Design choices

All vars must have annotated static types

Use clojure.core.typed/ann to assign types to vars

eg. Assign my-fn in the current namespace the type [Any -> Any] (a function of one argument).

(ann my-fn [Any -> Any])

Type checking is separate to compilation and must be explicitly run

Use clojure.core.typed/check-ns to type check the current namespace. This can be done at the REPL.

Note: Global annotations like ann are only valid when found in a namespace currently being checked with check-ns, or wrapped in a cf. A raw ann in a REPL has no effect. Global annotations should be top-level forms or inside a (possibly nested) top-level do.

All function arguments need to be annotated, or default to Any

Use clojure.core.typed/ann-form to annotate a function.


(ann-form #(+ 1 %) [Number -> Number])

Everything is type checked, but core.typed can ignore certain expressions

core.typed is early in development and there are Clojure idioms it cannot currently type check. Wrap top-level expressions in clojure.core.typed/tc-ignore to ignore them.

Suggestion: If porting a namespace to core.typed, initially use tc-ignore liberally to ignore problematic code while determining the types for expressions. Once most vars are annotated, revisit these sites to determine the issue.


clojure.core.typed/print-env takes a debug string and prints the local type environment at the current expression.

Use cf to experiment at the REPL

clojure.core.typed/cf takes an expression and optionally an expected type and type checks the expression, returning its inferred type.


clojure.core.typed=> (cf (fn [a]
                           {:pre [(number? a)]}
                           (inc a)))
[(Fn [Any -> java.lang.Number]) {:then tt, :else ff}]

If cf returns a vector of results, the first element is the static type.

Use ann-form to ensure expressions are particular types

clojure.core.typed/ann-form can be used as a kind of static assert.

clojure.core.typed=> (cf (let [a (+ 1 2)
                               _ (ann-form a clojure.lang.Symbol)]
#<AssertionError java.lang.AssertionError: Assert failed: 6: Local binding a expected type clojure.lang.Symbol, but actual type clojure.core.typed/AnyInteger
(or (not expected) (subtype? t (ret-t expected)))>

core.typed understands assertions and conditionals

Normal "untyped" Clojure code often use type predicates combined with assertions or conditionals to direct control flow. core.typed uses them to gain type information about the current environment.

(let [a (ann-form 1 Number)
      _ (print-env "before assert")
      _ (assert (integer? a))
      _ (print-env "after assert")])
; "before assert"{:env {a java.lang.Number}, 
;                 :props ()}
; "after assert"{:env {_28338 nil, _ nil, a clojure.core.typed/AnyInteger}, 
;                :props ((is clojure.core.typed/AnyInteger a) (when (! (U false nil) _) ff) (when (! (U false nil) _) ff) (when (! (U false nil) _28338) ff))}

The :env map is maps local bindings to their current types. :props is a list of propositions currently in scope (can usually be ignored, mostly useful for internal debugging purposes).

Notice the local binding a has a more accurate type after the assert expression.

Note: core.typed operates on a hygienic AST, so shadowed bindings will have gensymed names.

Typing core constructs

core.typed understands datatype definitions

Use clojure.core.typed/ann-datatype to give a datatype an expected type.

Use defprotocol> instead of defprotocol

core.typed currently cannot understand protocol definitions. Simply replace references to defprotocol with clojure.core.typed/defprotocol>

core.typed understands simple multimethods

core.typed can infer accurate types for multimethods that dispatch on simple things like keywords or class. Just assign an expected type to the multimethod's var with ann and core.typed will use it to infer accurate types in each defmethod.

If in doubt whether a multimethod is being inferred properly, use the debugging techniques to double check. core.typed may not throw an exception if the dispatch is too complex to type check currently.

Macros & Macro Definitions

Macro definitions are ignored. The type checker operates on the macroexpanded form from the Compiler's analysis phase.

Type Syntax

Types use the current global scope of the namespace

Simply adding an (:import ...) to the ns declaration as usual in Clojure brings the class name into scope. Otherwise, refers to classes via their fully qualified name.