Clojure Q&A - Recent questions tagged macroexpansion

Macro expansion lose type hints

Mon, 03 Nov 2025 13:50:41 +0000

The following snippet demonstrate how type hints when put before a macro expression are being lost.
On the first expression, the compiler correctly complains about the wrong type hint while it gets accepted if you wrap the hinted expression on a macro.

$ clj 
Clojure 1.12.3

user=> (import 'java.util.Date)

user=> (.setHours (Date.) ^long (.getHours (Date.)))
Syntax error (UnsupportedOperationException) compiling fn* at (REPL:1:1).
Cannot coerce int to long, use a cast instead

user=> (.setHours (Date.) ^long (-> (.getHours (Date.))))
nil

specifying `:let` as the first element of `for` causes an macro expansion error

Sun, 10 Aug 2025 02:40:33 +0000

We can use :let in for to define local variable like this

(for [i (range 5)
      :let [inc-i (inc i)]]
  [i inc-i])
;;=> ([0 1] [1 2] [2 3] [3 4] [4 5])

Otherwise, if we specified :let as first element, it causes an macro expansion error.

(for [:let [range-end 5]
      i (range range-end)
      :let [inc-i (inc i)]]
  [i inc-i])
;;=> Syntax error macroexpanding for at (*cider-scratch*:555:1).
;;   Can't pop empty vector

Of course, in this case, we should have wrapped for in let. However, I'm curious whether making :let available only for second and subsequent elements is intended behavior as part of the public interface.

defn rejects arity with args vector [&form]

Mon, 19 Aug 2024 21:05:00 +0000

Here's the smallest case, but it seems to happen no matter how many other arities are provided or whether list or vector syntax is used for the arity.

user=> (clojure-version)
"1.12.0-rc1"
user=> (defn f [&form])
Unexpected error (IndexOutOfBoundsException) macroexpanding defn at (REPL:1:1).
null
user=> *e
#error {
 :cause nil
 :via
 [{:type clojure.lang.Compiler$CompilerException
   :message "Unexpected error macroexpanding defn at (1:1)."
   :data #:clojure.error{:phase :macroexpansion, :line 1, :column 1, :source "NO_SOURCE_PATH", :symbol defn}
   :at [clojure.lang.Compiler macroexpand1 "Compiler.java" 7551]}
  {:type java.lang.IndexOutOfBoundsException
   :message nil
   :at [clojure.lang.RT subvec "RT.java" 1634]}]
 :trace
 [[clojure.lang.RT subvec "RT.java" 1634]
  [clojure.core$sigs$asig__5505 invoke "core.clj" 235]
  [clojure.core$sigs invokeStatic "core.clj" 259]
  [clojure.core$defn__5514 invokeStatic "core.clj" 317]
  [clojure.core$defn__5514 doInvoke "core.clj" 294]
  [clojure.lang.RestFn applyTo "RestFn.java" 149]
  [clojure.lang.Var applyTo "Var.java" 707]
  [clojure.lang.Compiler macroexpand1 "Compiler.java" 7525]
  [clojure.lang.Compiler macroexpand "Compiler.java" 7598]
  [clojure.lang.Compiler eval "Compiler.java" 7684]
  [clojure.lang.Compiler eval "Compiler.java" 7655]
  [clojure.core$eval invokeStatic "core.clj" 3232]
  [clojure.core$eval invoke "core.clj" 3228]
  [clojure.main$repl$read_eval_print__9240$fn__9243 invoke "main.clj" 437]
  [clojure.main$repl$read_eval_print__9240 invoke "main.clj" 437]
  [clojure.main$repl$fn__9249 invoke "main.clj" 459]
  [clojure.main$repl invokeStatic "main.clj" 459]
  [clojure.main$repl_opt invokeStatic "main.clj" 523]
  [clojure.main$main invokeStatic "main.clj" 668]
  [clojure.main$main doInvoke "main.clj" 617]
  [clojure.lang.RestFn invoke "RestFn.java" 400]
  [clojure.lang.AFn applyToHelper "AFn.java" 152]
  [clojure.lang.RestFn applyTo "RestFn.java" 135]
  [clojure.lang.Var applyTo "Var.java" 707]
  [clojure.main main "main.java" 40]]}

Macro-expansion hooks similar to clojure spec

Tue, 23 May 2023 14:18:10 +0000

clojure.spec.alpha currently holds a privileged position as a core library for Clojure core, being able to instrument macros as well as functions. Compiler.java relies on clojure.spec.alpha in this function, calling macroexpand-check specifically. This is a neat interaction and macroexpand-check is simple and obvious.

Other tools (such as Malli) can provide instrumentation for Clojure functions through alter-var-root (as seen in malli.instrument/-strument!), but they lack the ability to hook into macro-expansions.

It would be nice if Compiler.checkSpecs had some hook to allow for tools such as Malli to be used to instrument macros.

Macro symbols in &env don't get updated when shadowed, still contain the outer symbol instead.

Mon, 30 Jan 2023 03:13:31 +0000

(defmacro inspect
  [& body]
  (println (keys &env) (map meta (keys &env)))
  `(do ~@body))

(let [^:1 a 1]
  (let [^:2 a 2]
    (let [^:3 a 3]
      (inspect "hello"))))

;;=> (a) ({:1 true})
"hello"

The &env form returns a map of {sym LocalBinding} and the symbols on the keys of the map are supposed to be the local symbols, but as we see in the above code, when a local symbol is shadowing another, it seems the map does not contain the shadowing symbol, but instead contains the shadowed symbol. We can see that because it is returning the metadata of the outermost symbol, and not the inner one that is shadowing it.

In my opinion, this is a bug, that said, you can still get at the actual shadowing symbol by calling .-sym on the LocalBinding, in which case you get the inner shadowing symbol and it will have the right metadata.