Variant

So far, most of ReScript's data structures might look familiar to you. This section introduces an extremely important, and perhaps unfamiliar, data structure: variant.

Most data structures in most languages are about "this and that". A variant allows us to express "this or that".

type myResponse =
  | Yes
  | No
  | PrettyMuch

let areYouCrushingIt = Yes

myResponse is a variant type with the cases Yes, No and PrettyMuch, which are called "variant constructors" (or "variant tag"). The | bar separates each constructor.

Note: a variant's constructors need to be capitalized.

Variant Needs an Explicit Definition

If the variant you're using is in a different file, bring it into scope like you'd do for a record:

// Zoo.res
type animal = Dog | Cat | Bird

// Example.res
let pet: Zoo.animal = Dog // preferred
// or
let pet2 = Zoo.Dog

Constructor Arguments

A variant's constructors can hold extra data separated by comma.

type account =
  | None
  | Instagram(string)
  | Facebook(string, int)

Here, Instagram holds a string, and Facebook holds a string and an int. Usage:

let myAccount = Facebook("Josh", 26)
let friendAccount = Instagram("Jenny")

Labeled Variant Payloads (Inline Record)

If a variant payload has multiple fields, you can use a record-like syntax to label them for better readability:

type user =
  | Number(int)
  | Id({name: string, password: string})

let me = Id({name: "Joe", password: "123"})

This is technically called an "inline record", and only allowed within a variant constructor. You cannot inline a record type declaration anywhere else in ReScript.

Of course, you can just put a regular record type in a variant too:

type u = {name: string, password: string}
type user =
  | Number(int)
  | Id(u)

let me = Id({name: "Joe", password: "123"})

The output is slightly uglier and less performant than the former.

Pattern Matching On Variant

See the Pattern Matching/Destructuring section later.

JavaScript Output

A variant value compiles to 3 possible JavaScript outputs depending on its type declaration:

• If the variant value is a constructor with no payload, it compiles to a number.

• If it's a constructor with a payload, it compiles to an object with the field TAG and the field _0 for the first payload, _1 for the second payload, etc.

• An exception to the above is a variant whose type declaration contains only a single constructor with payload. In that case, the constructor compiles to an object without the TAG field.

• Labeled variant payloads (the inline record trick earlier) compile to an object with the label names instead of _0, _1, etc. The object might or might not have the TAG field as per previous rule.

Check the output in these examples:

type greeting = Hello | Goodbye
let g1 = Hello
let g2 = Goodbye

type outcome = Good | Error(string)
let o1 = Good
let o2 = Error("oops!")

type family = Child | Mom(int, string) | Dad (int)
let f1 = Child
let f2 = Mom(30, "Jane")
let f3 = Dad(32)

type person = Teacher | Student({gpa: float})
let p1 = Teacher
let p2 = Student({gpa: 99.5})

type s = {score: float}
type adventurer = Warrior(s) | Wizard(string)
let a1 = Warrior({score: 10.5})
let a2 = Wizard("Joe")

Tips & Tricks

Be careful not to confuse a constructor carrying 2 arguments with a constructor carrying a single tuple argument:

type account =
  | Facebook(string, int) // 2 arguments
type account2 =
  | Instagram((string, int)) // 1 argument - happens to be a 2-tuple

Variants Must Have Constructors

If you come from an untyped language, you might be tempted to try type myType = int | string. This isn't possible in ReScript; you'd have to give each branch a constructor: type myType = Int(int) | String(string). The former looks nice, but causes lots of trouble down the line.

Interop with JavaScript

This section assumes knowledge about our JavaScript interop. Skip this if you haven't felt the itch to use variants for wrapping JS functions yet.

Quite a few JS libraries use functions that can accept many types of arguments. In these cases, it's very tempting to model them as variants. For example, suppose there's a myLibrary.draw JS function that takes in either a number or a string. You might be tempted to bind it like so:

// reserved for internal usage
@bs.module("myLibrary") external draw : 'a => unit = "draw"

type animal =
  | MyFloat(float)
  | MyString(string)

let betterDraw = (animal) =>
  switch animal {
  | MyFloat(f) => draw(f)
  | MyString(s) => draw(s)
  }

betterDraw(MyFloat(1.5))

Try not to do that, as this generates extra noisy output. Alternatively, define two externals that both compile to the same JS call:

@bs.module("myLibrary") external drawFloat: float => unit = "draw"
@bs.module("myLibrary") external drawString: string => unit = "draw"

ReScript also provides a few other ways to do this.

Variant Types Are Found By Field Name

Please refer to this record section. Variants are the same: a function can't accept an arbitrary constructor shared by two different variants. Again, such feature exists; it's called a polymorphic variant. We'll talk about this in the future =).

Design Decisions

Variants, in their many forms (polymorphic variant, open variant, GADT, etc.), are likely the feature of a type system such as ReScript's. The aforementioned option variant, for example, obliterates the need for nullable types, a major source of bugs in other languages. Philosophically speaking, a problem is composed of many possible branches/conditions. Mishandling these conditions is the majority of what we call bugs. A type system doesn't magically eliminate bugs; it points out the unhandled conditions and asks you to cover them*. The ability to model "this or that" correctly is crucial.

For example, some folks wonder how the type system can safely eliminate badly formatted JSON data from propagating into their program. They don't, not by themselves! But if the parser returns the option type None | Some(actualData), then you'd have to handle the None case explicitly in later call sites. That's all there is.

Performance-wise, a variant can potentially tremendously speed up your program's logic. Here's a piece of JavaScript:

JS
let data = 'dog'
if (data === 'dog') {
  ...
} else if (data === 'cat') {
  ...
} else if (data === 'bird') {
  ...
}

There's a linear amount of branch checking here (O(n)). Compare this to using a ReScript variant:

type animal = Dog | Cat | Bird
let data = Dog
switch data {
| Dog => Js.log("Wof")
| Cat => Js.log("Meow")
| Bird => Js.log("Kashiiin")
}

The compiler sees the variant, then

1. conceptually turns them into type animal = 0 | 1 | 2

2. compiles switch to a constant-time jump table (O(1)).