Errors

Sere exceptions are values with a class hierarchy rooted at Exception. try needs except and/or finally. Optional else runs when nothing was raised.

try:
    raise TypeError("nope")
except TypeError as e:
    print(e.message)

except Type matches that class and its subclasses. Bare except: catches everything. as e binds an instance with .message.

Raising

class Boom(ValueError):
    pass

raise Boom("bad")
raise "boom"                 # Exception
raise TypeError              # empty message
assert False, "fail"         # AssertionError, catchable

raise stringifies the first constructor argument (or a bare str). Bare raise uses an empty Exception. panic("msg") still aborts — it is not catchable control flow.

Builtin classes

All subclass Exception:

SyntaxError, IndentationError, NameError, AttributeError, TypeError, IndexError, ImportError, ValueError, AssertionError, PermissionError, RuntimeError, RecursionError, NotImplementedError.

The compiler also uses these names as diagnostic codes: error[NameError]: unknown name 'foo'. That is the same catalog, two surfaces. See Diagnostics.

Prelude macros

todo!("not yet")
unreachable!()
dbg!(total)          # prints and yields total

todo! and unreachable! are for paths that must not run. They are not a substitute for raise.

defer vs finally

defer runs on function return, in reverse order. finally runs when leaving the try, including via raise. Use finally for exception-safe cleanup; use defer for the happy-path / every-return path inside one function.