Download Wiley Curl Programming Bible

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P
Curl Content
Language
Fundamentals
A
R
T
I
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In This Part
Chapter 1
Introducing Curl
Chapter 2
How Curl Works
Chapter 3
Building Your First
Curl Applet with the
SurgeTM Lab IDE
Chapter 4
Working with Text
Chapter 5
Building Tables
Chapter 6
Adding Images,
Backgrounds, and
Borders
Chapter 7
Working with
Variables
Chapter 8
Controlling Program
Flow
Chapter 9
Creating and Using
Procedures
Chapter 10
Creating Classes
Chapter 11
Inheritance
Chapter 12
Strings and Arrays
Chapter 13
Handling Exceptions
in Curl
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Introducing Curl
T
he 13 chapters in Part I provide basic information about
Curl, its architecture, and its fundamental programming
constructs. This chapter explains what Curl is, what businesses
would benefit from using it, and how to phase it into your
development process. Successive chapters discuss Curl’s
architecture, the development tools, plus the usage of text,
images, variables, control structures, procedures, and classes.
Part II, Chapters 14–18, discusses how to build a user interface
with Curl. Part III, Chapters 19–22, discusses Curl’s graphics
and multimedia capabilities. Part IV, Chapters 23–25, discusses
files and server communication. Part V, Chapters 26–28,
discusses deploying applications. Part VI, Chapters 29–30,
discusses XML and Web services.
1
C H A P T E R
✦
Curl Corporation provides the Surge plug-in and the Surge
Lab IDE, for viewing and creating content written in the Curl
Content Language. Users need the Surge plug-in to view Curl
content in their browser. Authors will also need the Surge Lab
IDE to create Curl content.
The terms Curl Surge plug-in, Surge plug-in, and plug-in are
used interchangeably, as are the terms Curl Content language,
and just Curl. Curl, Surge, and Surge Lab are trademarks of
Curl Corporation.
This chapter is an overview of each component. Successive
chapters cover each component in detail.
Curl content language
Curl combines the worlds of document markup, scripting, and
object-oriented programming into a single language. Curl
sprung from MIT’s famed Laboratory for Computer Science
and later was spun off into Curl Corporation (www.curl.com).
✦
✦
In This Chapter
Understanding Curl
technology
Learning when to use
Curl
Integrating Curl into
your development
process
✦
What Is Curl Technology?
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Part I ✦ Curl Content Language Fundamentals
Curl was specifically designed to have a “gentle slope” learning curve. This means
that no matter what your current knowledge of Curl, learning the next level of functionality is only slightly harder.
At one end of the learning curve, Curl is a simple markup language that you can use
in place of HTML. Authors can learn this aspect of Curl in a few minutes. Sliding up
the curve a notch, you can create simple interface elements such as buttons and
drop-down menus, with variables and local data validation. At the higher end of the
learning curve, you can have object-oriented client/server applications with 3D
graphics and other advanced tools.
Borrowing from Dylan, an object-oriented language, Curl combines the flexibility of
dynamic languages (Smalltalk, Scheme) with the speed of static-typed languages
(C++, Eiffel). This combination also provides a gentle slope for development styles.
When prototyping, Curl’s support of typeless variables and class variables provides
flexibility and speed of development. For commercial code, you can add more static
typing to improve correctness and execution speed.
Surge Lab IDE
The Surge Lab provides all the tools an author or developer needs to create Curl
content. In theory, Curl content could be written in any text editor, but the Surge
Lab provides much more functionality and it’s free.
The Surge Lab includes its own syntax-aware Source Editor, API Viewer, Debugger,
Object Inspector, and Release Tool for packaging Curl code. The Lab also allows
you to run applets locally without needing a Web server. See Chapter 3 for a tutorial
on the Surge Lab.
Surge plug-in
The Surge plug-in provides a sophisticated runtime environment for running Curl
code securely. The plug-in includes a just-in-time (JIT) compiler and a unique
security model that protects the user’s computer from the issues that have
plagued mobile code in the past. Developers can affect the executable code by
sending directives to the JIT compiler. These directives can control the trade-offs
between the freedom of typeless programming versus code-safety and speed.
The Surge plug-in also permits a single user to compile and run multiple versions
of Curl. If the applet needs a newer version of Curl, the plug-in gives the user the
option of downloading the newer module. This solves the thorny problem of
backward compatibility that haunts many other plug-ins.
Finally, the plug-in acts as the traffic cop that enforces many of Curl’s security
features. These include
✦ Verifying an applet’s right to include or import Curl files from the host and
comparing package versions to ensure compatibility
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Chapter 1 ✦ Introducing Curl
✦ Preventing unprivileged applets from accessing local files
✦ Protecting intellectual property by prohibiting users from viewing the Curl
source code
See Chapter 2 for more details on Curl’s security features.
Surge Control Panel
The Surge Control Panel allows the user to control runtime options for security,
debugging, and general preferences. This feature is most frequently used to allow
applets from a particular host or directory to run with privileged access.
Console window
The Console window shows system warning and error messages. Also, any messages using the output command (normally, only used while debugging) appear in
the Console window.
Platform Independence
Curl has been explicitly designed as a cross-platform language, suitable for reaching
a wide audience across the Web. Surge only accepts Curl source code, either raw
or compressed and preprocessed in .pcurl files, and compiles each applet on the
target machine before running it. This local compilation, rather than reliance on
externally produced executables or intermediate byte-codes, permits true platform
independence.
By compiling Curl on the client machine, Surge always knows the exact run-time
environment the applet will face when executing. This allows Surge to take advantage of local machine resources, such as hardware graphics rendering, when
possible, but does not depend on them. This late-binding aspect of the Curl/Surge
architecture yields better platform independence and produces faster executables.
Curl Corporation’s business approach to platform independence is equally crucial.
They are creating the Surge run time for each platform themselves. This allows Curl
Corporation to ensure that Surge works uniformly across all platforms and to avoid
the Java debacle of “write once, debug everywhere.”
Surge currently works on the vast majority of client machines, which is to say
machines that run Microsoft Windows. Curl Corporation plans to make Surge
available on Linux and Macintosh computers; eventually, it will be on PDAs and
cell phones. Visit www.curl.com for updates on platform availability.
Interoperability
You don’t have to throw out your current Web site to use Curl. Curl content can be
used alongside, or in place of, HTML. Curl can interoperate with other tools such as
ASP, CGI scripts, and other development tools.
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Part I ✦ Curl Content Language Fundamentals
Curl comes with the SAX (Simple API for XML) parser for XML documents. This
means that Curl speaks XML as a native language, which makes Curl applets excellent front ends for XML data. This makes Curl an ideal user interface for the growing
body of SOAP/Web services. See Chapter 29 for details on using SAX with Curl.
See the CurlExamples site (www.curlexamples.com) for specific examples of working with other tools. Also, see the “Fitting into Your Infrastructure” section, later in
this chapter, for tips on applying Curl to your current situation.
Types of Curl Technology Solutions
From a technical point of view, nearly any client-side application can be improved by
using Curl. However, there are some significant usability and business considerations.
Usability
Curl’s interactive features and local response times generally make applications significantly easier and faster to use. This improved usability can give your application
an edge against the competition. It may even permit you to charge premium pricing
(until your competitor adopts Curl). Improved usability is one of the key reasons
companies use Curl.
As with any plug-in, however, there are the issues of startup time and installation
that must be considered. In the 1.0 release, it can take 20 or more seconds, depending on your computer, to launch the Surge plug-in the first time (of course, you can
always configure the plug-in to launch on computer startup). Furthermore, each
user needs to have downloaded and installed the Surge plug-in, which takes a few
minutes, depending on your connection speed.
As Curl/Surge becomes more ubiquitous and Surge launch times decrease (the 2.0
version of the plug-in, due out in the summer of 2002, is supposed to launch 5 times
faster than the current version), these will not be issues. For now, the Surge launch
time needs to be considered as part of the application’s usability. Public sites that
expect traffic from a wide range of users would probably not want to put Curl
content on their home page, for example.
Business models
Before discussing when to use Curl, it is useful to segment the types of application
in use today. There are two basic kinds of revenue-producing methods on the Web:
transactions fees (eBay, Amazon, Dell) and subscription fees (Wall Street Journal,
Communispace).
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Chapter 1 ✦ Introducing Curl
Things are never so neatly defined in practice, of course. A “free” site is essentially
a subscription site with a fee of zero or with a marketing area designed to sell subscriptions or transactions; and an advertising site is essentially a transaction-fee
site, except that the transactions that yield money are advertising impressions to
the user.
Sites can be further categorized by whether they allow public access or private
access. These two dimensions segment Web applications into four quadrants
shown in Table 1-1. The sections below discuss how each relates to Curl.
Table 1-1
Business Models
Transaction Revenue
Subscription Revenue
Public Users
Amazon, eBay, Dell, etc.
Wall Street Journal, Yahoo!*, etc.
Private Users
Fidelity, E*Trade, etc.
Communispace, Paychex, Intranets*, etc.
*Note that “free” sites, such as Yahoo!, or a corporate intranet, can be thought of as subscription sites with a fee
of zero. Free corporate sites are marketing areas designed to sell subscriptions or transactions (offline or online).
Public transaction sites
Public transaction sites include such Web poster children as Amazon, eBay, and
Dell. These sites are open to any Web user and make money when a user completes
a transaction. From a technical and operational viewpoint, this category stands to
gain the most from a Curl solution. Curl could save these high-traffic sites tremendous amounts of money in reduced server and bandwidth costs.
Unfortunately, downloading the Surge plug-in presents the biggest challenge to
public transaction sites. With so many new users each day, they simply can’t insure
that the majority of users will have Surge installed on their computers.
Short term, these sites may want to start a small Curl project and create a members-only segment of their site with a premium interface. Repeat users would be
enticed to download Surge to gain access to more sophisticated features. In effect,
this converts a segment of these sites to private transaction sites for frequent
users. Every user they entice to use the Curl application saves them money. Equally
important, as Surge becomes more prevalent, they will already have a Curl application waiting. They can maintain their market leadership while making a smooth
transition.
Public subscription sites
Public subscription sites include paid sites, such as the Wall Street Journal online,
and sites such as Yahoo! (Think of it as a free subscription.) Again, technically and
operationally these sites would benefit tremendously from Curl.
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For usability, the more the subscription costs, the more likely the user will be interested in Curl. When the information is free and often available in multiple places,
the user’s primary concern is usually saving time. Launching Surge slows them
down. When the information is expensive, the user is more likely to be concerned
about finding the best information. This is where the improved response time and
usability of a Curl application is worth the time it takes to launch Surge.
Free sites will probably have to wait until Surge is more readily available or until
they can create a members-only area as discussed previously.
Private transaction sites
Private transaction sites include financial sites such as Fidelity, E*Trade, and home
banking. Although these companies have a public component to their Web site, this
category refers to the features provided for registered users. Here, users can examine their account, request information, and move money around or make other
transactions. This is an excellent setting for Curl. Aside from the usual technical
and operational benefits, most users are repeat users and would be happy to have a
richer interface.
Private subscription sites
Private subscription sites include application service providers (ASPs) such as
Communispace and Paychex. In these settings, users pay a flat fee to rent the ASP’s
online application. Because users have already paid for the service, they are highly
motivated to get the best use from the site. Installing Surge is easier since the users
are known in advance.
Since ASP providers have a fixed revenue stream each month, any reduction in cost
is pure profit for them. This gives ASPs a strong incentive to use Curl.
Corporate intranets fit into this category as well. Again, think of them as a subscription site that charges the flat fee of zero each month. This is further incentive to
keep the cost down and the user productivity high because any wasted user time is
wasting your own company’s money.
Private subscription sites are the ideal business and technical situation to use Curl.
Summary of Web business uses
Web businesses fall into four main categories. Technically and operationally, all four
categories could benefit from using Curl today. The viability of using Curl for your
business depends on how prevalent Surge is among your users.
Sites with private users (ASP services, intranets, financial services, and so on) are
perfect for using Curl today. Public subscription sites, particularly those that
charge premium prices, are also good Curl candidates. Public transaction sites,
such as Amazon, would be better using Curl in a members-only capacity until Surge
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is more ingrained with the public. Once converted to Curl, however, public transaction sites may have the biggest cost saving.
Fitting into Your Infrastructure
Adding any new tool to your arsenal requires both business and technical planning.
As this is a technical book, most of the chapters focus on technical learning and
planning. However, we start by discussing business planning here since it often sets
the parameters and goals for technical planning.
Business planning
The gentle slope aspect of learning Curl makes it relatively easy to add it into your
current development environment. Naturally, you will want to weigh the costs and
benefits of using Curl. Many of the benefits of Curl were already covered in this
book’s introduction, but a quick summary would be that Curl allows your applications to
✦ Provide a richer interface.
✦ Provide faster response time for your users in many cases.
✦ Provide better security for your applications.
The idea of a richer interface merits a brief elaboration. Curl permits direct manipulation of the interface by your users, enabling them, for example, to resize tables,
sort data, use configurators, and rotate graphics. This creates stickier application
sessions.
On the cost side, Curl has many advantages. In the heady dot-com boom days of
the late 1990s, revenue was king and nobody cared about the cost, let alone profits.
Those days are gone, gone, gone. Back in the real world, cost is a crucial aspect of
any business’s profitability. When examining the cost of switching to a new technology, you would normally consider
✦ Cost of hiring and training Curl developers
✦ Cost of supporting dual development environments during transition
✦ Cost of Curl development tools
✦ Cost in time and dollars to develop and code projects in Curl
✦ Cost in time and dollars to test Curl applications
✦ Cost of deploying and operating a Curl application
✦ Cost of maintenance to fix problems and extend a Curl application
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The following sections examine each of these costs in more detail.
Hiring and training
The big win with Curl is that your development staff can focus on becoming expert
in one client-side language rather than average in many languages. The cost of
teaching developers Curl might be offset by not having to train each person in
multiple languages.
Curl is relatively easy to learn, so you won’t need to hire Curl-only developers.
Anyone can quickly learn Curl’s text formatting features; HTML experience is not
required. To use the programmatic aspects of Curl, experience with any objectoriented language (Java, C++, Eiffel, Smalltalk, Dylan) would suffice.
Curl’s gentle slope learning curve means that whatever a person’s current experience with Curl, learning the next feature is only slightly harder. This is a remarkable
accomplishment as compared to other languages, such as C++, that require huge
time investments before one can become proficient.
In addition to books like this, developers can learn Curl from the resources listed in
“Finding Curl Information,” later in this chapter. Curl Corporation also offers handson training.
Dual environment costs
Often, switching to new tools comes with the hidden cost of maintaining your
existing system while developing the new one. This means that, during the transition, you need to have staff expertise with many tools. You may even need to have
separate hosting environments. Since Curl can usually be phased into your existing
architecture, there is rarely a need to support dual development environments. (An
exception might be public transaction sites.) Thus, this cost is often zero, or very
low. “Technical planning,” later in this chapter, explains why this is true.
Development costs
Using Curl, development times are similar to — or faster than — normal, once you
have trained developers. Partly, this is because Curl is a powerful language and
partly this is because your developers spend less time working around the quirks
of using multiple languages. Figure 1-1 shows how fast complexity grows when
there are more tools involved.
The graph in Figure 1-1 shows that accounting for the interactions between two
tools is 10 times easier than having five tools and 21 times easier than having seven
tools. For enhancements and bug fixes, having a uniform language makes it easier
for one developer to modify another developer’s code. Plus, Curl development tools
are free.
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45
40
Number of Interactions
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30
25
20
15
10
5
0
1
2
3
4
5
6
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Number of Tools
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Figure 1-1: How more tools increase development complexity
Another benefit is that Curl-based architectures are an extension of the well-studied
client/server architectures that have been around for many years. This means that
you are not on the bleeding edge of system architecture. Instead, you can benefit
from years of industry experience in this area, further reducing risk on your project.
Testing costs
Testing current Web applications is a combinatorial mess. Figure 1-1 applies to
testers as well as to developers. If testers tried to verify a complex application for
every combination of operating system manufacturer and version, browser manufacturer and version, JVM version, JavaScript version, and so on, they would never
finish. This is not slighting testers, but recognition that testing is an economic
choice; testing each combination produces diminishing returns. How much are you
willing to spend to be sure? How long is the market willing to wait for your product?
What is the cost if you are wrong?
With Curl applications, the major browser combinations were already tested by
Curl Corporation when it tested Surge. Also, many Curl customers have gone before
you. Yes, it’s a nightmare, but it’s their nightmare. Since Curl version heralds ensure
your applet uses the correct version of Surge, all you need to do is test that your
application works with a specific version of Surge. You can spend more time testing
your application’s features and less on platform combinations.
Operational costs
It is generally cheaper to run Curl applications than applications based on other
Web technologies. However, your saving will depend on how many of the following
bullets apply to your site. Does your Web site:
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✦ Deliver large pages that may include rich content?
✦ Do data-validation on the server?
✦ Dynamically create pages from templates on the server?
✦ Deliver pages that interact locally with the user (e.g., JavaScript)?
✦ Provide multiple views of the same data?
✦ Require large amounts of bandwidth to satisfy user demand?
✦ Require multiple servers to handle user load?
✦ Make money via subscription or transaction fees?
The more yes answers you had to these questions, the more money Curl is likely to
save you. Typically, for complex applications, Curl drastically reduces the amount
of data being served from your hosts and uses less CPU time. This translates
directly into lower monthly hosting and bandwidth costs.
If you answered no to all of those questions, you probably have a simple online
brochure or business card site with mostly static content. If so, then Curl may not
be the best choice for your site. If your site is already small enough that you are
using a single host with standard bandwidth, you will not save money on hosting or
bandwidth. Then again, it’s possible you might be small enough to not trigger Curl
Corporation’s billing threshold at all, in which case Curl would be free.
Technical planning
This section focuses on how best to phase Curl into your existing architecture. It is
tempting to start with the minimal disruption to your development process and
simply start by recoding your HTML pages into Curl. While this has simplicity on
its side, it might not benefit you much, which could incorrectly give a negative first
impression of Curl.
Because, in the initial version of Curl, it can take a number of seconds to launch
Surge, you want to make sure your users get a tangible benefit whenever they
launch it. It generally would not make sense, for example, to have a Curl home page
unless you expected your visitors to use your Curl-based features.
Where does it hurt?
To decide where to start, you should first identify the areas where Curl could provide
the greatest benefits. Benefits come in many flavors: faster response times, richer
features, cost savings, and so on. Naturally, you will have to decide which goal(s) to
pursue, but as a guide you should gather the following information about your site:
✦ Which pages do users view the most?
✦ Which pages are the largest to download?
✦ Which features require the most CPU time on your servers?
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✦ Which features require the most disk accesses on your servers?
✦ Which features are currently most awkward for your users?
✦ Which pages have the most local user interaction?
✦ Which use cases require the most round-trips to the server?
The answers create a “pain list” of what “hurts” most about each aspect of your
site. Often, users are more elated by an improvement in a current feature than by
the addition of a new one. (Note that Marketing usually feels the opposite way.)
Users already expect new features to be perfect, so if you deliver a great new feature, they will be only modestly impressed. Users are more apt to be impressed
when you fix a daily frustration. Existing features also have the advantage of being
well understood, so you are less likely to stall on endless brainstorming aiming at
perfection. Pages or features that appear on multiple pain lists should be strongly
considered for your first foray into Curl.
Analyzing the pain
Now that you’ve identified the painful aspect of your site, you need to decide if Curl
can help. If you have high-bandwidth pages, can Curl reduce them by using clientstored templates that only get loaded once? Where your server is using CPU time,
could Curl use the client’s CPU? If your server accesses its disk to store user-state
data, could Curl store the state information on the client? If you have multiple
servers, what would it take to eliminate one of them? Most systems have a specific
bottleneck that causes problems. Curl should be used to address one of the bottlenecks, if possible.
Solve an easy one
This does not mean that you should find the hardest problem you can find and
tackle it first. The road of technology adoption is littered with roadkill from teams
that overpromised and fell short. If your first attempt at Curl is a failure, you may
not get a second chance.
Instead, you should take the pain list and estimate how easy or hard it would be to
solve the problem using Curl. Choose something that you think is easy or mediumhard for your first project. If everything on your pain list is hard to solve in Curl,
pick a less critical feature even if it’s not on the list. Choosing a hard project for
your first usage of any new tool is begging for trouble. The first project is always
harder than it appears.
Consider the longer term
Since Curl can incrementally replace parts of your site, update the pain list periodically to help pick future priorities. Keep in mind that the notion of solving a current
problem was only intended to cover your first use of Curl. After you are comfortable with Curl, new features or enhancements should be considered as well.
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Finding Curl Information
As Curl has expanded into daily use around the world, various resources have
emerged to help you understand Curl further. Below is a list of resources that can
help you expand your knowledge of Curl.
Curl Corporation
First and foremost, you can get definitive information at the Curl Corporation Web
sites. It you want it straight from the horse’s mouth, here is where to go.
General information
See www.curl.com for general information about Curl Corporation. This includes
product information, press releases, Curl demos, and other general information
about Curl Corporation.
Developer information
See www.curl.com/html/developers/developers.jspto connect directly to the
Curl Developer section of the site. The Curl Developer area contains FAQs, tutorials,
documentation, and discussion forums. The discussion forums are particularly
helpful now that Internet newsgroups have devolved into spam. It is not unusual to
get a response directly from Curl’s internal development team.
curlBreaker.com
curlBreaker.com is an independent organization that is committed to being “the
place to go for Curl Content Language news, tips, etc.” They offer several different
resources for Curl developers.
Web magazine
The curlBreaker.com Web magazine is an independent ‘zine for Curl developers.
The magazine covers news from the Curl world with feature articles on various Curl
topics. This is a great place to go to get independent news and analysis on Curlrelated topics. See www.curlBreaker.com for details.
Developer listserv
For Curl questions and discussions, curlBreaker.com also sponsors an e-mail listserv. This is a great way to rapidly push questions to a wide audience of Curl
developers. Many Curl Corporation developers belong to this list and are frequent
contributors. See www.curlBreaker.com/curlBreaker-l/devlist.html for
more information.
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Chapter 1 ✦ Introducing Curl
Open source
curlBreaker.com serves as the focal point for various open source projects using
Curl. Open source project teams are always looking for interested developers. This
is a great way to learn Curl and connect with other developers. Projects such as
curlMail, a completely cross-platform e-mail client, and others can be found at
www.curlBreaker.com/opensource/.
CurlExamples
CurlExamples is an independent community site for sharing, learning, and understanding Curl technology. As the name implies, this site is a treasure trove of small
working examples that show off various aspects of Curl. Examples are searchable
by title and categorized in many useful ways such as most recent, most viewed, and
top rated. There is also a hierarchical list of topics such as Business, Games,
Utilities, and so on. If you learn by example, this is the site for you. See
www.curlexamples.com for more information.
Open Directory Project
The Open Directory Project, also known as DMOZ (Directory Mozilla), contains
links to various Curl-related sites. In addition to listing the above sites, DMOZ also
has a list of sites that use Curl. Since the DMOZ directory is always being updated,
it’s a good idea to check back periodically to see what’s new. See DMOZ at
dmoz.org/Computers/Programming/Languages/Curl/.
Summary
Curl permits a fundamental change in the way we think about Web applications,
while still catering to the business realities of fitting in with current practices. Curl is
an excellent choice today for private-user systems and might be a good choice for
segments of public-user systems. Currently, designers should give particular thought
to how the user’s launch of Surge fits into their application’s usability. As the
installed base of the Surge plug-in increases, good uses for Curl will increase as well.
Curl can phase into your current development process, making it a smooth, lowrisk, transition. The simplest implementations of Curl, while tempting, may not
provide large enough benefits to justify the transition costs. You will be better off if
you first create a pain list of potential improvements and compare them against the
benefits of solving them in Curl. Then you can choose a relatively easy — but
worthy — task for your first Curl project.
Curl has become increasingly popular and many resources have sprung up, inside
and outside of Curl Corporation, to help you succeed as a Curl developer.
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