Errors

An error is a named failure a protocol function can raise. It has a required message and, optionally, fields.

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/// Raised when a message is sent to an unknown recipient.
error RecipientNotFound {
    message = "no recipient named {self.name}"
    name: str
}

message

A string with {dotted.path} placeholders that are substituted when the error is rendered — {self.name} reads the error's own name field. Write a literal brace by doubling it: {{ and }}.

message is expected on every error.

Fields

After message, an error takes fields exactly like a struct — typed, optionally optional, with defaults and docstrings. They carry the data the message placeholders read.

Raising one

A function declares the errors it can raise with ! ErrorName:

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protocol Mail {
    function send(message: Message) -> bool ! RecipientNotFound;
}

This is a bare reference — binding values into the error's fields at the raise site is not part of the language yet.