How useful are Lisp macros?

What sorts of things do people actually end up doing with macros?

Writing language extensions or DSL's.

To get a feel for this in Lisp-like languages, study Racket, which has several language variants: Typed Racket, R6RS, and Datalog.

See also the Boo language, which gives you access to the compiler pipeline for the specific purpose of creating Domain-Specific Languages through macros.

Here are some examples:

Scheme:

• define for function definitions. Basically it makes a shorter way to define a function.
• let for creating lexically scoped variables.

Clojure:

• defn, according to its docs:

  Same as (def name (fn [params* ] exprs*)) or (def name (fn ([params* ] exprs*)+)) with any doc-string or attrs added to the var metadata

• for: list comprehensions
• defmacro: ironic?
• defmethod, defmulti: working with multi-methods
• ns

A lot of these macros make it much easier to write code at a more abstract level. I think of macros as being similar, in many ways, to syntax in non-Lisps.

The plotting library Incanter provides macros for some complex data manipulations.

Macros are useful to embedded some patterns.

For example, Common Lisp doesn't define the while loop but has do which can be used to define it.

Here is an example from On Lisp.

(defmacro while (test &body body)
  `(do ()
       ((not ,test))
     ,@body))

(let ((a 0))
  (while (< a 10)
    (princ (incf a))))

This will print "12345678910", and if you try to see what happens with macroexpand-1:

(macroexpand-1 '(while (< a 10) (princ (incf a))))

This will return:

(DO () ((NOT (< A 10))) (PRINC (INCF A)))

This is a simple macro, but as said before, they're usually used to define new languages or DSLs, but from this simple example you can already try to imagine what you can do with them.

The loop macro is a good example of what macros can do.

(loop for i from 0 to 10
      if (and (= (mod i 2) 0) i)
        collect it)
=> (0 2 4 6 8 10)
(loop for i downfrom 10 to 0
      with a = 2
      collect (* a i))
=> (20 18 16 14 12 10 8 6 4 2 0)               

Common Lisp has another kind of macros called reader macro which can be used to modify how the reader interprets the code, i.e. you can use them to use #{ and #} has delimiters like #( and #).

I mostly use macros for adding time-saving new language constructs, that would otherwise require a bunch of boilerplate code.

For example, I recently found myself wanting an imperative for-loop similar to C++/Java. However, being a functional language, Clojure didn't come with one out of the box. So I just implemented it as a macro:

(defmacro for-loop [[sym init check change :as params] & steps]
  `(loop [~sym ~init value# nil]
     (if ~check
       (let [new-value# (do ~@steps)]
         (recur ~change new-value#))
       value#)))

And now I can do:

 (for-loop [i 0 , (< i 10) , (inc i)] 
   (println i))

And there you have it - a new general-purpose compile-time language construct in six lines of code.