The Explorer
The Adventures of a Pythonista in Schemeland/9
by Michele Simionato
October 31, 2008

 Summary
 This episode is entirely devoted to Scheme macros from a personal point of
 view. Pattern matching is introduced as the fundamental mechanism on which
 macros are built.
_________________________________________________________________________

## A minimal introduction to Scheme macros

Scheme macros have many faces. You can see them as a general mechanism to extend
the syntax of base Scheme, and also as a mechanism to reduce boilerplate. On the
other hand, if you focus your attention on the fact that they work at compile
time, you can see them as a mechanism to perform arbitrary computations at compile
time, including compile time checks.

I think the correct way of looking at macros is to see them as a general facility
to write compilers for micro-languages - or Domain Specific Languages, DSL -
embedded in the Scheme language. The languages defined through macros can be very
different from Scheme; for instance you can define object oriented languages
(object systems such as TinyCLOS or Swindle are typical examples) or even
languages with static typing (the new language Typed Scheme, built on top of PLT
Scheme, is such an example).

In order to address such use cases, Scheme macros have to be extremely advanced,
much more than Lisp macros and any other kind of macros I know of; as a
consequence, they also have a reputation for complexity. Unfortunately, on top of
the intrinsic complexity, Scheme macros also suffers from accidental complexity,
due to history (there are too many standard macro systems) and to the tradition of
not caring about readability (many Scheme constructs are made to look more complex
than they actually are).

Scheme has two macro systems included in the de jure standard - syntax-rules,
which allows to define hygienic macros only, and syntax-case, which has the full
power of Lisp macros and much more - plus a de facto standard based on
define-macro system, which is available in all implementations and it is well
known to everybody because of its strict similarity to Common Lisp defmacro
system.

## Which macrology should I teach?

Since there are so many macro systems it is difficult to decide from where to
start in a pedagogical paper or tutorial. If you look at the original Italian
version of this paper, you will see that I did talk about syntax-rules macros
first. However, after a lot of thinking, I have decided to go my own way in this
English series of the Adventures. Here I will not discuss syntax-rules, nor I will
discuss syntax-case: instead, I will discuss my own version of Scheme macros,
which I called sweet-macros.

Why I am doing that? After all, why my readers should study my own version of
macros when they surely will be better served off by learning the standard
macrology used by everybody? I have spent years debating with myself this very
question, but at the end I have decided to go this way for a series of reasons:
1. I regard the existence of two separate macro systems in the same standard as a
   wart of Scheme and as a mistake made by the R6RS editors: they should have
   included in the language syntax-case only, leaving syntax-rules as a
   compatibility library built on top of syntax-case;
2. I really don't like the syntax-case syntax, it is by far too verbose and
   unreadable; I find there is a strong need for some sugar on top of it and that
   is what sweet-macros are for;
3. sweet-macros are very close to syntax-case macros, so once you understand them
   you will understand syntax-case too; from there, understanding syntax-rules is
   a breeze;
4. starting from sweet-macros is much better from pedagogical purposes,
   especially for readers with a Common Lisp background, since it is easy to
   explain the relation with defmacro and the hygiene issue;
5. my target readers are programmers coming from the scripting languages
   (Perl/Python/Ruby) world. For this kind of public, with no previous exposition
   to Scheme, bare syntax-case is just too hard, so I needed to dress it in nice
   clothes to make it palatable;
6. sweet-macros are intended to easier to use than syntax-case macros, but they
   are also more powerful, since they provide introspection and debugging
   capabilities as well as guarded patterns, so they should look attractive to
   experienced users too; however, this is a nice side effect and not the main
   motivation for the library;
7. sweet-macros were written expressely for this series of papers, since I did
   not want to litter my explanation of Scheme macros with endless rants. So, I
   took action and I wrote my own library of macros made "right": this is also a
   tribute to the power of Scheme macros, since you can "fix" them from within
   the standard macro framework in fifty lines of code.

If you are an advanced reader, i.e. a Schemer knowing syntax-case/syntax-rules or
a Lisper knowing defmacro, I a am sure you will ask yourself what are the
differences of sweet-macros with respect to the system you know. I will make a
comparison of the various systems in the future, in episode #12 and later on. For
the moment, you will have to wait. I do not want to confuse my primary target of
readers by discussing other macro systems right now. I also defer to episode #12
the delicate question are macros a good idea?. For the moment, focus on what
macros are and how you can use them. Then you will decide if they are a good idea
or not.

## Enter sweet-macros

My sweet-macros library is a small wrapper around the syntax-case macro system. I
release it under a liberal BSD licence. You can do whatever you want with it, just
keep the attribution right.

The primary goal of sweet-macros is semplicity, so it only exports three macros,
def-syntax, syntax-match and syntax-expand:
 * def-syntax is a macro used to define simple macros, which is similar to
   defmacro, but simpler and strictly more powerful.
 * syntax-match is a macro used to define complex macro transformers. It is
   implemented as a thin layer of sugar on top of syntax-case.
 * syntax-expand is a macro which acts as a debugging facility to expand macros
   defined via def-syntax or syntax-match.

It should be mentioned that standard Scheme macros do not provide debugging and/or
introspection facilities and that every implementation provides different means of
debugging macros. This is unfortunate, since debugging macros is usually difficult
and it is done often, since it is uncommon to get a macro right the first time,
even if you are an experienced developer.

I wanted to provide my readers with the tools to understand what they are doing,
without relying on the details of the implementation they are using. Therefore
macros defined via syntax-match (and that includes macros defined via def-syntax)
provide out of the box introspection and debugging features.

Of course, readers who want to rely on the debugging tools of their implementation
can do so; for instance I hear that DrScheme has a pretty good macro stepper but I
have not tried it since I am an Emacs-addict.

First of all, you should download and install the right sweet-macros library.
Unfortunately the R6RS module system does not really solve the portability issue
(to my endless frustration) so I had to write different versions of the same
library :-( I you are using Ikarus you should download the single file version of
the library

   $ wget http://www.phyast.pitt.edu/~micheles/scheme/sweet-macros.sls

and put it everywhere in you IKARUS_LIBRARY_PATH. If you are using PLT Scheme (you
need a version of PLT newer than 4.0 for R6RS support) you must download the zip
file version

   $ wget http://www.phyast.pitt.edu/~micheles/scheme/sweet-macros.zip
   $ unzip sweet-macros.zip
   $ mv sweet-macros <your collects directory>

and install it in your collects directory, which on my machine is

   $HOME/.plt-scheme/4.0/collects.

Actually, the multifile version of the library works also with Ikarus if you have
a recent enough version (right now I am using the trunk, version 0.0.3+, revision
1654). I have not tried the library on Larceny; I have tried it in Ypsilon Scheme
which however has a small bug so that it does not run there (the bug is already
fixed in the trunk). You should always keep in mind than R6RS implementations are
pretty young and that implementors are still working to make them really
compatible. I have also prepared an R5RS version which should work in Chicken
Scheme, at least in the interpreter:

   $ wget http://www.phyast.pitt.edu/~micheles/scheme/sweet-macros.scm

However I have developed and tested sweet-macros in Ikarus only (caveat emptor!).
Still, since the title of this blog is The Explorer, I think it is fine if we deal
with exploratory code.

You can check that the installation went well by importing the library:

   $ ikarus
   > (import (sweet-macros))

and by trying to define a macro.

An example: multi-define

Here is a multi-define binding construct which allows to define many identifiers
at once:

   (def-syntax (multi-define (name ...) (value ...))
     #'(begin (define name value) ...))

As you see, Scheme macros are based on pattern matching: we are giving
instructions to the compiler, specifying how it must acts when it sees certain
patterns. In our example, when the compiler sees a multi-define expression
followed by two sequences with zero o more arguments, it must replace it with a
begin expression containing a sequence of zero or more definitions. You can check
that this is exactly what happens by means of syntax-expand:

   > (syntax-expand (multi-define (a b) (1 2)))
   (begin (define a 1) (define b 2))

Notice that (multi-define () ()) is valid code expanding to a do-nothing (begin)
expression; if you want to reject this corner case, you should write your macro as

   (def-syntax (multi-define (name1 name2 ...) (value1 value2 ...))
     #'(begin (define name1 value1) (define name2 value2) ...))

so that multi-define requires one or more arguments. However, it is often useful
to accept degenerate corner cases, because they may simplify automatic code
generation (i.e. multi-define could appear in the expansion of another macro).

multi-define works as you would expect:

   > (multi-define (a b) (1 2)) ; introduce the bindings a=1 and b=2
   > a
   1
   > b
   2

I have just scratched the surface of Scheme macros here: I leave the rest for the
next episode, don't miss it!

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