core.typed is intended to be used at the REPL for rapid feedback, however there are a few things to look out for.

Checking mode

core.typed only has effect when in checking mode. cf and check-ns are two ways of starting checking mode.

This is most apparent when type checking a definition at the REPL.

clojure.core.typed=> (ann my-inc [Number -> Number])
nil
clojure.core.typed=> (defn my-inc [a] (inc a))
#'clojure.core.typed/my-inc
clojure.core.typed=> (cf (my-inc 1))
Type Error (clojure.core.typed:1:34)
Unannotated var clojure.core.typed/my-inc
Hint: Add the annotation for clojure.core.typed/my-inc
via check-ns or cf
in: clojure.core.typed/my-inc


ExceptionInfo Type Checker: Found 1 error 
clojure.core/ex-info (core.clj:4327)

To correctly feed forms to be checked, wrap each form in cf.

clojure.core.typed=> (cf (ann my-inc [Number -> Number]))
Any
clojure.core.typed=> (cf (defn my-inc [a] (inc a)))
(Var [Number -> Number])
clojure.core.typed=> (cf (my-inc 1))
Number

Experimenting at the REPL

core.typed is designed for this workflow:

• check a namespace with check-ns
• keep the type environment (if checking is successful) for REPL interaction
• use cf for flexible small experiments, possibly producing a “dirty” environment
• another check-ns will wipe away previous type state

If you want even more flexibility, check-ns accepts a :collect-only keyword argument. This collects type annotations for the target namespace (which must be provided) and does not perform type checking.

This is useful if you want to jump straight to REPL interactions for debugging purposes. Perhaps you need to diagnose a particular type error, but check-ns keeps throwing type errors.

clojure.core.typed=> (check-ns *ns* :collect-only true)
Start collecting clojure.core.typed
Finished collecting clojure.core.typed
Collected 1 namespaces in 686.003946 msecs
Checked 0 namespaces (approx. 0 lines) in 686.818295 msecs
:ok

More resources

For more advice on how to effectively leverage core.typed, join the core.typed mailing list or #typed-clojure on Freenode and start a conversation.