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Errors and missing values

The functions to return a failure or "no value" that the caller has to check, grouped by what you want to do. The From column says what a program needs before it can call one:

• built-in: nothing; it is always available.
• std/outcome: #import "std/outcome", then using outcome to call it by its bare name.

Jump to: Return a value or the reason it failed · Return a value that may be missing · Catch runtime errors in a handler · Find out what went wrong · Raise an error yourself

Return a value or the reason it failed

Function	What it does	From
Result	Holds either a value or the reason it could not be made; a new one counts as failed.	std/outcome
Ok	Returns a successful Result holding the value; both types must be written out.	std/outcome
Fail	Returns a failed Result holding the reason; both types must be written out.	std/outcome
Result.IsOk	Returns true if the Result holds a value rather than a reason for failure.	std/outcome
Result.Value	Returns the value by reference; raises an error if the Result failed.	std/outcome
Result.Failure	Returns the reason by reference; raises an error if the Result succeeded.	std/outcome
Result.ValueOr	Returns the value, or the fallback you pass if the Result failed.	std/outcome

Return a value that may be missing

Function	What it does	From
Optional	Holds one value or nothing at all; a new one is empty.	std/outcome
Some	Returns an Optional holding the value, its type taken from the argument.	std/outcome
None	Returns an empty Optional of the type you write out.	std/outcome
Optional.HasValue	Returns true if the Optional holds a value.	std/outcome
Optional.Value	Returns the value by reference; raises an error if the Optional is empty.	std/outcome
Optional.ValueOr	Returns the value, or the fallback you pass if the Optional is empty.	std/outcome
Optional.Clear	Empties the Optional, releasing whatever its value held.	std/outcome

Catch runtime errors in a handler

Function	What it does	From
On Error	Sets a label to jump to when a runtime error happens; On Error Goto 0 turns it off.	built-in
Local	Marks an On Error handler as belonging to the procedure it is set in.	built-in

Find out what went wrong

Function	What it does	From
Err	Returns the number of the last runtime error, or sets it without jumping.	built-in
Erl	Returns the source line of the last runtime error, or 0 if there was none.	built-in
Erfn	Returns a pointer to the name of the failing procedure; null unless built with -exx.	built-in
Ermn	Returns a pointer to the name of the source file where the last error happened.	built-in

Raise an error yourself

Function	What it does	From
Error	Raises a runtime error by number: jumps to the handler, or ends the program.	built-in

See Also

• Result guide
• Optional guide
• Error handling functions

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