[erlang-questions] If you are homesick for object.selector

Darach Ennis <>
Sat Jan 26 14:05:52 CET 2013


Hi guys,

My apologies for not introducing what a Kaysian transform is. It's basically
a technique discovered by Michael Kay (long time XPath / XSLT guy,
wrote/leads
Saxon) for efficient set intersection and difference with XPath. It's a
neat little efficiency
trick. It's documented in the following (page 3), with other neat tricks:

http://www.xml.org//sites/www.xml.org/files/xslt_efficient_programming_techniques.pdf

On the schemey/lispy approach? I like it. If the structures are reasonably
stable with respect to
change over time then some artful hiding behind accessor functions is good
enough. I do it myself
for things like gen_server state (accessors) or process options (mutators).
I agree with Joe too.
If a parse transform compiles away some inefficiency (human or systemic)
then that's a better
accessor.

But would I maintain the structures for a family of complex financial
derivative instruments
in erlang today and/or represent them as erlang records? No, not with the
same accessors and/or
code generators I'd be happy to write and use for other isolated tasks. The
reason is simple. The
number of, complexity of, frequency of change, and continuous arrival and
departure of newly
invented structures would mean a lot of maintenance and upheaval. It's a
significant source of pain
in capital markets and the problem is not exclusive to any one language. At
some point you're
going to end up compromising and accept a certain level of maintenance.

So maybe there isn't one perfect solution. Maybe there's a few good partial
solutions and each
is optimal given a certain presumed level of maintenance. But to presume
all environments are
dealing with fairly trivial structures like the examples thus far, or that
they are few in volume,
or that the frequency of change of those structures is low? That is an auto
selection problem.

I see more than one good approach but no right one here.

Cheers,

Darach.


On Sat, Jan 26, 2013 at 10:15 AM, <> wrote:

> I wrote:
>
>
> > So I don't _want_ to write AXP.a.x.p because that code
> > frightens me with its fragility.  The only x.a.b or
> > x->a->b I could find in a quick find+grep over my own
> > C code on this laptop were two lines in some code to
> > handle doubly linked lists.
>
> Imagine me with egg all over my face.
> Somehow that find+grep missed an entire directory,
> containing a project with lots of code by me and
> nobody else, containing hundreds of examples of
> what I said I didn't do.
>
> Some of the code is actually benign.  It's not *really*
> spanning multiple records.  I have a lot of tagged unions,
>    struct Thingy {
>       enum Thingy_Tag tag;
>       union {
>          int i;
>          Unique_String s;
>          ...
>       } u;
>    }
> and occurrences of thingy->u.s really count as a single
> field reference.
>
> However, there were also about 300 lines containing
> foo->bar->ugh and I have spent a couple of hours
> fixing as many of them as I could.
>
> In many cases, the code was (to my mind) simplified
> and clarified by introducing
>     gvar const o = m->owner;
>     ... o->whatever ...
> instead of m->owner->whatever, because it turned out
> that o was actually used several times in a function
> containing such a double step.  Simply, if I was using
> foo->bar->ugh I was almost always _also_ using
> foo->bar->zoo and/or plain foo->bar itself.  Before C
> compilers got good at optimising, this change would
> also have made the object code smaller and faster;
> even these days, C's aliasing rules probably mean there's
> some benefit there.
>
> I've spotted this in other people's Erlang code too.
> Function bodies that contain lots of field references
> can often be shortened (and to my mind simplified) by
> replacing field references by pattern matching in the
> head.
>
> Of the rest, quite a few would have benefited from
> virtual fields, had C supported them.  Some of the
> ones that are left would have been simpler and easier
> for the compiler to type-check had I *not* been using
> C format strings.  (The whole program basically doubled
> in speed when I stopped using <stdio.h> for most I/O;
> the only stdio code left is for error messages.)
>
> But to me the most interesting thing was recalling,
> as I worked over the code, that these had been the
> most error prone parts of the program.  foo->bar->ugh
> really _had_ been a warning sign that I had not
> understood years ago when writing this stuff.
>
> Not only that:  the foo->bar->ugh antipattern turned up
> a lot in code that needs a rewrite in order to hold
> Unicode in certain internal data structures, and I'd
> been putting that off and putting it off because fixing
> this stuff was so scary.  I hadn't understood _why_ it
> was scary, but now I think I do.
>
> Why has it taken me so long to learn not to be so stupid?
>
>
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> 
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>
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