Pony doesn’t feature exceptions as you might be familiar with them from languages like Python, Java, C++ et al. It does, however, provide a simple partial function mechanism to aid in error handling. Partial functions and the
error keyword used to raise them look similar to exceptions in other languages but have some important semantic differences. Let’s take a look at how you work with Pony’s error and then how it differs from the exceptions you might be used to.
Raising and handling errors¶
An error is raised with the command
error. At any point, the code may decide to declare an
error has occurred. Code execution halts at that point, and the call chain is unwound until the nearest enclosing error handler is found. This is all checked at compile time so errors cannot cause the whole program to crash.
Error handlers are declared using the
try callA() if not callB() then error end callC() else callD() end
In the above code
callA() will always be executed and so will
callB(). If the result of
callB() is true then we will proceed to
callC() in the normal fashion and
callD() will not then be executed.
callB() returns false, then an error will be raised. At this point, execution will stop and the nearest enclosing error handler will be found and executed. In this example that is, our else block and so
callD() will be executed.
In either case, execution will then carry on with whatever code comes after the
Do I have to provide an error handler? No. The
try block will handle any errors regardless. If you don’t provide an error handler then no error handling action will be taken - execution will simply continue after the
If you want to do something that might raise an error, but you don’t care if it does you can just put in it a
try block without an
try // Do something that may raise an error end
Is there anything my error handler has to do? No. If you provide an error handler then it must contain some code, but it is entirely up to you what it does.
What’s the resulting value of a try block? The result of a
try block is the value of the last statement in the
try block, or the value of the last statement in the
else clause if an error was raised. If an error was raised and there was no
else clause provided, the result value will be
Pony does not require that all errors are handled immediately as in our previous examples. Instead, functions can raise errors that are handled by whatever code calls them. These are called partial functions (this is a mathematical term meaning a function that does not have a defined result for all possible inputs, i.e. arguments). Partial functions must be marked as such in Pony with a
?, both in the function signature (after the return type) and at the call site (after the closing parentheses).
For example, a somewhat contrived version of the factorial function that accepts a signed integer will error if given a negative input. It’s only partially defined over its valid input type.
fun factorial(x: I32): I32 ? => if x < 0 then error end if x == 0 then 1 else x * factorial(x - 1)? end
Everywhere that an error can be generated in Pony (an error command, a call to a partial function, or certain built-in language constructs) must appear within a
try block or a function that is marked as partial. This is checked at compile time, ensuring that an error cannot escape handling and crash the program.
Prior to Pony 0.16.0, call sites of partial functions were not required to be marked with a
?. This often led to confusion about the possibilities for control flow when reading code. Having every partial function call site clearly marked makes it very easy for the reader to immediately understand everywhere that a block of code may jump away to the nearest error handler, making the possible control flow paths more obvious and explicit.
Partial constructors and behaviours¶
Class constructors may also be marked as partial. If a class constructor raises an error then the construction is considered to have failed and the object under construction is discarded without ever being returned to the caller.
When an actor constructor is called the actor is created and a reference to it is returned immediately. However, the constructor code is executed asynchronously at some later time. If an actor constructor were to raise an error it would already be too late to report this to the caller. For this reason, constructors for actors may not be partial.
Behaviours are also executed asynchronously and so cannot be partial for the same reason.
In addition to an
else error handler, a
try command can have a
then block. This is executed after the rest of the
try, whether or not an error is raised or handled. Expanding our example from earlier:
try callA() if not callB() then error end callC() else callD() then callE() end
callE() will always be executed. If
callB() returns true then the sequence executed is
callB() returns false then the sequence executed is
Do I have to have an else error handler to have a then block? No. You can have a
then block without an
else if you like.
Will my then block really always be executed, even if I return inside the try? Yes, your
then expression will always be executed when the
try block is complete. The only way it won’t be is if the
try never completes (due to an infinite loop), the machine is powered off, or the process is killed (and then, maybe).
with expression can be used to ensure disposal of an object when it is no longer needed. A common case is a database connection which needs to be closed after use to avoid resource leaks on the server. For example:
with obj = SomeObjectThatNeedsDisposing() do // use obj end
obj.dispose() will be called whether the code inside the
with block completes successfully or raises an error. To take part in a
with expression, the object that needs resource clean-up must, therefore, provide a
class SomeObjectThatNeedsDisposing // constructor, other functions fun dispose() => // release resources
It is possible to provide an
else clause, which is called only in error cases:
with obj = SomeObjectThatNeedsDisposing() do // use obj else // only run if an error has occurred end
Multiple objects can be set up for disposal:
with obj = SomeObjectThatNeedsDisposing(), other = SomeOtherDisposableObject() do // use obj end
The value of a
with expression is the value of the last expression in the block, or of the last expression in the
else block if there is one and an error occurred.
Language constructs that can raise errors¶
The only language construct that can raise an error, other than the
error command or calling a partial method, is the
as command. This converts the given value to the specified type if it can be. If it can’t then an error is raised. This means that the
as command can only be used inside a try block or a partial method.
Comparison to exceptions in other languages¶
Pony errors behave very much the same as those in C++, Java, C#, Python, and Ruby. The key difference is that Pony errors do not have a type or instance associated with them. This makes them the same as C++ exceptions would be if a fixed literal was always thrown, e.g.
throw 3;. This difference simplifies error handling for the programmer and allows for much better runtime error handling performance.
else handler in a
try expression is just like a
catch(...) in C++,
catch(Exception e) in Java or C#,
except: in Python, or
rescue in Ruby. Since exceptions do not have types there is no need for handlers to specify types or to have multiple handlers in a single try block.
then block in a
try expression is just like a
finally in Java, C#, or Python and
ensure in Ruby.
If required, error handlers can “reraise” by using the
error command within the handler.