Function userError

A user error is an error caused by the programmer.

There are two types of user errors:

• Providing illegal input to the interface, that is, providing input that's never accepted and is considered malformed regardless of the current state.
• In the case where the interface user has sole ownership of the state: Providing input to the interface that is invalid in the current state.

Many interface contracts may not report user errors and instead choose the route of undefined behaviour with a potentially poisonous outcome. Check the policy of a specific interface for confirmation.

A perfect type system would remove the need for user errors to ever be reported at runtime and instead be caught at compile-time.

Example

const sqrt = (x) => {
  if (x < 0) {
    throw user_error("x must be non-negative");
  }

  return Math.sqrt(x);
};
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