Myrddin: Error Handling

Error Handling

pkg std =
        type option(@a) = union
                `Some @a
                `None
        ;;

        type result(@a, @b) = union
                `Ok @a
                `Err @b
        ;;

        $noret const fatalv : (fmt : byte[:], ap : valist# -> void)
        $noret const fatal  : (fmt : byte[:], args : ... -> void)
        const assert    : (cond : bool, fmt : byte[:], args : ... -> void)
        const die           : (msg : byte[:] -> void)
        const suicide   : ( -> void)

        generic try : (v : result(@a, @b) -> @a)
        generic tryv : (v : result(@a, @b), d : @a -> @a)
        generic get : (v : option(@a) -> @a)
        generic getv : (v : option(@a), d : @a -> @a)
;;

Overview

Myrddin does not have exceptions. By convention, code will abort on programmer errors, such as passing invalid values where valid ones were expected -- for example, calling std.fmt("{}") with the wrong number of arguments int the list.

For recoverable error conditions that depend on the environment, and not the developer making a mistake, one of the branched return types are conventionally used.

For conditions where something can be either present or not, the option(@a) type is used. For places where there can be either a result or an error, the result(@a, @e) type is used.

Generally, by convention, the type returned for the result should have a custom that converts it to something directly displayable to the user.

Types

type option(@a) = union
        `Some @a
        `None
;;

As mentioned in the overview, option(@a) is a type that wraps up a result and error type. It is typically used in places where a missing value is the only exceptional condition.

type result(@a, @b) = union
        `Ok @a
        `Err @b
;;

The type result(@a, @e) is used to signal either success or an error condition. The first type parameter, @a is what is returned on success, and the second, @b is returned on failure.

Functions

$noret const fatalv : (fmt : byte[:], ap : valist# -> void)
$noret const fatal  : (fmt : byte[:], args : ... -> void)

Both fatal and fatalv exit the program with an error message, formatted as in std.fmt. They do not return.

They exit with a failure status. On Unix, this status is 1. On Plan 9, the status is the failure message that it prints out before exiting.

const assert    : (cond : bool, fmt : byte[:], args : ... -> void)

Assert checks that condition is true. If it is not true, then the message is printed as in std.fmt, and the program is aborted with suicide().

const suicide   : ( -> void)

Suicide aborts a program. It does not print any message, it simply sends the program a SIGABRT or segfaults it. All threads are terminated, and the program goes away.

generic try : (v : result(@a, @b) -> @a)
generic get : (v : option(@a) -> @a)

Try and get both return the value from the successful branch of their type: try returns the value contained in `std.Ok, and get returns the value in `std.Some.

If this does not match the union, a diagnostic message is printed and the program is aborted.

generic tryv : (v : result(@a, @b), default : @a -> @a)
generic getv : (v : option(@a), default : @a -> @a)

Try and get both return the value from the successful branch of their type: try returns the value contained in `std.Ok, and get returns the value in `std.Some.

If this does not match the union, the default value is returned to the caller, as though the type had contained `Some default or `Ok default.