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Introducing Type "Kinds"

· 3 min read

Further adding to the type calculus of the CQL language we introduced the ability to encode the "kind" of primitive types. This can be used in a number of ways -- like "units" for natural things and like a "type" for synthetic keys and other such. It's easier to illustrate by example.

declare job_id type long<job_id>;
declare person_id type long<person_id>;

declare j job_id;
decalre p person_id;

set p := j; -- this is an error

With the above in place, other expressions like p == j would also produce errors as these long values are no longer type compatible. This is a great way to add enforcement to your schema and procedures. Likewise you can use these annotations to add "units" to your data types. e.g.

declare meters type real<meters>;
declare grams type real<grams>;

declare m meters;
declare g grams;

Variables of type grams (e.g. g) are not compatible with variables of type meters (e.g. m) even though both are real.

Likewise, attemping to insert grams into a column that is typed to meters will give errors. Of course SQLite doesn't know about any of this so all the <> stuff is removed in the generated SQL. This is just about type enforcement at compile time.

Enumerations like:

declare enum surface integer (paper, canvas);
declare enum writer integer (pen, paper, brush);

enable this:

declare s surface;                  -- s is now of type integer<surface>
declare w writer; -- w is now of type integer<writer>
set s := surface.paper; -- ok
set s := writer.pen; -- error
set w := writer.pencil; -- ok
case when s == w then 1 else 0 end; -- error (w/s not comparable)
set w := s; -- error again

additionally in DML/DDL:

create table draw_action(
w writer,
s surface

insert into draw_action values(w, s); -- ok
insert into draw_action values(s, w); -- error!

So the type kinds can be quite helpful when dealing with loose variables.

The notion of specific types was added to the language nearly two years ago to support the object type because there was a great desire to prevent object<dictionary> being assigned from object<list> but this "type kind", whether it's with units (e.g. "meters", "grams") or a type name (e.g. "job_id") adds a lot of high value type checking.

The kind can be added, stripped, or changed with a cast operation and the type system allows a constant or variable with no kind (e.g. "1") to mix and match with any kind so long as the base type is compatible as usual. So you get the most value by using the specific type consistently but you won't go insane adding test cases that use constants for instance.

As of this writing the expression kinds are checked for compatibility everywhere plus or minus bugs. There are extensive tests.