I just heard that core.typed is ready for production. Which is great timing because I'm working with a new team and they like types.

So I've had a cursory play and discovered that if I create a file like so:

( ns typed-clojure-test.core ( :use clojure.core.typed )) ( ann my-inc [ Number -> Number ]) ( defn my-inc [ n ] ( let [ internal-number ( Integer/parseInt "1" )] ( + n internal-number )))

And then from a repl run:

( use 'clojure.core.typed ) ( check-ns 'typed-clojure-test.core )

I will be told:

Start collecting typed-clojure-test.core Finished collecting typed-clojure-test.core Collected 1 namespaces in 31.183 msecs Start checking typed-clojure-test.core Checked typed-clojure-test.core in 33.359 msecs Checked 1 namespaces (approx. 8 lines) in 65.576 msecs :ok

But if I remove Integer/parseInt and introduce a type error deep within my program:

( ns typed-clojure-test.core ( :use clojure.core.typed )) ( ann my-inc [ Number -> Number ]) ( defn my-inc [ n ] ( let [ internal-number "1" ] ( + n internal-number )))

And re-run the type checker (it automatically uses the latest version of the code, no need to use require :reload-all or anything):

( check-ns 'typed-clojure-test.core )

Then I get a really detailed description of the error with an accurate line and column location.

Start collecting typed-clojure-test.core Finished collecting typed-clojure-test.core Collected 1 namespaces in 26.436 msecs Start checking typed-clojure-test.core Checked typed-clojure-test.core in 102.541 msecs Type Error (typed-clojure-test.core:7:5) Static method clojure.lang.Numbers/add could not be applied to arguments: Domains: AnyInteger AnyInteger Number Number Arguments: Number (Value "1") Ranges: AnyInteger Number with expected type: Number in: (clojure.lang.Numbers/add n internal-number) in: (clojure.lang.Numbers/add n internal-number) ExceptionInfo Type Checker: Found 1 error clojure.core/ex-info (core.clj:4327)

I'm super impressed.

I've written a few libraries myself and I try really hard to make them easy for others to work with. But it can be difficult to get feedback more detailed than "I checked it out, it was cool". So in that spirit I've recorded my first 20 minutes with core.typed. It's mostly me failing to get it to work, and may only be of interest to Ambrose himself.

Since having had this first look I've watched the Typed Clojure video from the 2012 conj. In it Ambrose goes into depth about the capabilities of the type system and how to use it. I also spotted the Quickstart section from the readme. Either of these are probably better resources for figuring out how to begin than my frantic scan of the api docs and tutorials.