gleam/docs/runtime-errors.md

Runtime errors

There are several runtime errors that Gleam code can throw. This documentation lists them and their runtime properties.

On Erlang runtime errors are Erlang maps thrown with erlang:error/1, having at least these properties:

Key	Type	Value
gleam_error	Atom	See individual errors
message	String	See individual errors
module	String	The module the error occured in
function	String	The function the error occured in
line	Int	The line number the error occured on

On JavaScript runtime errors are instances of the JavaScript Error class, having at least these properties added to them:

Key	Type	Value
gleam_error	String	See individual errors
message	String	See individual errors
module	String	The module the error occured in
function	String	The function the error occured in
line	Number	The line number the error occured on

Todo

A panic that indicates that the code has not yet been completed, intended for use in development.

todo
todo as "some message"

Key	Erlang Value	JavaScript Value
gleam_error	todo	"todo"
message	The given message	The given message

Panic

An explicit panic to unconditionally error.

panic
panic as "some message"

Key	Erlang Value	JavaScript Value
gleam_error	panic	"panic"
message	The given message	The given message

Let assert

An inexhaustive pattern match, erroring if the pattern does not match.

let assert Ok(x) = something()

Key	Erlang Value	JavaScript Value
gleam_error	let_assert	"let_assert"
message	The given message	The given message
value	The unmatched value	The unmatched value