Built in Types

Mint comes with several built in types. These are used in control expressions: Promise, Result, Void, Never. The Maybe type represents a nillable value.


The promise type represents a JavaScript promise, an asynchronous computational task that might fail.

In Mint promises have two parameters Promise(error, result) :

  • error - the value if the task failed

  • result - the value if the task succeeded

A good example is a HTTP request which in Mint looks like this:

Promise(Http.ErrorResponse, Http.Response)

where the Http.ErrorResponse is a record containing information about the error that happened while Http.Response is a record containing the response of the request.

Promises are used in a sequence and parallel expressions.


The result type, represents a synchronous task that might fail. It is defined as an enum:

enum Result(error, value) {

In Mint, results have two parameters Result(error, value):

  • error - the type of the error

  • value - the type of the result

For example, converting a String to a Number:

  • If the conversion fails we get an error:

    |> Number.fromString()
    |> Result.isError() # true
  • If the conversion succeeds we get a value

    |> Number.fromString()
    |> Result.isOk() # true

Results are used in sequence, parallel and try expressions.


The void type represents a side-effect: an expression which cannot have a value, either because it's not synchronous or because it causes a state change.

  • Void is used to handle events coming from the DOM or any outside source.

  • Void can be explicitly returned with the void keyword.


The never type is used to describe tasks (promises or results) that can never fail. For example, here is the type of task that can never fail: SomeTask(Never, Value)

These tasks don't need to be caught in sequence, parallel and try expressions.


The maybe type represents a value which may or may not exist. It's defined as an enum:

enum Maybe(value) {

For example here is a user who may or may not have a car:

record Car {
type : String,
engine : String
record User {
name : String,
car : Maybe(Car)