Hello,

Here is the latest Caml Weekly News, week 11 to 18 March, 2003.

1) Ocaml-beginners ML
2) monads for dummies
3) poll - need for a good introductory OCaml book
4) Beta release of lablgtk2
5) OCaml popularity

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1) Ocaml-beginners ML
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Ernesto Torresin said:

Since you're talking about ocaml beginners, I can give a quick report of the
ocaml-beginners mailing list that I've started about one year ago:

Subscribers: 167
Messages: 883 right now

An interesting point is that as far as I see now the only expert contributing
actively and continuously has been Rémi Vanicat.

Translation: if *you* already know Ocaml, you can take a ride on the teaching
side by subscribing to ocaml_beginners. Be a senior camel ;-))

Cheers
Ernesto Torresin
PS You can see the archives here:
http://groups.yahoo.com/group/ocaml_beginners/
(there is also a ads-free mhonarc version up to 11/10/02 at
http://membres.lycos.fr/ipotesi/OCAML/ocaml-beginners_archive/index.html ).

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2) monads for dummies
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James Woodyatt lectured:

I'm a self-taught "programmer" with no formal education in mathematics
or computer science, and I found the monad concept to be fairly
difficult to learn.  It took a leap of consciousness that feels about
the same level of difficulty as what I remember needing to get through
my high school calculus class.

I've been seeing a lot of attempts to explain the theory lately, but
most of them remind me of the kind of thing that I found more confusing
than helpful when I was trying to get my head around the practical
matters.  What I really wanted is a Monads For Dummies tutorial.

Here's what I remember being the crucial tip that got me over the hump:
if you are working with immutable data structures (and there are at
least three in the Ocaml standard library: Map, Set and List), then the
monadic programming style can be really useful for composing
complicated functions for manipulating the contents of those data
structures from assemblies of simpler functions.

Without the monadic programming style, you end up concocting huge
specialized functions for passing into the higher-order standard
library functions, e.g. fold, map, filter, etc., and achieving any kind
of modularity requires careful design work.  The problem can quickly
become hard enough that learning monadic programming starts to seem
like a decent trade.

Once you start using immutable data structures in complex ways (e.g.  
doing lots of weird transformations on 'a list values), you may find   
that writing your functions as monads will help you modularize your
program, allowing you to encapsulate and reuse computations that would
otherwise be duplicated in scattered fragments of code all over the  
place.  (Alternatively, you may want to use mutable data structures
instead, which is always a popular way out of this problem.)

If you discover your immutable data structure manipulations are getting
out of hand, and you want to reorganize your code for better
encapsulation and reusability-- without changing them to use mutable
data structures instead-- then I recommend learning about the
continuation monad first.

Here is the signature of a module I use for the continuation monad in
Ocaml:

  type ('x, 'a) t = ('a -> 'x) -> 'x

  (* let ( >>= ) m f x = m (fun a -> f a x) *)
  val ( >>= ): ('x, 'a) t -> ('a -> ('x, 'b) t) -> ('x, 'b) t

  val return: 'a -> ('x, 'a) t          (* let return x f = f x
     *)
  val lift: 'x -> ('x, 'a) t            (* let lift x _ = x
     *)
  val cont: ('x -> 'x) -> ('x, unit) t  (* let cont f g = f (g ())
     *)
  val eval: ('x, unit) t -> 'x -> 'x    (* let eval m x = m (fun () ->
x)    *)

By using this module, I can make new continuations (functions of type
'x -> 'x) by composing appropriate instances of the continuation monad
type and binding them with functions that chain the results of previous
computations into the subsequent computations.

For a very simple example: if you have a bunch of continuation
functions, e.g. f1, f2, f3 each of type 'x -> 'x, then you can compose
them in sequence like so:

        let m: ('x, unit) t =
          cont f1 >>= fun _ ->
          cont f2 >>= fun _ ->
          cont f3

In fact, it might be easier to represent f1, f2 and f3 as their monads
to begin with:

        let m1: ('x, unit) t = cont f1
        let m2: ('x, unit) t = cont f2
        let m2: ('x, unit) t = cont f3

        let m: ('x, unit) t =
          m1 >>= fun _ ->
          m2 >>= fun _ ->
          m3

This defines a new instance of the continuation monad type, but it
isn't exactly the composed continuation function yet.  You *evaluate*
the monad to get the continuation it represents:

        let f: 'x -> 'x = eval m

The 'return' function is also sometimes called the 'unit' function (but
'unit' is a reserved word in Ocaml, so I like to avoid it).  It
constructs a monad that passes a value through the ( >>= ) operator to
the function that constructs a new monad value with it.  It's good for
use in monad functions that take input from "outside" the encapsulation
and pass it along to other monads.

The 'lift' function is a variant of the 'cont' function which ignores
the 'x value that is input from any previous computation and produces
the 'lifted' value as output.  It's good for use in monad functions
that initialize (or reinitialize) the encapsulated value of the monad.
I probably should have named it 'init' instead.

If I were writing a book on using monads for dummies, I'd launch here
into a series of useful examples showing how to use monads to simplify
very complicated list manipulation functions.  The example above is too
simple to show the power of monadic programming adequately.  I don't
have the time, but it's certainly something that needs to be done.

The continuation monad is only the beginning.  There are several
fundamental types of monad, and it's possible to combine them into
composites to support more operations, e.g. state manipulation,
exception handling, backtracking, etc.  Once you understand how to use
monads in a systematic design, you can take better advantage of the
support for pure functional programming available in the Ocaml language.

In lieu of my writing a long list of examples, here is a paper that I
found immensely helpful in the learning process:

        http://www.math.chalmers.se/~augustss/AFP/monads.html

The examples are written in Haskell, but I found them pretty easy to
translate in my head to the equivalent Ocaml.

Warning: you don't have to go down this path very far before you will
begin to chafe at the "operator overloading" problem.  You can't define
a ( >>= ) operator that will be good for all monad types.  I don't know
how to explain exactly why.  You just can't.  Some folks have proposed
that "extensional polymorphism" would be very helpful  in helping solve
the problem.  I buy that.

Warning2: since Ocaml strictly evaluates all the arguments to a
function before executing it (unless the argument is of type 'a
Lazy.t), you really can't define a proper ( >> ) operator for your
monads.  It won't work the same as the one in Haskell, because Ocaml
evaluates the right operand before it's needed, and you don't buy
anything by fixing it so that it takes a Lazy.t value instead.

If what I've written isn't very helpful to you, then please tell me so
I know not to try to explain this stuff to people.  On the other hand,
if it *is* helpful, that would be nice to know too.

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3) poll - need for a good introductory OCaml book
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This post started a long thread, that one can read here:
http://caml.inria.fr/archives/200303/msg00128.html and here:
http://caml.inria.fr/archives/200303/msg00182.html

Graham Guttocks said:

There seems to me clearly, a lack of a good, tutorial style,   
introductory book about OCaml in English.  Something akin to
``Programming Perl'' (as someone mentioned).

I'm talking to a publisher at Manning (http://www.manning.com) about
the possibility of such a book.  They are intrigued, but would like to
see some indication that people are interested.

Would you like to see such a book?  Something you could pick up in the
technical section of your local bookshoppe.  If so, speak up, and I'll
collect the responses to feed back to the publisher.  Thanks.

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4) Beta release of lablgtk2
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Jacques Garrigue announced:

We have just released LablGTK2 beta.

It can be found at
   http://wwwfun.kurims.kyoto-u.ac.jp/soft/olabl/lablgtk.html
   ftp://ftp.kurims.kyoto-u.ac.jp/pub/lang/olabl/lablgtk2-beta.tar.gz

While not being feature-complete yet, it is now relatively stable.

Along with support for the new text widget, it also provides support
for various extensions: gtkglarea (using lablgl), glade2, gnomecanvas,
and librsvg.

Hopefully porting applications from lablgtk1 should not be too hard,
however there are important changes in font and text handling.

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5) OCaml popularity
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A huge thread was started about OCaml popularity here:
http://caml.inria.fr/archives/200303/msg00070.html

Graham Guttocks said:

I discovered OCaml on Doug Bagley's computer shootout page where he
gives it a rave review over all the other languages he evaluated.

After looking into it further, I'm just surprised that OCaml isn't
more popular.  It seems to have all the rapid development features of
a scripting language like Python, but unlike scripting languages
offers fast native code like a compiled language.  Seemingly the best
of both worlds.

Any ideas why OCaml isn't more well known?  Is it just because the
language is not as old as something like Python, or perhaps because
the syntax is more difficult to learn?

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Old cwn
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Alan Schmitt