# bool.fromEnvironment constructor

const bool.fromEnvironment(String name, { bool defaultValue: false })

Returns the boolean value of the environment declaration `name`.

The boolean value of the declaration is `true` if the declared value is the string `"true"`, and `false` if the value is `"false"`.

In all other cases, including when there is no declaration for `name`, the result is the `defaultValue`.

The result is the same as would be returned by:

    (const String.fromEnvironment(name) == "true")
        ? true
        : (const String.fromEnvironment(name) == "false")
            ? false
            : defaultValue

Example:

    const loggingFlag = const bool.fromEnvironment("logging");

If you want to use a different truth-string than `"true"`, you can use the String.fromEnvironment constructor directly:

    const isLoggingOn = (const String.fromEnvironment("logging") == "on");

## Implementation

    // The .fromEnvironment() constructors are special in that we do not want
    // users to call them using "new". We prohibit that by giving them bodies
    // that throw, even though const constructors are not allowed to have bodies.
    // Disable those static errors.
    //ignore: const_constructor_with_body
    //ignore: const_factory
    external const factory bool.fromEnvironment(String name,
        {bool defaultValue: false});