[Haskell-cafe] Asynchronous computations and error handling in the free monad

Yesterday I posted the following question to SO: http://stackoverflow.com/questions/40495304/abstract-result-types-in-free-mo... As discussion with Benjamin ensued, in which he stated that it is better to be explicit about the fact that we want parts of our code to run asynchronously. I am not so sure. I also have a similar question about how would you deal with fatal errors that could happen at the interpreter. I guess this could be modeled in the free monad by having a command returning a `Maybe` or `Either` type. However we're adding details about the interpreters in our DSL, since we are stating that certain command could cause fatal errors. So my question is what approach do you or would you use?