An interesting aspect of type checking a Clojure dialect is dealing with host interop.
In Typed Clojure, Java interop is handled by utilising Java type information. For the collected Java types to be sound in Typed Clojure, we then transform them to deal with corner cases in the Java type system like covariant arrays and null.
Checking DOM interactions
Take this helper function
(ann by-id [string -> (U nil js/HTMLElement)]) (defn by-id [id] (.getElementById js/document id))
Typed Clojurescript can statically verify the expected annotation of
by-id if it knows
js/documentis of type
- an instance of
js/Documenthas a method of type
[string -> (U nil js/HTMLElement)].
This information needs to be provided explicitly, and the current implementation of Typed Clojurescript provides a small base environment.
Clearly the amount of work to manually annotate the entirety of the DOM is not worth the effort. However, there are several sources of inspiration for automating this effort.
- TypeScript annotates the DOM and provides type checking on its usage.
- The DOM4 spec includes detailed type signatures for the DOM.
This work is an important deliverable for the Typed Clojure crowdfunding campaign. The longer I can work on Typed Clojure, the more viable Typed Clojurescript will become.