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

[Damian Nadales]

Yesterday I posted the following question to SO:
    http://stackoverflow.com/questions/40495304/abstract-result-types-in-free-monads

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?