return throw

In a programming language with exceptions (like Suneido), one of the API design questions is when to return an error, and when to throw it.

It's helpful to distinguish "queries" and "commands". (Command-query separation) A query returns a result. A command performs an action, it may or may not have a return value.

For dynamically typed languages like Suneido query functions can easily return an error value like false or a string. If you forget to check for errors it will usually cause a runtime error. But if you forget to check for errors with a command function, they'll be lost. It might cause a problem later but that can be hard to debug.

From the perspective of using the API, both approaches have pros and cons.

• checking a return value is generally faster and less code than try-catch
• it's too easy to forget to check a command return value, especially if failure is rare
• you can't forget to check an exception (unless you have a try-catch somewhere that deliberately ignores exceptions)
• return values have to be checked on every call, exceptions can be handled for larger scopes

C++ and C have nodiscard and Java has JSR-305 @CheckReturnValue annotation but these are static compile time checks, not a good fit for a dynamic language like Suneido.

I came up with the idea of "return throw" (avoiding the need for a new keyword). It returns a value like a regular return. But if the return value is not used (discarded) then it throws an exception.

As I started to use this, I realized it could be more dynamic. A successful return could be just a regular (ignorable) return. whereas error returns could be return throw.

if error
    return throw false
return true

That got a little repetitive so I changed return throw to automatically treat true and "" as success. i.e. a return throw of true or "" would be treated as a normal return and the result could be ignored. But a return throw of any other value e.g. false or "invalid foo" would throw if the result was ignored.

Another issue was if F() did return throw, and G() did return F() that shouldn't be treated as "using" the result. Luckily that turned out to be relatively easy to handle.

I made a bunch of the built-in functions return throw and that has helped track down a few issues. Otherwise it isn't too popular yet. Hopefully it will prove worthwhile in the long run.