Type declarations: Constructor functions?

[ChristianGruen]

I’ve tagged this as a discussion issue:

Would it make sense to declare constructor functions for the new XQuery type declarations, similar to what we now have for records, or do we believe that’s over the top?

It might improve typing in complex code, and it would allow us to write things like:

declare type world:continent as enum('Africa', 'America', 'Asia', 'Australia', 'Europe');
world:continent('Africa')

For the type above, an implicit constructor function would be created that would simply coerce its argument to the declared type:

declare function world:continent($value) as world:continent { $value };

One current drawback is that this only works for prefixed types (see #657), whereas it’s possible to define types without prefix, and reference them in local type declarations:

declare type continent as enum('Africa', 'America', 'Asia', 'Australia', 'Europe');
let $c as continent := 'Africa'
return $c