First page of the Elm guide covers it pretty well:
<a href="http:&#x2F;&#x2F;guide.elm-lang.org&#x2F;" rel="nofollow">http:&#x2F;&#x2F;guide.elm-lang.org&#x2F;</a><p>&gt; No runtime errors in practice. No null. No undefined is not a function.<p>&gt; Friendly error messages that help you add features more quickly.<p>&gt; Well-architected code that stays well-architected as your app grows.<p>&gt; Automatically enforced semantic versioning for all Elm packages.