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Clojure: &env and &form

02.07.2011
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Inside the body of defmacro you can call &env and &form to get a bit of interesting information that may or may not be helpful.

Here's a few examples that demonstrate how &env and &form can be used.
(note: I'm using Clojure 1.2)

&env
By default &env is nil.

user=> (defmacro show-env [] (println &env))
#'user/show-env
user=> (show-env)
nil
However, if any bindings exist, &env gives you the names of the bindings as the keys of a map.
user=> (let [band "zeppelin" city "london"] (show-env))
{city #<LocalBinding clojure.lang.Compiler$LocalBinding@78ff9053>, band #<LocalBinding clojure.lang.Compiler$LocalBinding@525c7734>}
Okay, now we're getting somewhere. What's a Compiler$LocalBinding? I'm not exactly sure, and I've never bothered to look into it. I've been told that the 'values' from &env may change in the future, so I wouldn't rely on them anyway. Since I can't rely on them, I haven't found the need to look into what they are.

Back to the keys. They sure look like symbols.
user=> (defmacro show-env [] (println (keys &env)) (println (map class (keys &env))))
#'user/show-env
user=> (let [band "zeppelin" city "london"] (show-env))
(city band)
(clojure.lang.Symbol clojure.lang.Symbol)
As the example shows, they are definitely symbols. However, these symbols don't have a namespace.
user=> (defmacro show-env [] (println (keys &env)) (println (map namespace (keys &env))))
#'user/show-env
user=> (let [band "zeppelin" city "london"] (show-env))
(city band)
(nil nil)
Since the symbols don't have a namespace there didn't seem to be much fun I could do with them; however, you can use the symbols in your macro to print the values, as the following example shows.
user=> (defmacro show-env [] (println (keys &env)) `(println ~@(keys &env)))                      
#'user/show-env
user=> (let [band "zeppelin" city "london"] (show-env))
(city band)
london zeppelin
Printing the values of bindings can be a helpful trick while you are debugging.
&form
&form can be used to get the original macro invocation.
user=> (defmacro show-form [] (println &form))                               
#'user/show-form
user=> (show-form)
(show-form)
Okay, not very interesting so far. It gets a bit more interesting when your macro takes a few arguments.
user=> (defmacro show-form [a b] (println &form))       
#'user/show-form
user=> (show-form 50 100)
(show-form 50 100)
user=> (show-form a 100)
(show-form a 100)
So, you can get the arguments. Notice you can grab both 50 and 100.
user=> (defmacro show-form [a b] (println (next &form)))
#'user/show-form
user=> (show-form 50 100)
(50 100)
user=> (defmacro show-form [a b] (println (map class (next &form))))
#'user/show-form
user=> (show-form 50 100)
(java.lang.Integer java.lang.Integer)
Interesting. So I have a few integers I can work with, if I wish. What about 'show-form'?
user=> (defmacro show-form [a b] (println (map class &form)))       
#'user/show-form
user=> (show-form 50 100)
(clojure.lang.Symbol java.lang.Integer java.lang.Integer)
'show-form' is a symbol, as expected. Which brings us back to a previous example, shown again below.
user=> (defmacro show-form [a b] (println (map class &form)))
#'user/show-form
user=> (show-form a 100)
(clojure.lang.Symbol clojure.lang.Symbol java.lang.Integer)
Okay, 'a' is also a symbol, unsurprising, but perhaps it's interesting since 'a' doesn't exist anywhere except in our invocation. You can probably do some interesting things here, like allow people to specify enum values and append the enum yourself.
user=> (ns user (:import [java.util.concurrent TimeUnit]))                                                     
java.util.concurrent.TimeUnit
user=> (defmacro time-units [& l] (->> (next &form) (map (partial str "TimeUnit/")) (map symbol) (cons 'list)))
#'user/time-units
user=> (time-units SECONDS MILLISECONDS)
(#< SECONDS> #< MILLISECONDS>)
Would you want to use &form instead of just using the arguments (stored in l)? Probably not. This isn't an exercise in what you should do, but it does demonstrate what you could do.

So &form must be returning a list, right?
user=> (defmacro show-form [a b] (println (map class &form)) (println (class &form))) 
#'user/show-form
user=> (show-form 50 100)
(clojure.lang.Symbol java.lang.Integer java.lang.Integer)
clojure.lang.PersistentList
A list. Correct.

And, I'm in a macro, so I can do anything I want with this list. Maybe I just want to print the arguments. Easy enough.
user=> (defmacro show-form [a b] `(println ~@(next &form)))
#'user/show-form
user=> (show-form 50 100)
50 100
Of course, there are a million things I could do. You get the idea.

One other interesting thing to note is that &form has metadata.
user=> (show-form 50 100)                                 
{:line 132}
Perhaps you don't care about line numbers, but they definitely can come in handy when you are writing a testing framework.

I use &form in expectations and I believe LazyTest uses &env. I guess you never know what you're going to need...

From http://blog.jayfields.com/2011/02/clojure-and.html

Published at DZone with permission of Jay Fields, author and DZone MVB.

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)

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Comments

Thomas Kern replied on Thu, 2012/09/06 - 10:48am

I think the main reason that &form exists, is so that you can access metadata that was applied to the macro call. Otherwise that would get lost when the macro was expanded.

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