Not only the purpose of this article to prove that XSLT can be
considered a functional programming language has been fulfilled by
providing XSLT implementation for the most major FP design patterns
and examples from John Hughes article "Why Functional Programming
matters" (this article contains the code of 35 functions), but as a
side effect we have now available what can be considered the first
XSLT functional programming library. The full library code,
together with test examples demonstrating the use of individual
functions, is available at the downloads page of TopXML.COM as specified
in the links at the start of this article.
The fxsl library and its next versions are (will be) available also as part of the
evaluation version of the Xselerator IDE, which can be downloaded
at
http://www.topxml.com/xselerator/ . This is done with the
purpose to assure that the new fxsl library releases would be made
constantly available to the thousands of highly experienced XSLT
programmers, who are users of Xselerator. Anyone, who is interested
to study and start using the fxsl library, can do this
now.
Discussion
We have provided a multitude of examples illustrating the
usefulness of higher-order functions in XSLT. Most of the examples
from John Hughes article "Why Functional Programming matters"
have been implemented and the XSLT code demonstrated. Many of them
(e.g. foldl, foldr, foldl-tree, map, buildListWhile,
buildListWhileMap) are important functional programming design
patterns that can generate unlimited types of new functions,
depending on the functions passed to them as parameters.
All this is more than an ample proof that XSLT is in fact
a full-pledged functional programming language.
On the other side, the XSLT code of those functions
seems too-verbose compared to the corresponding Haskell code. The
process of writing functional XSLT code can be made much more
straightforward and easier by providing support for higher-order
functions in XPath and XSLT, thus definitely improving even further
the compactness, readability and reliability of XSLT functional
code. It is the ideal time right now for the W3C XPath 2.0 working
group to make the decision to provide the necessary support for
higher-order functions as part of the standard XPath 2.0 specification.
In case this golden opportunity is missed, then generic templates and libraries
will be used in the years to come.
Resources
[1]Why
Functional Programming Matters
By John
Hughes
A brilliant paper, explaining the essence and beauty of the most
important FP concepts. Multiple, very nice examples. Beautiful and
elegant -- this is a "must read" for everyone who needs to
understand what FP is all about.
[2]
Functional Programming and XML
By Bijan Parsia: Describes support for XML in some
FP languages: HaXML, LambdaXML and Erlang.
[3]
What kind of language is XSLT?
By Michael Kay:("Note: Although XSLT is based on
functional programming ideas, it is not as yet a full functional
programming language, as it lacks the ability to treat functions as
a first-class data type.")
[4]
A formal semantics of patterns in XSLT (PDF)
By Philip Wadler
[5] Philip Wadler's
home page
A researcher with major contributions in the area of formal
definition of the semantics of XSLT and XPath
[6] Haskell home
page
The home page of the FP language Haskell.
[7] Haskell,
The Craft of Functional Programming, Second Edition
By Simon Thompson
[8] XSL
Transformations (XSLT) Version 1.0W3C Recommendation 16 November
1999
[9] Personal communication with Corey Haines, through messages
in the xsltalk
forum.
[10] A Gentle
Introduction to Haskell, Version 98
By Paul Hudak,
John
Peterson,
Joseph Fasel
About the author
Dimitre Novatchev is the author of the popular tool for learning
XPath -- the XPath Visualizer, and he developed the first
systematic implementation of the concept of "generic
templates". His background (and PhD, a long time ago) is in
computer science and mathematics. He has been working at Fujitsu
Australia on various aspects of OODBMS, including XML support for
Jasmine. At the time of writing Dimitre is a software development
manager at the UN Centre in Vienna, Austria.
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