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Re: [Caml-list] HTTP-server written in ocaml
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Date: -- (:)
From: Benjamin Geer <ben@s...>
Subject: Re: [Caml-list] HTTP-server written in ocaml
Jurjen Stellingwerff wrote:
> More info on: http://camlserv.sf.net

I've worked on several web application frameworks and templating systems 
(in languages other than Caml).  After a brief look at Camlserv, I have 
a few suggestions.

1. A templating system can be useful for generating many things besides 
HTML.  For example, I've used templating systems to generate email 
(automated replies), SQL, other sorts of program source code, 
configuration files, etc.  Therefore, if you want to provide a 
templating system, I'd recommend making it a general-purpose one, not 
tied to the HTTP server or to HTML.

Two examples of such a system (in Java) are Velocity:

    http://jakarta.apache.org/velocity

and FreeMarker (I was the original author):

    http://www.freemarker.org

2. A templating system, when used to generate web pages, should have a 
syntax which web designers will find easy to use.  The XML-like syntax 
you propose is, in my view, rather cumbersome.  Many web designers are 
familiar with JavaScript, whose syntax is more concise and readable. 
Here again, I'd recommend looking at Velocity's syntax.  Your 
conditional syntax looks like this:

    <when test="user_is_logged_in">
        ....
    <else>
        ....
    </when>

Try reading that out loud: "When test equals user is logged in...".  It 
doesn't sound like what it means.  Web designers will find this confusing.

Now compare the syntax for a conditional in Velocity:

    #if ($user_is_logged_in)
       ...
    #else
       ...
    #end

If you read that out loud, you get: "If user is logged in...".  It 
sounds like what it means.

Cheetah (http://www.cheetahtemplate.org), a Python-based templating 
system, has also adopted this syntax.

You might consider using a camlp4-like approach, to allow your template 
engine to use different syntaxes.

3. I notice that you have some syntax for connecting to a database.  It 
appears to be very MySQL-specific.  Also, your mechanism allows people 
to include SQL in their templates.  This is generally considered to be 
poor design.  Most web application frameworks follow the 
model-view-controller approach, in which templates contain *only* 
presentation logic (e.g. HTML, and any control structures needed for 
controlling the way data is presented).  A template gets data from a 
'model', which is simply a data structure provided by another module or 
program.  That data could come from a relational database, from files, 
from LDAP, or from anywhere else.  The 'controller' is a module which, 
given a request, decides which model and view should be used to handle 
the request.

This allows you to change the way the model is implemented without 
changing the presentation layer (the 'view').  It also means that 
templates are not cluttered with data access syntax (e.g. SQL); this 
makes life easier for web designers.

Most importantly, this approach allows for code reuse.  View components 
can be reused with many different models.  For example, you might create 
a general-purpose tabular HTML view, with typical features such as 
allowing the user to sort on any column by clicking on the column's 
heading.  This view could then be used with models containing data from 
many different sources, including databases.

Similarly, model components can be reused with many different views. 
You might have different versions of your web site for different users, 
e.g. a printable version, or a low-graphics version for disabled users. 
 More generally, a model describing a document could be rendered by an 
HTML view, a LaTeX view, a plain-text view, etc.

Hope this helps,
Benjamin

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