Re: Variables and constants

* John Lam
|
| This is more of a philosophical question. Since I haven't worked
| with functional programming languages before, can anyone explain to
| me the rationale for not having true variables a'la procedural
| programming languages (i.e. you can re-assign the value of an
| existing variable)?

XSLT is not alone in not having assignment, in fact there is a whole
family of programming languages called the functional programming
languages that work this way. The best-known are perhaps Standard ML
and Haskell. (No, Lisp does not belong here. Lisp is imperative, just
like the mainstream languages.)

The difference between the traditional imperative languages and the
functional ones is a deep one and not easily understood. At the
deepest level it has to do with whether change (that is, time) is
allowed in a program or not.

If change is banished, functions always return the same values and
reasoning about what is going on in the program becomes enormously
much simpler.

The best description you are likely to find of what this really means
appears in 'The Structure and Interpretation of Computer Programs', by
Abelson, Abelson and Sussman. (The book is definitely recommended for
anyone who wants to to serious programming, BTW.)
 
| Can anyone shed any light on why variables == constants in XSLT?

I'll let this one pass; others can probably answer it much better than
me.

--Lars M.


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