In article <[EMAIL PROTECTED]>,
Erik Bruchez <[EMAIL PROTECTED]> wrote:
[snip]
> Note that other beginners have also found it curious that some
> processors don't execute if you don't connect their outputs. In this
> case, it may be that they are less familiar with the lazy evaluation
> model, of that the documentation is lacking, or...
An interesting comment! I find that most students have difficulty with
-either- the concept of lazy evaluation -or- the concept of always
evaluating. There are definitely two mind sets involved here!
> Another example: XSLT beginners can sometimes jump to the roof when
> they find out that you can't modify XSLT "variables". However, it
> makes perfect sense considering XSLT from the point of view of
> functional languages.
Yes! - .and most students find functional programming quite
mind-boggling. I would, however, rather use the term 'binding' rather than
variable - in teaching xslt I find this helps considerably.
[snip]
> If you find the time, and if you hit the appropriate use case, it
> would be nice for us to see another example of how a pipeline written
> with XPL would look like with the changes you suggest, i.e.:
> 1. All processors have at least one output.
> 2. There is a syntactical construct to "ground" outputs.
What you both seem to be discussing is an extension of three-valued
logic as used in the formal methods world. Here, however, we have a 'value'
result from some processing and at the same time a 'status' result - which
is how POSIX interfaces are defined - there is a result status and a result
value (or an empty value) - have a look at the language-independent POSIX
specs. In practice this is often implemented at the CPU level by a 'flags
word' and a result.
I think that at this very much higher level the concept of a status
(or control if you prefer the term) AND a value result therefore makes
perfect sense - it is just an extension of the principle already well-known
in other contexts.
Forgive me if I have entered the discussion from a different point of
view - I just thought that the parallels were interesting.
Keith Hopper
--
City Desk
Waikato University
[PGP key available if desired]
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