It was mentioned elsewhere in the comments, but I believe you're thinking of Meta Programming System: https://www.jetbrains.com/mps/ https://www.jetbrains.com/m

It was mentioned elsewhere in the comments, but I believe you're thinking of Meta Programming System: https://www.jetbrains.com/mps/ https://www.jetbrains.com/mps/ I regret that I have not had enough real world experience with MPS to comment on how it might relate to what Facebook is doing. But as for real world, a JetBrains employee told me that they wrote YouTrack (https://youtrack.jetbrains.com https://youtrack.jetbrains.com) in MPS, presumably using an "issue tracker DSL" but he wouldn't elaborate further.