Macroexpanding a macro with `fn` crashes with "fn did not conform to spec"

Question

The following code crashes:

(defmacro m []
  `(fn [x]))

(macroexpand '(m))

The error is:

Call to clojure.core/fn did not conform to spec
[...]
:reason "Extra input"

The following code does not crash and works as expected:

(defmacro m []
  `(asdfasdf [x]))

(macroexpand '(m))

How does the first snippet crash when the macro should just be returning a quoted piece of code? Why does it run fn at all?

Answer 1

Not sure what version of Clojure or editor you're using but Clojure 1.10.3 gives more useful output in the standard repl:

user=> (macroexpand '(m))
Syntax error macroexpanding clojure.core/fn at (REPL:1:1).
(user/x) - failed: Extra input at: [:fn-tail :arity-1 :params] spec: :clojure.core.specs.alpha/param-list
user/x - failed: vector? at: [:fn-tail :arity-n :params] spec: :clojure.core.specs.alpha/param-list

As you can see it is not running your code but (recursively) macroexpanding when the problem is encountered. The first expansion will become:

(clojure.core/fn [user/x])

clojure.core/fn is itself a macro that is expanded. During that expansion, the fn macro is checked against the fn spec (this is on by default for macros). The spec finds user/x and sees it as "extra input" in the :params because it does not conform to any expected parameter form (unqualified symbols usually, but also all recursive destructuring forms). Really, the problem here is user/x instead of x.

You can fix this with:

(defmacro m []
  `(fn [~'x]))

Comments

Much appreciated, that does make sense.

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I don't get why x evaluated to user/x when it's syntax quoted though. How is that different from unquoting and then quoting again?

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Syntax quote resolves all symbols in terms of the current namespace context. Unquote turns on evaluation, then evaluated a quoted unresolved symbol.

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I see thanks again

Answer 2

Try macroexpand-1 instead - then the mistake should become obvious.