How do I dynamically find metadata for a Clojure f

问题:

Say I have the following code:

(defn ^{:graph-title "Function 1"} func-1
  [x]
  (do-something-with x))

(defn get-graph-title 
  [func]
  (str
    ((meta func) :graph-title))) 

I expect this to return "Function 1", but it returns nil. I think this stems from the following difference, which I don't totally comprehend:

(meta func-1)
=>  {:ns some-ns-info, :name func-1}
(meta #'func-1)
=>  {:ns some-ns-info, :name func-1, :graph-title "Function 1"}

Can someone explain this to me?

回答1:

The metadata is attached to the var, not to the function.

Thus, to get the graph title, you have to get the entry :graph-title from the meta of the var. How do you like your macros ?

(defmacro get-graph-title
  [func]
  `(:graph-title (meta (var ~func))))

(get-graph-title func-1)
=> "Function 1"

回答2:

There's metadata on the function func-1, metadata on the Var #'func-1, and metadata on the symbol 'func-1. The Clojure reader macro ^ adds metadata to the symbol, at read time. The defn macro copies metadata from the symbol to the Var, at compile time.

Prior to Clojure 1.2, functions did not support metadata. In Clojure 1.2, they do, and defn also copies some standard Var metadata to the function:

Clojure 1.2.0
user=> (defn ^{:foo :bar} func-1 [] nil) 
#'user/func-1
user=> (meta func-1)
{:ns #<Namespace user>, :name func-1}
user=> (meta #'func-1)
{:foo :bar, :ns #<Namespace user>, :name func-1, ...

However, in current Clojure 1.3 snapshots, defn does not copy any metadata to the function:

Clojure 1.3.0-master-SNAPSHOT
user=> (defn ^{:foo :bar} func-1 [] nil) 
#'user/func-1
user=> (meta func-1)
nil
user=> (meta #'func-1)
{:foo :bar, :ns #<Namespace user>, :name func-1, ...

In general, if you want to get at the metadata of a definition, you want metadata on the Var.

回答3:

The metadata you specify on the symbol func-1 in your source code is copied to the var named func-1 by the def special form. See the documentation for def in http://clojure.org/special_forms

When you evaluate func-1 where that's a symbol bound to a var, you get the value of the var (which is the function object in this case). See http://clojure.org/vars

The function object itself does not automatically recieve the metadata manually specified on the symbol / var.

So, the information you want is not in the function at all. It's in the var, and you have to specify that you really want the var func-1 itself instead of its value. That's what (var func-1), and the equivalent short-cut #'func-1 does.