throw

You can throw errors as in JavaScript.

If you throw a string, it will be converted to an Error for you. It has to be a string literal though — throw a won't work if a happens to be a String.

throw "This code throws a fit!"

assert

Usually, you'll only need to throw an exception under certain conditions.

assert true
forbid =? 0 1
|| `throw` splits the line in two between the condition and the error.
assert false throw "Custom error"

If an explicit throw isn't given, the compiler tries to create a descriptive error message.

For example, when asserting the result of a function call, the error message will include the function called and its arguments.

except

You can save yourself from an exception with except.

The error given to to catch is by default , but you could also provide a name.

except
	try
		throw 'oh-no!
	|| same as:
	|| catch foo
	||	foo
	catch
		_.message
	|| `finally` is optional
	finally
		pass

Usually you only need to catch a certain type of exception.

A type on the caught exception means that the exception will not be caught unless it is of that type.

Omitting a variable name before the type makes it _.

You can only provide one untyped catch and it has to come at the end, much like the else of a switch.

except
	try
		throw new TypeError 'type-no-good
	catch r:RangeError
		r
	catch :TypeError
		_
	catch some-other-error
		some-other-error

Usually you only need to catch errors in a few lines.

Code in else happens after the code in try but errors in it are not caught.

This helps you make clear which specific code is dangerous, moving everything else into else.

except
	try
		|| `try` is for dangerous code that throws errors!
		foo = 'success
	catch
		'failure
	else
		|| `else` is for using values defined in `try`.
		|| If there are errors here, they won't be caught.
		foo

If there are any errors in try, else never runs.

You can read code as: try (something dangerous)... catch (these errors)... else (if nothing was caught)...

else can use values defined in try, but catch can't. Why? Because catch only runs if something went wrong when defining those values.