Compile-time variables

When building an executable, you may wish to customize certain aspects of the program. Take the following program for example:

let PATH = '/usr/share/example'

fn load_assets_from(path: String) {
  # ...
}

class async Main {
  fn async main {
    load_assets_from(PATH)
  }
}

This program assumes the assets are located in /usr/share/example. But what if we instead want to store them in /usr/local/share/example? We could change the source code, but then we'd have to maintain a fork of the project just for this one change.

Changing constants at compile-time

Inko's compiler provides a solution to this problem: we can override the value at compile-time, without changing the source code. For this to work, the constant must meet the following requirements:

1. It must be a public constant
2. The value must be of type String, Int or Bool

In the above case we just need to make the constant public like so:

let pub PATH = '/usr/share/example'

fn load_assets_from(path: String) {
  # ...
}

class async Main {
  fn async main {
    load_assets_from(PATH)
  }
}

To specify a custom value, you use the -d/--define option when running inko build. This option takes a value in the following format:

module.name.CONSTANT=VALUE

Here module.name is the fully qualified module name, such as std.string or std.net.socket, CONSTANT is the name of the constant and VALUE is the value to assign to the constant. To set PATH to /usr/local/share/example, we'd build the program as follows:

inko build --define main.PATH=/usr/local/share/example

If multiple constants need to have their values adjusted, specify the -d/--define option multiple times:

inko build --define main.PATH=/usr/local/share/example --define foo.bar.EXAMPLE=42

Type requirements and conversions

The value assigned to the constant is interpreted according to the type of its original value.

If the constant's default value is a String, the provided value is interpreted as an UTF-8 string. If the default value is an Int, the value assigned by the --define option must be a decimal number, other number formats such as hexadecimal numbers aren't supported. For Bool constants the only two valid values are true and false:

inko build --define foo.bar.BOOLEAN=true

If the value is invalid for the constant, or the constant's value can't be overwritten (e.g. because it's a private constant), a compile-time error is produced and the compilation process is stopped.