Most of time in Reason, you'd be using record to group named values together. In certain niche situations, however, you might want to use a similar feature called object; they're a bit more flexible and come with different tradeoffs.

If you come from JavaScript, you're mostly likely not looking for plain Reason objects. Skip right to the Tip & Tricks section below.


Type Declaration

An object doesn't need a type declaration, though it can have one. It looks like a record, except with a .:

type tesla = { . color: string };

The dot at the beginning indicates that this is a "closed" object type, which means that an object based on this type must have exactly this shape.

type car('a) = { .. color: string } as 'a;

Two dots, also called an elision, indicate that this is an "open" object type, and therefore can also contain other values and methods. An open object is also polymorphic and therefore requires a parameter.



type tesla = { . color: string, }; let obj: tesla = { val red = "Red"; pub color = red; }; Js.log(obj#color) /* "Red" */

Here we have a simple object with the method color and the property red. This method takes no arguments and returns the private property red. Because the method color is a public method we can access it using object notation. Remember, objects only export methods and all properties are private.


type tesla = {. drive: int => int }; let obj: tesla = { val hasEnvy = ref(false); pub drive = (speed) => { this#enableEnvy(true); speed }; pri enableEnvy = (envy) => hasEnvy := envy };

This object is of object type tesla and has a public method drive. It also contains a private method enableEnvy that is only accessible from within the object.

As you can see, a Reason object can also access this. JavaScript object's this behavior can be quirky; Reason this always points to the object itself correctly.

The following example shows an open object type which uses a type as parameter. The object type parameter is required to implement all the methods of the open object type.

type tesla('a) = { .. drive: int => int } as 'a; let obj: tesla({. drive: int => int, doYouWant: unit => bool}) = { val hasEnvy = ref(false); pub drive = (speed) => { this#enableEnvy(true); speed }; pub doYouWant = () => hasEnvy^; pri enableEnvy = (envy) => hasEnvy := envy };

You can use the above object like so: