Using a loop inside a macro

Question

Consider this minimal example

(defmacro foo []
  `(list ,@(for [i [1 2 3]] (+ 1 i))))

I expect (foo) to expand to (list 2 3 4)

But instead, I get a bunch of indecipherable errors when I try (macroexpand-1 (foo)

Syntax error macroexpanding clojure.core/let at (*cider-repl <filepath>:localhost:43203(clj)*:366:22).
test/i - failed: simple-symbol? at: [:bindings :form :local-symbol] spec: :clojure.core.specs.alpha/local-name
test/i - failed: vector? at: [:bindings :form :seq-destructure] spec: :clojure.core.specs.alpha/seq-binding-form
test/i - failed: map? at: [:bindings :form :map-destructure] spec: :clojure.core.specs.alpha/map-bindings
test/i - failed: map? at: [:bindings :form :map-destructure] spec: :clojure.core.specs.alpha/map-special-binding

What's going wrong? Can't I use for loops inside my macro definitions?

amalloy · Accepted Answer · 2020-08-01 02:43:23Z

3

,@ should be ~@.

user=> (defmacro foo []
  `(list ~@(for [i [1 2 3]] (+ 1 i))))
#'user/foo

user=> (macroexpand-1 '(foo))
(clojure.core/list 2 3 4)

user=> (foo)
(2 3 4)

edited Aug 1, 2020 at 2:43

amalloy

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answered Jul 31, 2020 at 20:01

Alex Miller

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1 Comment

amalloy Over a year ago

The # is unrelated. There's no qualifying of variables needed here, since they're not introduced into the caller's scope. You've in fact just named a variable i#, which is legal but confusing. The only important change is the switch from , to ~.

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