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In absent.diesel:

The mb[T] parameterized type represents either a value of type T or the value absent, akin to ML's option datatype. mb[T] differs from maybe[T] in that mb[T] doesn't introduce a level of indirection, but cannot store absent as a non-absent value. (The current Diesel typechecker also has some limitations in its handling of or-types like mb[T], such as that such a type cannot be directly declared to be subtypes of comparable[mb[T]].)

extend module Stdlib;

module Absent;

The distinguished absent value:

object absent;

mb[T] is either absent or a T:

synonym mb[T] = T | absent;

Testers:

fun is_present(:mb[`T]):bool;
fun is_absent(:mb[`T]):bool;

``Typecase'' operations:

fun if_present(:mb[`T], if_present:&(T):`R, if_absent:&():`R):R;
fun if_present(:mb[`T], if_present:&(T):void):void;
fun if_absent (:mb[`T], if_absent:&():`R, if_present:&(T):`R):R;
fun if_absent (:mb[`T], if_absent:&():void):void;

Checked ``downcast'' operations:

fun as_present(:mb[`T]):T;
fun as_present(:mb[`T], :&():T):T;

end module Absent;