Question

I am trying to create a little Clojure macro that defs a String with a type hint:

(defmacro def-string [name value]
  `(def ^String ~name ~value))

(def-string db-host-option "db-host")

When I macroexpand it, the type hint is lost:

(macroexpand '(def-string db-host-option "db-host"))
;=> (def db-host-option "db-host")

Never mind the wisdom of type hinting this.

Why is the macro losing the metadata? How do I write this macro, or any that includes metadata?

Answer 1

^ is a reader macro. defmacro never gets to see it. The hint is put on the list (unquote name). Compare for example (meta ^String 'x) with (meta ' ^String x) to see the effect.

You need to put the hint on the symbol.

(defmacro def-string
  [name value]
  `(def ~(vary-meta name assoc :tag `String) ~value))

And the usage:

user=> (def-string foo "bar")
#'user/foo
user=> (meta #'foo)
{:ns #<Namespace user>, :name foo, :file "NO_SOURCE_PATH", :line 5, :tag java.lang.String}