Error Handling

Puck's error handling is shamelessly stolen from Swift. It uses a combination of Option/Result types and try/catch statements, and leans somewhat on Puck's metaprogramming capabilities.

There are several ways to handle errors in Puck. If the error is encoded in the type, one can:

1. match on the error
2. compactly match on the error with if ... of
3. propagate the error with ?
4. throw the error with !

If an error is thrown, one must explicitly handle (or disregard) it with a try/catch block or risk runtime failure. This method of error handling may feel more familiar to Java programmers.

Errors as Monads

Puck provides Option[T] and a Result[T, E] types, imported by default. These are union types and so must be pattern matched upon to be useful: but the standard library provides a bevy of helper functions. Two in particular are of note. The ? operator unwraps a Result or propagates its error up a function call (and may only be used in type-appropriate contexts). The ! operator unwraps an Option or Result directly or throws an exception in the case of None or Error.

pub macro `?`[T, E](self: Result[T, E]) =
  quote:
    match `self`
    of Okay(x): x
    of Error(e): return Error(e)

pub func `!`[T](self: Option[T]): T =
  match self
  of Some(x): x
  of None: raise EmptyValue

pub func `!`[T, E](self: Result[T, E]): T =
  of Okay(x): x
  of Error(e): raise e

The utility of the provided helpers in std.options and std.results should not be understated. While encoding errors into the type system may appear restrictive at first glance, some syntactic sugar goes a long way in writing compact and idiomatic code. Java programmers in particular are urged to give type-first errors a try, before falling back on unwraps and try/catch.

A notable helpful type is the aliasing of Result[T] to Result[T, ref Err], for when the particular error does not matter. This breaks try/catch exhaustion (as ref Err denotes a reference to any Error), but is particularly useful when used in conjunction with the propagation operator.

Errors as Catchable Exceptions

Errors raised by raise/throw (or subsequently the ! operator) must be explicitly caught and handled via a try/catch/finally statement. If an exception is not handled within a function body, the function must be explicitly marked as a throwing function via the yeet prefix (name to be determined). The compiler will statically determine which exceptions in particular are thrown from any given function, and enforce them to be explicitly handled or explicitly ignored.

Despite functioning here as exceptions: errors remain types. An error thrown from an unwrapped Result[T, E] is of type E. catch statements, then, may pattern match upon possible errors, behaving similarly to of branches.

try:
  ...
catch "Error":
  ...
finally:
  ...

This creates a distinction between two types of error handling, working in sync: functional error handling with Option and Result types, and object-oriented error handling with catchable exceptions. These styles may be swapped between with minimal syntactic overhead. Libraries, however, should universally use Option/Result, as this provides the best support for both styles.

Errors and Void Functions

Some functions do not return a value but can still fail: for example, setters. This can make it difficult to do monadic error handling elegantly: one could return a Result[void, E], but...

pub func set[T](self: list[T], i: uint, val: T) =
  if i > self.length:
    raise IndexOutOfBounds
  self.data.raw_set(offset = i, val)

Unrecoverable Exceptions

There exist errors from which a program can not reasonably recover. These are the following:

• Assertation Failure: a call to an assert function has returned false at runtime.
• Out of Memory: the executable is out of memory.
• Stack Overflow: the executable has overflowed the stack.
• any others?

They are not recoverable, but the user should be aware of them as possible failure conditions.

References: Error Handling in Swift