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Chicken for Ruby programmers

If you don't know much about Chicken yet, please take a moment to read the introductory part of The User's Manual. You're back? Good!

Paradigm independence

The most important design feature of Ruby is that it is purely object-oriented; everything is an object. Scheme is not an object-oriented language. In fact, it does not commit to any particular programming paradigm -- it offers complete and total freedom to the programmer. If you decide (a part of) a program is best implemented in an object-oriented fashion, you can choose to use one of the many object systems. Have a quick glance at Eggs Unlimited#object-oriented-programming to get an impression of the diversity of styles of object oriented programming you can use with Chicken. By the way, the list on that page shows all the available eggs for Chicken. We'll explain all about these later.

Besides object-oriented programming, you can also program in a procedural fashion (like you would in Pascal, for example) or in a functional style (a bit like Haskell or ML) and you can even experiment with message-passing like in Erlang, logic programming like you would with Prolog, or stack languages like Forth and Factor.


Ruby's origins are firmly rooted in Lisp. It takes many things and ideas from Lisp (symbols, lambdas, eval, metaprogramming, DSLs etc). What it doesn't take from Lisp it takes from Smalltalk, which was itself inspired by Lisp's clean syntax. All this means that once you're over the initial hump of grokking the syntax, you'll find yourself in pretty familiar territory.

Originally Ruby began as an implementation-defined language. That is, there was only one implementation (Matz's) and whatever that implementation did was the language spec. Nowadays, Ruby has rubyspec, an attempt to standardize the Ruby language.

Likewise, Scheme is a specification-defined language. There is one official language specification which says what Scheme is and how it works. Chicken is simply an implementation of that specification. There is one thing that is important to know right now: The Scheme specification is extremely minimal in design. It tries to define as few language constructs as possible, but these few should be so powerful that you will not need any more to make programs. This results in an extremely small spec and a very elegant and clean language with very few rules to remember, which makes it easy to learn.

However, in the real world you will need much more than just programming constructs, you need things to interact with the operating system, networking libraries, etc. That's where the difference between Scheme and other languages comes in: Every implementation of Scheme defines the things it thinks are necessary to build real programs with. Unfortunately, this means most Scheme programs are inherently not portable from one implementation to another, but it also means that Scheme implementations are free to experiment how they want and explore new language territory. This gives each Scheme implementation its uniqueness.

Fortunately, most Scheme implementations including Chicken are fairly portable to modern hardware architectures and operating systems. You may or may not be able to port a Chicken program to another Scheme, but you can port it from Windows to Mac OS X or Linux, or from the 32-bit Intel 386 to the 64-bit SPARC architecture, because Chicken runs in all those environments. Usually "porting" just means "running it on that system", because just like Ruby Chicken has procedures that are platform-independent, with multiple implementations where the platform itself differs.

Chicken's power is in how it extends the Scheme standard. It has a very comfortable interface to C that does not require you to touch a single line of C code in order to create bindings to existing C libraries, but it also gives you the freedom to embed Chicken in C code or C in Chicken code as you want. It offers a TCP/IP networking layer, it has great POSIX interoperability so you can interact with the OS. And most importantly: It can compile Scheme code to very efficient C code which can itself be compiled to machine code, giving you the best of both worlds: a dynamic language which allows you to program in the flexible way you are used to with Ruby, but this can be compiled for maximum efficiency.


The basics

The one thing that is most strikingly different between Ruby and Scheme is of course the syntax. Ruby has an extremely baroque syntax that allows you many freedoms in how you would like to write down things. Scheme, on the other hand, has only one way in which to write a given expression. Let's start by looking at an example. First we start by firing up an irb session and typing a little program:

 irb(main):001:0> # My first program
 irb(main):002:0* [1, 2, 3, 4].map{|x| x + 1}
 => [2, 3, 4, 5]

Now, we fire up a csi (chicken scheme interpreter) session:

 #;1> ; My first program
 (map add1 (list 1 2 3 4))
 (2 3 4 5)

In Scheme, lists are delimited with parentheses with its elements separated by spaces. As we can see, everything in Scheme is a list, even the expressions that you use to tell it what to do! An expression in Scheme is always a list with the operator on its first position and the operands following it. Procedures that accept no arguments are simply a list with only the procedure in it, for example (newline), which simply displays a newline character.

This simple rule also means that every parenthesis has a meaning. You can't add more parentheses or leave off parentheses like you can in most Ruby expressions. Adding extra parentheses simply applies the resulting expression as if it were a procedure:

 #;2> ((newline))
 Error: call of non-procedure: #<unspecified>
         Call history:
         <syntax>                ((newline))
         <syntax>                (newline)
         <eval>          ((newline))
         <eval>          (newline)       <--

If (newline) returned a procedure, it would be called. But as it happens, newline simply returns an unspecified value which is not a procedure and thus can't be applied. We can also use the result of calling a procedure in the call to another procedure:

 #;3> (add1 2)
 #;4> (- 10 (add1 2) 1)

We see that arithmetic is a little different from Ruby, as a result of the simple rules Scheme has for its syntax. This may be a little awkward, especially with complex calculations:

 #;5> (* (+ 1 (- 6 2) 2) (- 10 5))

In Ruby (and other languages with algebraic syntax) this would have been

 irb(main):002:0> (1 + (6 - 2) + 2) * (10 - 5)
 => 35
 irb(main):003:0> # Alternatively:
 irb(main):004:0* (1 + 6 - 2 + 2) * (10 - 5)
 irb(main):005:0> # or even:
 irb(main):006:0* (((1) + ((((6 - 2)))) + 2) * (((10) - ((5)))))
 => 35

Both types of syntax have their advantages and disadvantages: The Ruby-like syntax is more natural, but you have to think about operator precedence rules. Chicken does not need operator precedence rules because the precedence can be determined from the way it's nested, but it's less natural for most people (though you get used to it very quickly).

Actually, right now you know almost all there is to know about Scheme's syntax! Chicken has a couple of extensions to the basic Scheme syntax, but we won't go into detail here. Later you'll see a couple of handy shortcuts, but this is basically it.


Variables are names for things. Chicken has vary lax rules for naming variables. Actually, any string is a valid identifier as long as you quote it correctly.


 #;1> x = 10
 => 10
 #;2> x
 => 10
 #;3> x-y-z = 10
 NameError: undefined local variable or method `x' for main:Object
       from (irb):2


 #;1> (define x 10)
 #;2> x
 #;3> (define x-y-z 10)
 #;4> x-y-z
 #;5> (define %x% 1)
 #;6> %x%
 #;7> (define |a b c| 5)
 #;8> |a b c|

As you can see, because of Scheme's operator rules, symbols that would normally be separate tokens designating operators have no special meaning so we can use it in the middle of a name. The convention in Scheme is to use the minus sign as word separator (in Ruby, you would use an underscore for separating words). The final example shows how any string is allowed as a variable name: if the string contains syntax that would mean something else to Scheme you can enclose the variable name in pipe symbols. The pipe symbol itself can be escaped with a backslash, if you need it to be part of a variable.

To assign to a pre-existing variable we can also use set!:

 #;1> (define x 10)
 #;2> x
 #;3> (set! x 20)
 #;4> x

Top-level variables can also be overwritten by simply redefining them, but in some cases you need set!. However, set! is a typical artifact of an imperative programming style and in clean code you want to avoid using it.

Scheme also allows the binding of local variables. Bound variables behave like the defined variables above, however, they are only valid within a local scope. The top-level variables we've seen are the equivalent of a global in Ruby.

The most common binding constructs are let and let*. let allows for any number of bindings, none of which are related.

(let ((a 5)
      (b 10))
  (+ a b))

let* is like let, except that the bindings are evaluated in order, so subsequent bindings can reference earlier bindings.

(let* ((a 5)
       (b (* 2 a)))
  (+ a b))

There are other binding forms, such as letrec and the so-called named let. More information about these forms can be found in the Scheme specification.


Of course using simple expressions like this is not enough. You'll need procedures too. In Ruby, named procedures are actually methods on objects, but we can forget about that little detail for now:


def pythagoras(a, b)
  Math.sqrt(a**2 + b**2)


(define pythagoras
  (lambda (a b)
    (sqrt (+ (* a a) (* b b)))))

Now that's interesting! Procedures are just regular variables in Scheme (a bit like functions in Javascript). We assign a lambda to it. We can do that in Ruby too, but it's not pretty:


 some_class.send(:define_method, :pythagoras) {|a, b| Math.sqrt(a**2 + b**2) }

Just like in Ruby the def foo is shorter than the above, in Scheme we have a convenient shorthand for defining procedures too:

  (define (pythagoras a b)
    (sqrt (+ (* a a) (* b b))))

Recursion and tail-call optimization

In Scheme, recursion is a very important tool. In fact, it is so important that the Scheme standard demands tail call optimization (TCO), which basically means that you can have infinite recursion as long as the recursive procedure does not need to do anything after it returns. That sounds a bit strange, so here's an example:


 irb(main):010:0> def add_up_to(num)
 irb(main):011:1>   if
 irb(main):012:2>     0
 irb(main):013:2>   else
 irb(main):014:2*     add_up_to(num - 1) + num
 irb(main):015:2>   end
 irb(main):016:1> end
 => nil
 irb(main):017:0> add_up_to(4)
 => 10
 irb(main):018:0> add_up_to(9999)
 SystemStackError: stack level too deep


 #;2> (define (add-up-to x)
        (if (zero? x)
            (+ (add-up-to (sub1 x)) x)))
 #;3> (add-up-to 4)
 #;4> (add-up-to 9999)

Note the + in front of the (add-up-to (sub1 ...)). That is a cue that this is not tail-recursive code: each level of recursion must eventually come back 'up a level' in order to complete the addition, and so the program must keep a live reference to every level of recursion until the final result is computed.

In most other Schemes, however, this will break just like in Ruby, because when the (+ (add-up-to (sub1 x)) x) expression is evaluated, the recursive call to add-up-to creates a new stack frame so that when it returns the x can be added to the result.

[This code will 'break' in Chicken too, but only for much larger numbers. Although Chicken doesn't have an arbitrary stack depth, if you try (add-up-to) on a large enough number, you'll use up all your system memory before getting an answer. Read on for a better way to write it.]

All Schemes know that when there is nothing that needs to be done after a procedure returns, there is no point in returning to the procedure that called it at all: instead it can just return directly to the procedure that called the current procedure. So the call can be optimized to become a goto, replacing the current stack frame.

Here is a tail-recursive version. Ruby still can't handle it:

 irb(main):027:0> def add_up_to(x)
 irb(main):028:1>   def inner(x, y)
 irb(main):029:2>     if
 irb(main):030:3>       y
 irb(main):031:3>     else
 irb(main):032:3*       inner(x-1, x+y)
 irb(main):033:3>     end
 irb(main):034:2>   end
 irb(main):035:1>   inner(x, 0)
 irb(main):036:1> end
 => nil
 irb(main):037:0> add_up_to(9999)
 SystemStackError: stack level too deep

But Scheme can (this works in all Schemes):

 #;2> (define (add-up-to x)
        (define (inner x y)
          (if (zero? x)
              (inner (sub1 x) (+ y x))))
        (inner x 0))
 #;3> (add-up-to 4)
 #;4> (add-up-to 9999)

Note that the recursive call to inner isn't nested inside another function call, such as the (+ (add-up-to ...)) in the first version. This is the hallmark of a tail-recursive program. (The astute reader might note that it actually *is* nested inside an (if ...) procedure, but conditional forms like if are handled intelligently in tail-recursion. The if statement itself is not nested inside a procedure call, so all is well.)

As you'll notice, this version is a lot faster in Chicken too because it does not have to travel back through all those empty "stack frames". In the first example, Chicken's memory usage increases upon every recursion: for large numbers, it will break because it can't allocate any more. But in the second example, memory usage will stay constant and simply loop forever.


Ruby programmers will be familiar with blocks. Classic example in Ruby is the map method used to iterate over a collection, executing a block of code for each item in the collection.


 >> [1, 2, 3, 4, 5].map { |x| x * x }
 => [1, 4, 9, 16, 25]

Scheme also contains blocks, though we call them anonymous procedures usually. Procedures are created using the (lambda args body...) body form. This syntax is a little more verbose than Ruby's, but the trade off is that more than one procedure can be passed as an argument, whereas Ruby generally only allows one.


 #;1> (map (lambda (x) (* x x)) '(1 2 3 4 5))
 (1 4 9 16 25)

A more complicated example involves opening and closing files. Say we wanted to create a utility like wc -l that counts the number of lines in a file. In Ruby, it might look something like:

count = 0"myfile", 'r') { |fh|
    count += 1 while fh.gets
puts count

Similarly, Scheme uses anonymous procedures to create the same behavior:

(with-input-from-file "filename" 
  (lambda () (port-fold (lambda (line lines-so-far) (+ 1 lines-so-far)) 0 read-line)))

This Scheme code also showcases some typical functional style, using a fold operation instead of incrementing the value of a variable.

Data types

Now we have a basic grasp of Scheme's syntax, we can have a look at the different data types Chicken has to offer. We will do this from a Ruby perspective.


In Ruby we use arrays for storing lists of things. The obvious Scheme equivalent type is the list, you'd think. This is sort of true:


x = [1, 2, 3, 4]{|y| y + 10 }
x.each{|y| puts y }


(define x '(1 2 3))
(map (lambda (x) (+ x 10)) x)
(for-each print x)

Note that Scheme does not have the block scoping bug. Another thing that we should note is the first line. We create a list by quoting it. This allows us to enter the list in such a way that Chicken knows the list is just that; a list, and not a procedure application of the procedure called 1 on the arguments 2 and 3. The apostrophe takes care of that.

However, we must always remember that the Scheme list is more like a linked list. This means that it is very flexible in how we can add things to it and alter it, but it also means that traversing it takes more time as more items are added to it. Accessing an element is an O(n) operation, where n is the position of the element.

If we want O(1) operations on our lists, we can use a vector:

 #;1> (define x (vector 1 2 3 4))
 #;2> (vector-ref x 2)
 #;3> (define y (list 1 2 3 4))
 #;4> (list-ref y 2)

Adding new elements to a vector requires resizing or even copying the vector, just like it would in Ruby. So whenever you're contemplating using a list type, think about the properties you want the list type to have. This may sound odd, but in fact this gives you much more flexibility than Ruby, where you have the choice of using an Array, or... using an Array. However, as Knuth famously said: "Premature optimization is the root of all evil", and you should probably take the list solution until it's proven that you need vectors. Also, because Lisp was built on lists, it is very good at manipulating them, so they're most likely the most convenient datatype.

Chicken also offers you several other types of array-like types, each with their own unique time and space properties. Which you'll use depends on the task at hand and the situations your system will be used under.

List procedures

Lists are, as mentioned before, linked lists. This means they always consist of two parts: a head and a tail. We've seen the list procedure which creates lists, but this works on lower primitives:

 #;1> (list 1)
 #;2> (cons 1 '())

The () is the empty list. It is itself a list, but it is also a single symbol. It serves as the end of list marker. That's why the list construction procedure, cons, can create longer lists too:

 #;1> (list 1 2 3 4)
 (1 2 3 4)
 #;2> (cons 1 (cons 2 (cons 3 (cons 4 '()))))
 (1 2 3 4)

To take the head/tail of these lists we have two procedures:

 #;1> (car '(1 2 3 4))
 #;2> (cdr '(1 2 3 4))
 (2 3 4)
 #;3> (cdr (cdr '(1 2 3 4)))
 (3 4)
 #;4> (car (cdr (cdr '(1 2 3 4))))
 #;5> (caddr '(1 2 3 4)) ; combination of car cdr cdr
 #;6> (car (car '(1 2 3 4)))
 Error: (car) bad argument type: 1
 #;7> (cdr (cdr '(1)))
 Error: (cdr) bad argument type: ()

Actually, cons just sticks two things together, so we could also stick together two numbers:

 #;1> (cons 1 2)
 (1 . 2)
 #;2> (car (cons 1 2))
 #;3> (cdr (cons 1 2))

Two things stuck together are called a pair. By sticking together more things without an end of list marker, we can create an improper list:

 #;1> (cons 1 (cons 2 (cons 3 4)))
 (1 2 3 . 4)

You should not use lists like these unless you know what you're doing, because all list library procedures expect proper lists: lists with end markers. Chicken supports the full SRFI-1 out of the box. Have a look at that document and compare it to the Ruby standard Enumerator and Array methods. Most of the procedures in srfi-1 will look very familiar to you. Here are some examples:

 #;1> (use srfi-1)  ;; Not needed in Ruby
 ; loading library srfi-1 ...
 #;2> ;; [1, 2, 3] + [4, 5, 6] / [1, 2, 3].concat([4, 5, 6])
 (append '(1 2 3) '(4 5 6))
 (1 2 3 4 5 6)
 #;3> (map add1 '(1 2 3 4)) ;; [1, 2, 3, 4].map{|x| x + 1}
 (2 3 4 5)
 #;4> ;; No equivalent because map works on one object:
 (map + '(1 2 3 4) '(5 6 7 8))
 (6 8 10 12)
 #;5> ;; [1, 2, 3, 4].each{|x| puts x}
 (for-each (lambda (x) (printf "~A\n" x)) '(1 2 3 4))
 #;6> ;; [1, 2, 3, 4, 5, 6].select{|x| (x % 2).zero? }
 (filter even? '(1 2 3 4 5 6))
 (2 4 6)
 #;7> ;; [1, 2, 3, 4].inject(""){|str, x| str + x.to_s}
 (fold (lambda (x str) (conc str x)) "" '(1 2 3 4))


Luckily, you are a Ruby programmer, so we will not have to go through the whole "explaining what symbols exactly are" again :) Actually, Ruby borrowed symbols from Lisp.




(string->symbol "foo")
(symbol->string 'foo)

As we can see, a symbol is only a quoted variable name! This is the origin of symbols and also the reason you can send symbols representing method names to objects in Ruby. Symbols have all the same semantics as Ruby's symbols: they can be compared in constant time and they take up very little memory space.


Strings are simple. Just like in Ruby, we have strings enclosed by double quotes: "foo" works the same in Ruby as it does in Chicken. Chicken's double quoted strings work more like Ruby's single-quoted strings, though. There is no string interpolation and other things; a string is just a string.


x = 10
y = "x contains #{x}"
z = "x contains " + x.to_s


  (define x 10)
  (define y (sprintf "x contains ~A" x))
  (define z (conc "x contains " x))
  ; Conc automatically converts its arguments to strings. We also could do:
  (define z (string-append "x contains " (->string x)))

Note that ->string is simply the name of a procedure, including the arrow.

It may be important to know that Scheme also has a character data type, unlike Ruby:


 irb(main):001:0> "foo"[0]
 => 102
 #;1> (string-ref "foo" 0)
 #;2> (char->integer #\f)

You will probably not need this data type for your first few Scheme programs so we won't go into it deeper here.

String procedures

Chicken comes shipped with SRFI-13, which is a library of string procedures which is intended to be a lot like SRFI-1, which we already looked at a few sections ago:

 #;1> (use srfi-13) ;; Not needed in Ruby
 ; loading library srfi-13 ...
 #;2> ;; "abc" + "def"
 (string-append "abc" "def")
 #;3> ;; "abcdef"[-3..-1]
 (string-take-right "abcdef" 2)
 #;4> ;; "abcdef".rjust(10)
 (string-pad "abcdef" 10)
 "    abcdef"
 #;5> ;; ["this", "is", "very", "cool"].join(" ")
 (string-join '("this" "is" "very" "cool"))
 "this is very cool"
 #;6> ;; "this is very cool".split(" ")
 ;; NOT from srfi-13 but chicken's extras unit:
 (string-split "this is very cool" " ")
 ("this" "is" "very" "cool")

Regular expressions

Just like in Ruby, there's a Regex data type, but in Chicken there is no special syntax for it:


 irb(main):001:0> /(.)(.)(\d+)(\d)/.match("THX1138.").to_a
 => ["HX1138", "H", "X", "113", "8"]


 #;1> (use regex)
 ; loading library regex ...
 #;2> (string-search "(.)(.)(\\d+)(\\d)" "THX1138.")
 ("HX1138" "H" "X" "113" "8")

The string-search procedure automatically transforms the first string to a regexp. You can also do that yourself:

 #;3> (string-search (regexp "(.)(.)(\\d+)(\\d)") "THX1138.")
 ("HX1138" "H" "X" "113" "8")

The advantage of doing this is that when you need to match several strings you can use the same regexp so it doesn't have to precompile the regexp every time you call string-search.


The final datatype we use a lot in Ruby is the Hash. In Chicken there are two datatypes you could use instead of the Ruby Hash; association lists (or alists for short) or hash tables.

Association Lists

Association lists are the simpler Hash like structure in chicken. Effectively, alists are standard lists of pairs, where the first item in the pair is the key and the second item is the value. Consequently, alists have a nice literal form:

'((foo  1) (bar 42) (baz 101))

To lookup a value in the alist, use assoc. For example to check if the pair (bar 42) is in the alist:

 #;1> (assoc 'bar '((foo  1) (bar 42) (baz 101)))
 (bar 42)

If the pair is not in the list, you would get the boolean false (#f). If you need more stringent checks, you can also use assq or assv, learning more about these procedures is an exercise for the reader.

Alists may simplistic, and inquisitive readers may notice that lookup is O(n) time. However, they are convenient and adding new items is a constant time operation. You may find they work in may places that you might use a small Hash in Ruby.

Hash tables

For more complex hashing operations, Chicken supplies true hash tables.

 #;11> (define h (make-hash-table))
 #;12> h
 #;13> (hash-table-set! h 'foo 12)
 #;14> (hash-table-set! h 'bar 101)
 #;15> (hash-table-ref h 'bar) 
 #;16> (hash-table-delete! h 'bar)
 #;17> (hash-table-ref h 'bar)
 Error: (hash-table-ref) hash-table does not contain key

Hash tables are more powerful overall, but do not offer convenient literal notation. If you need to convert from a hash table an alist you can use hash-table->alist. The alist->hash-table procedure converts in the opposite direction. For a complete list of supported procedures, check the hash table section in the manual.


Scheme has a boolean type where #f is false and #t is true. Its handling of truthiness is a lot like Ruby's; anything that is not #f is treated as being true:

 #;1> (if #f
          (print "WTF, it's true")
          (print "It's not true"))
 It's not true
 #;2> (if #t
          (print "Yes, it's really true")
          (print "No, it's not true"))
 Yes, it's really true
 #;3> (if "Some random other value than #f"
          (print "Yes, this is also true")
          (print "No, it's not true"))
 Yes, this is also true

Ruby's nil does not have a direct equivalent in Scheme. In the situations where a not present value is supposed to be returned, usually #f is used:

 #;1> (use srfi-1)
 ; loading library srfi-1 ...
 #;2> (find even? '(3 1 4 1 5 9))
 #;3> (find even? '(1 3 7 9))

In cases where a procedure really has no sensible thing to return, we use the special void value, returned by the void procedure:

 #;1> (define (say-hello)
        (print "Hello")
 #;2> (say-hello)

As we see, the interpreter understands that there is no proper value to return so it displays the prompt immediately without showing the result value. The example is a little contrived, because in real code we wouldn't explicitly call (void) because print already returns the void value.


Now we have the tools to make programs, let's look at a few larger programs to better appreciate how one would program in Chicken.


Chicken and the Real World

Programming is about more than having a pretty language, so let's look at what Chicken has to offer for real construction work.


Eggs are to chicken what gems are to Ruby: installable extensions like libraries and programs. The list of eggs is where you should look first when you are going to implement something big. You can install an egg almost like you install a gem, as follows:

 $ chicken-install matchable

This downloads and installs the egg with the name "matchable". This egg has no dependencies, but if it did it would have downloaded and installed them as well.

Meta programming

A hot topic in the Ruby community is meta programming and DSLs (Domain specific languages). These ideas originated from Lisp, which means you can just keep on trucking in Chicken!

Data is code and code is data

The most fundamental concept of Lisp is that code is data. As we've seen, procedure calls look like lists. We've also seen that we can quote lists to "block" Scheme from interpreting a list as a procedure call. We can also turn this around on its head and force a list to be evaluated as code:

 #;1> (define calculate '(+ 1 2 3 4))
 #;2> calculate
 (+ 1 2 3 4)
 #;3> (eval calculate)

"Big deal", you might say, "Ruby also has eval". But the difference is what matters: In Ruby you have to construct strings to be evaluated, which means you need to be able to parse strings if you want to change a Ruby-program-stored-as-string. In Scheme we can simply hack the list. The program is stored in a parsed state, so to speak.

If we would want to change the operator from + to *, we can simply do that:

 #;1> (eval (cons '* (cdr calculate)))

This is much more robust than any regexp hacking or ad-hoc parsing on strings you want to evaluate.


One of the coolest concepts, but also the one most easily abused is macros. Because Scheme stores code as data, you can change the code on-the-fly as described above. You can do that at run-time on random data through eval, but you can also do it at compile-time on your program, which is usually the best time to do it.

Scheme macros rewrite your code during compile time. They can range from simple to complex, with some macros defining entire "sublanguages" embedded in Scheme.

Some people call Rails' acts_as_foo methods "macros". This description is not wrong, as these methods do rewrite your classes in a similar way to Scheme macros, but they are not quite as powerful.

Here is a simple example of a task that is easy in Scheme, but much, much harder using Ruby's eval. Say you were debugging a program and found yourself printing out variables at certain points in the execution, along with the name of the variable so you could tell what you were looking at.

  (print "myvar: " myvar)

You decide that repeatedly typing the variable name twice (once to indicate which variable, once to get the value) is a waste of time. Using a macro, you can quickly and easily abstract away the common syntax into one place.

  (define-macro (ez-debug var)
    (let ((name (symbol->string var)))
      `(print ,name ": " ,var)))
  (define myvar '(this is a list))
  (ez-debug myvar)

This simply wouldn't be possible with a regular procedure. By the time a procedure is called, syntactic information like variables names has been optimized away.