title_body: Are there any successful conlangs using information density as a design goal?
upvoted_answer: Answering this question is tricky. As you obtain higher and higher levels of information density, you have to sacrifice some naturalism or some simplicity in order to get there.
  Marking the point where you've sacrificed too much naturalism or simplicity to be learnable is a judgment call.
  Short answer:
  Possibly guaspi, but it's obscure.
  
  One famous example of this is Ithkuil. Parts of Ithkuil are reasonably naturalistic, such as its phonology, which is less complex than Ubykh or Chechen (which also has a large number of vowels). Other parts are not, such as the very fine-grained case distinctions or how meanings are organized into roots, which is more complex than the system of triliteral roots in Semitic languages that it is based on.
  Is Ithkuil learnable? I really don't know. My guess is no but I don't have proof.
  
  An example of an informationally-dense written language is Classical Chinese. Classical Chinese is not a conlang, but it was written centuries after it stopped being spoken and uses a heavily abbreviated style where nearly everything that's pragmatically inferable is dropped.
  If you want to make an informationally dense conlang, Classical Chinese is a good starting point. It has pervasive zero marking (although you can optionally mark possessors and relative clauses with an overt marker) and the reconstructed roots are monosyllabic or sesquisyllabic.
  
  One possible example of an intentionally information dense language is guaspi, a tonal derivative of lojban.
  I don't know whether lojban is considered learnable or not and I don't know whether to consider guaspi successful or not, but efficiency is one of its stated goals.
  
  Gua\spi is efficient. Words are short, and extensive defaults on
  articles and modal cases eliminate the majority of structure words.
  
  In my personal opinion, so take it with a grain of salt, a human can speak &quot;informal&quot; lojban riddled with logic errors and this should count as speaking lojban.
  
  Closing remarks:
  Looking at some of these examples reveals an important design decision that creators of an informationally-dense language face: whether to allow the speaker to drop things that are pragmatically inferable and how to account for that when evaluating how well their language achieves its goals.
  For instance, suppose a language has very specific tenses that are marked on all verbs or person marking on all verbs. Tense information is frequently recoverable from context, as are pronominal arguments. If information is recoverable from context, does the language get any points for encoding it directly, however efficiently it may do so?