I'm using MPS to implement a really interesting DSL. What I'm curious to know, and haven't found anything on their site about, is whether I can, within the typesystem, issue an API call or query a database on the fly. So we would see that an equivalence test occurs and I want to be able to issue an API call or database query to see the feasibility of that equivalence test under further constraints and analysis.
You can call any Java code but it will probably make the editor and possibly other parts of MPS unresponsive since asynchronous calls are not supported.