From the discord blog posts it seems that elixir powers the chat system, with rust and python as the other two main languages in their stack. As for whatsapp,

From the discord blog posts it seems that elixir powers the chat system, with rust and python as the other two main languages in their stack. As for whatsapp, they are mainly a erlang shop and yesterday they open sourced a type checker for erlang: https://github.com/WhatsApp/eqwalizer https://github.com/WhatsApp/eqwalizer