Distinguishing between different types of error in Pony?

Question

Pony has a single exception value, which isn't parameterised.

Unfortunately frequently I have code which wants to throw different kinds of exception, and I need to know what they are in order to handle them properly --- for example, trivially, when halting the program it's important to give the user the right error message!

So what's the idiomatic way of doing this in Pony?

Answer 1

Currently the idiomatic way would be to not use errors and instead use union types of the normal return type and the error reasons. For example, if you have a function that returns ReturnType and can error with either Error1 or Error2, your function signature can be

fun foo(): (ReturnType | Error1 | Error2)

The caller function must then match on the union to see what happened.

If you want some real-world examples, the files package of the standard library uses this idiom extensively.

Also, for completeness, an RFC on adding typed errors is currently being discussed.