E4069

Unexpected main function in the non-main package.

In MoonBit, packages are divided into two types: main packages and non-main packages. Non-main packages are often used as libraries to provide functionality to other packages. Main packages, on the other hand, are the entry points of the program. Therefore, non-main packages should not contain a main function.

Sometimes, people use the main function for testing. In such cases, we recommend using the test block to write tests, instead of using the main function. See the test section of the MoonBit documentation for more information.

Erroneous example

Suppose in package lib:

lib/moon.pkg.json:

{} // "is-main" is by default false

lib/lib.mbt:

fn main { // Error: Unexpected main function in the non-main package.
  println("Hello, world!")
}

Suggestion

You can remove the main function from the non-main package:

// Remove the main function
//- fn main {
//-   println("Hello, world!")
//- }

Alternatively, you can set the package to be a main package by setting "is-main" to true in the package configuration file.

lib/moon.pkg.json:

{
  "is-main": true
}

You can also move the main function to a different package, say main:

main/moon.pkg.json:

{
  "is-main": true
}

main/main.mbt:

fn main {
  println("Hello, world!")
}