More concise way to build a configuration class using environment variables?

The most immediate change that comes to mind is to change new to be:

method new() {
    sub env(Str $envVar) {
        %*ENV{$envVar} // die "environment variable $envVar must exist"
    }

    return self.bless(
        config_string_a => env('CONFIG_STRING_A'),
        config_string_b => env('CONFIG_STRING_B'),
        config_flag_c => Bool(env('CONFIG_FLAG_C')),
    );
}

While // is a definedness check rather than an existence one, the only way an environment variable will be undefined is if it isn't set. That gets down to one mention of %*ENV and also of each environment variable.

If there's only a few, then I'd likely stop there, but the next bit of repetition that strikes me is the names of the attributes are just lowercase of the names of the environment variables, so we could eliminate that duplication too, at the cost of a little more complexity:

method new() {
    multi env(Str $envVar) {
        $envVar.lc => %*ENV{$envVar} // die "environment variable $envVar must exist"
    }
    multi env(Str $envVar, $type) {
        .key => $type(.value) given env($envVar)
    }

    return self.bless(
        |env('CONFIG_STRING_A'),
        |env('CONFIG_STRING_B'),
        |env('CONFIG_FLAG_C', Bool),
    );
}

Now env returns a Pair, and | flattens it in to the argument list as if it's a named argument.

Finally, the "power tool" approach is to write a trait like this outside of the class:

multi trait_mod:<is>(Attribute $attr, :$from-env!) {
    my $env-name = $attr.name.substr(2).uc;
    $attr.set_build(-> | {
        with %*ENV{$env-name} -> $value {
            Any ~~ $attr.type ?? $value !! $attr.type()($value)
        }
        else {
            die "environment variable $env-name must exist"
        }
    });
}

And then write the class as:

class Configuration {
    has $.config_string_a is from-env;
    has $.config_string_b is from-env;
    has Bool $.config_flag_c is from-env;
}

Traits run at compile time, and can manipulate a declaration in various ways. This trait calculates the name of the environment variable based on the attribute name (attribute names are always like $!config_string_a, thus the substr). The set_build sets the code that will be run to initialize the attribute when the class is created. That gets passed various things that in our situation aren't important, so we ignore the arguments with |. The with is just like if defined, so this is the same approach as the // earlier. Finally, the Any ~~ $attr.type check asks if the parameter is constrained in some way, and if it is, performs a coercion (done by invoking the type with the value).

So I mentioned this in a comment but I figured it would be good as an actual answer. I figured this would be useful functionality for anyone building a Docker based system so took Jonanthan's example code, added some functionality for exporting Traits Elizabeth showed me and made Trait::Env

Usage is :

use Trait::Env;
class Configuration {
    has $.config_string_a is env;
    has $.config-string-b is env(:required);
    has Bool $.config-flag-c is env is default(True);
}

The :required flag turns on die if not found. And it plays nicely with the is default trait. Attribute names are upper cased and - is replaced with _ before checking %*ENV.

I have a couple of planned changes, make it throw a named Exception rather than just die and handle Boolean's a bit better. As %*ENV is Strings having a Boolean False is a bit of a pain.