static-tagged-union

A simple, minimal, flexible, forgiving, static, serializable, tagged union utility.

Definition

Tagged unions, also known as sum types or union types, are useful for handling cases in a way that is nicer than switch/case or if/else.

Aside from tagged unions having cases that you define, there are specific unions that are commonly useful:

• Maybe (Nothing, Just)
• Either (Left, Right)
• Result (Ok, Err)

These types are similar and, in static-tagged-union, are combined into a single helper type:

• Maybe (N, Y)

Just like general TaggedUnions, Maybe instances can optionally contain params. That goes for both N and Y. In that sense, Maybe is closer to Either and Result.

Maybe is provided for convenience, since it is very common to have present/absent values, true/false values, and other binary possibilities.

Motivation

I looked at several similar libraries, really trying to find one that meets all my requirements, namely:

• static functions to handle cases, not instance functions on returned types
• serializable representation: JSON.parse(JSON.stringify(instance)) must work
• non-exhaustive case handling: although exhaustive case handling can be seen as an advantage, because the library prevents you from forgetting to handle a case, I actually prefer to be able to handle only some cases and ignore the others
• default case handling: be able to define a handler that gets called if a case does not match any of the handlers that you have defined.

All of the similar libraries that I looked at matched some, but not all, of those requirements:

• stags (repo here)
• daggy
• @joakin/sum-types
• results
• sum-types
• tagged-union
• union-type

I give credit to the libraries above. They are excellent work and definitely do more robust verifications of your code than what I provide here.

Although the initial version of static-tagged-union did no verifications, the current version provides optional checking that:

• the case handlers match the case type (see foldChecked, below)
• the case handlers match all the cases of the case type (see foldStrict, below)

Installation

Using npm:

npm i static-tagged-union

Using a script tag:

<script src="https://unpkg.com/static-tagged-union"></script>

Using the script tag exposes a TaggedUnion global, under which TaggedUnion, fold, etc. are provided.

API

• TaggedUnion(listOfCases)
• fold(handlerObject)(caseInstance)
• cases(listOfCases)(handlerFunction)
• TaggedUnionChecked(type, listOfCases)
• foldChecked(handlerObject)(checkedCaseInstance)
• foldStrict(handlerObject)(checkedCaseInstance)

That is the core API. For convenience, there is also Maybe:

• Maybe.N(params), Maybe.Y(params)
• bifold(fn, fy) -- shortcut for fold({ N: fn, Y: fy })
• map(fn)(maybeInstance)
  • if maybeInstance is N, returns maybeInstance
  • if maybeInstance is Y, returns Maybe.Y(fn(maybeInstance.params))
• unless(fn)(maybeInstance)
  • if maybeInstance is N, returns Maybe.N(fn(maybeInstance.params))
  • if maybeInstance is Y, returns maybeInstance
• bimap(fn, fy): shortcut for

    fold({
      N: params => Maybe.N(fn(params)),
      Y: params => Maybe.Y(fy(params))
    })

• contains(caseInstance)([caseInstance0, caseInstance1, ...])
  • if one of the caseInstances in the array is the same case as caseInstance, returns Maybe.Y(params), where params is the params contained by the first match
  • otherwise, returns Maybe.N()

Usage

Define a tagged union:

const Route = TaggedUnion(["Home", "Profile", "Login"])

The single argument to TaggedUnion is an array of strings defining the cases for the type. These will be properties of the returned object (Route in this example), and so the strings must be valid to be used as object properties.

Create instances of the tagged union:

const route1 = Route.Home()
const route2 = Route.Profile({ id: 42 })

For each case that you provided when creating the tagged union, you get a function that creates an instance. The function accepts a single optional argument that becomes the params stored in the instance. Each instance is a plain JavaScript object of the form { id: ..., params: ... }

Use fold to handle cases:

fold({
  Home: () => "Home",
  Profile: ({ id }) => `Profile ${id}`
})(route2)

// Returns "Profile 42"

You can provide just the cases you want to handle. If there is no match, the call returns undefined:

fold({
  Home: () => "Home"
})(route2)

// Returns undefined

You can also use _ to indicate a default handler that gets called when there is no match:

fold({
  Home: () => "Home",
  _: () => "No match was found"
})(route2)

// Returns "No match was found"

If you want to handle multiple cases in the same way, use cases for convenience:

fold(cases(["Home", "About"])(() => "Result"))

fold({
  ...cases(["Home", "About"])(() => "Result"),
  User: () => "User",
  _: () => "No match found"
})

Instead of {...cases()}, you can also pass multiple handler objects to fold:

fold(
  cases(["Home", "About"])(() => "Result"),
  { User: () => "User",
    _: () => "No match found"
  }
)

Credit: thank you James Forbes for this idea!

TaggedUnionChecked, foldChecked, and foldStrict

The API so far does not do any verifications on cases and handlers. Optionally, you can ensure that:

• the case handlers match the case type (see foldChecked, below)
• the case handlers match all the cases of the case type (see foldStrict, below)

First, use TaggedUnionChecked to create the tagged union:

const Route = TaggedUnionChecked(["Home", "Profile", "Login"])

Next, use foldChecked to make sure that your handlers match the case type:

// This works fine:
foldChecked({
  Profile: ({ id }) => `Profile ${id}`
})(Route.Profile({ id: 42 }))

// This throws an error because `User` is not part of the cases for `Route`:
foldChecked({
  User: ({ id }) => `User ${id}`
})(Route.Profile({ id: 42 }))

Finally, use foldStrict to make the same verifications as foldChecked plus requiring that the handler handles all the cases of the type. Note that _ cannot be used here; instead, use cases to handle multiple cases.

// This works fine:
foldStrict({
  Home: () => "Home",
  Profile: ({ id }) => `Profile ${id}`,
  Login: () => "Login"
})(Route.Profile({ id: 42 }))

// This also works fine:
foldStrict(
  cases(["Home", "Login"])(() => "Page"),
  { Profile: ({ id }) => `Profile ${id}` }
)(Route.Profile({ id: 42 }))

// This throws an error because `Login` is not handled:
foldStrict({
  Home: () => "Home",
  Profile: ({ id }) => `Profile ${id}`
})(Route.Profile({ id: 42 }))

Use Maybe for convenience:

Whenever you have a value that can be true/false, present/absent, and so on, you can use Maybe.Y and Maybe.N:

let data = Maybe.N()
// later...
data = Maybe.Y({ things: ... })

If you like, you can define an alias:

const Loaded = Maybe

let data = Loaded.N()
// later...
data = Loaded.Y({ things: ... })

As a shortcut to fold({ N: () => ..., Y: params => ... }), you can use bifold:

bifold(
  () => "Loading, please wait...",
  ({ things }) => `Here are the things: ${things}`
)

Note that N accepts params as well. For example, you might use this for an error message.

Maybe can also be used to denote the presence or absence of a value, and provide a convenient way to operate on a value only if it is present. Use map for this:

const name = getName() // returns Maybe.N or Maybe.Y
const upper = map(str => str.toUpperCase())(name) // only if name is present
bifold(
  () => "No name given.",
  name => `Hello, ${name}` // Hello, FRED
)(upper)

Finally, you can do something with the params in N and something with the params in Y with bimap:

const name = getName() // returns Maybe.N or Maybe.Y
const converted = bimap(
  err => `Error message: ${err}`,
  name => name.toUpperCase()
)
bifold(
  msg => console.err(msg),
  name => `Hello, ${name}`
)(converted)

Note that Maybe, map, bimap, and bifold are all just convenience shortcuts. You can achieve the same with just TaggedUnion and fold.

Credits

Special thanks to James Forbes for static-sum-type and stags. Definitely check those out if you want static tagged unions with verifications for types and exhaustive case handling.

The readme for stags also gives a great explanation of why the Maybe type is so useful.

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static-tagged-union is developed by foxdonut (@foxdonut00) and is released under the MIT license.