Semantics as metadata?

[airballking]

Kinematic, dynamic data-structure carry semantics which we can use for reasoning/configuring and checking. In C++ I always had to create a new data-structure for a numeric representation with semantics.

In Clojure, data can carry meta-data annotations. Would this be accessible to programs at runtime? Can we store the intended semantics in the meta-data of matrices/vectors to avoid creating new data-structures or bloating set/maps?

Yes, this is for example done in the clojure.zip (explanation video) core library that is used to navigate and manipulate tree data-structures.

There the functions that specify how to traverse the tree are meta annotated to the data of the tree itself.

In general it should be avoided though to use metadata. The reason for this is two-fold.

• Metadata is not considered during equality checks, and thus can break referential transparency.
• Metadata can be lost quickly. Even though a datum derived from an immutable datum by conj or assoc will inherit the metadata many functions use a paradigm of destructuring collections and then reassembling new ones.

(let [{x :x y :y} mymap]
    {:x (f x)
     :y (g y)}

So a little bit more context is needed on how this would be used to determine if it would be idiomatic :)