throw when an uneven length array is passed. Multiple answers are possible and some are more optimized than others.

Every

A helper that makes it easy to check if a value passes all patterns.

const gte200 = x => x >= 200
const lt300 = x => x < 300

patroon(
  every(gte200, lt300), 'Is a 200 status code'
)(200)
Is a 200 status code

Some

A helper to check if any of the pattern matches value.

const isMovedResponse = patroon(
  {statusCode: some(301, 302, 307, 308)}, true,
  _, false
)

isMovedResponse({statusCode: 301})
true

Multi

Patroon offers the multi function in order to match on the value of another argument than the first one. This is named multiple dispatch.

  patroon(
    multi(1, 2, 3), 'arguments are 1, 2 and 3'
  )(1, 2, 3)
arguments are 1, 2 and 3

Predicate

By default a function is assumed to be a predicate.

See the references section if you wish to match on the reference of the function.

const isTrue = v => v === true

patroon(
  isTrue, 'is true'
)(true)
is true

Could one combine predicates with arrays and objects? Sure one can!

const gt20 = v => v > 20

patroon(
  [[gt20]], 'is greater than 20'
)([[21]])
is greater than 20
const gt42 = v => v > 42

patroon(
  [{a: gt42}], 'is greater than 42'
)([{a: 43}])
is greater than 42

Matches

A pattern matching helper that can help with using patroon patterns in if statements and such.

const isUser = matches({user: _})
const isAdmin = matches({user: {admin: true}})

const user = {
  user: {
    id: 2
  }
}

const admin = {
  user: {
    id: 1,
    admin: true
  }
}

JSON.stringify([isUser(admin), isUser(user), isAdmin(admin), isAdmin(user)])
[true,true,true,false]

Custom Helper

It is very easy to write your own helpers. All the builtin helpers are really just predicates. Let's look at the source of one of these helpers, the simplest one being the _ helper.

_.toString()
() => true

Other more complex helpers like the every or some helper are also predicates.

every.toString()
(...patterns) => {
  const matches = patterns.map(predicate)

  return (...args) => matches.every(pred => pred(...args))
}

See the ./src/index.js if you are interested in the implementation.

Errors

Patroon has errors that occur during runtime and when a patroon function is created. It's important to know when they occur.

NoMatchError

The no match error occurs when none of the patterns match the value.

const oneIsTwo = patroon(1, 2)

oneIsTwo(3)
/home/ant/projects/patroon/src/index.js:96
      throw error
      ^

NoMatchError: Not able to match any pattern for arguments
    at /home/ant/projects/patroon/src/index.js:90:21

UnevenArgumentCountError

Another error that occurs is when the patroon function is not used correctly.

patroon(1)
/home/ant/projects/patroon/src/index.js:82
  if (!isEven(list.length)) { throw new UnevenArgumentCountError('Patroon should have an even amount of arguments.') }
                              ^

UnevenArgumentCountError: Patroon should have an even amount of arguments.
    at patroon (/home/ant/projects/patroon/src/index.js:82:37)

PatroonError

All errors patroon produces can be matched against the PatroonError using instanceof.

const isPatroonError = patroon(instanceOf(PatroonError), 'patroon is causing an error')

isPatroonError(new NoMatchError())
isPatroonError(new UnevenArgumentCountError())
patroon is causing an error

Examples

Patroon can be used in any context that can benefit from pattern matching.

Tests

./src/index.test.js - Contains some tests for edge cases and it defines some property based tests.

We also care about code coverage so we'll use nyc to generate a coverage report.

# Clean install dependencies.
npm ci &> /dev/null

# Run tests and generate a coverage report
npx nyc npm t | npx tap-nyc

# Test if the coverage is 100%
npx nyc check-coverage
    > patroon@1.5.3 test
    > tape ./src/index.test.js
    -------------|---------|----------|---------|---------|-------------------
    File         | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
    -------------|---------|----------|---------|---------|-------------------
    All files    |     100 |      100 |     100 |     100 |                   
     helpers.js  |     100 |      100 |     100 |     100 |                   
     index.js    |     100 |      100 |     100 |     100 |                   
     walkable.js |     100 |      100 |     100 |     100 |                   
    -------------|---------|----------|---------|---------|-------------------

  total:     31
  passing:   31

  duration:  9.3s

Changelog

See the CHANGELOG.md to know what changes are introduced for each version release.

Contribute

You may contribute in whatever manner you see fit. Do try to be helpful and polite and read the CONTRIBUTING.md.

Contributors

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npm i patroon

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Version

1.5.3

License

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