[Haskell-cafe] Using Product Algebraic types

[Crypt Master]

-------------
<sigh>
    I always get a bad feeling when people start to think about efficiency 
right
    from the beginning: First get your program correct and readable, then 
measure,
    and only then optimize (if at all). Programmers are notoriously bad when
    guessing about efficiency, which even more true for lazy functional 
programs.
</sigh>
------------


Sorry to have produced the *sigh* factor. I asked the question only to start 
learning,  which your answer has certainly helped me with.

Maybe I can present a differnt persepctive on performance though. I have to 
say up front though that if I was interested in squeezing the last drop of 
performance out of everything I wouldnt be learning haskell as all the web 
sites warn you about this :-) I do beleive in clarity, readbaility, 
correctness, time to develop etc.., it just that performance is one of those 
factors.

My day job is all mostly about performance (from iterative programming 
perspective), I make complex business applications useable from a 
perfromance perspective. While most people say "just buy more hardware" or 
make it "correct or readable" first, my experince shows this leads to a lot 
of wasted effort which could have been avoided with a few seconds thought. 
Have you ever tried telling an IT Director that his 500 000 - 1 000 000 
pound equipment purchase isnt cutting it for a "simple" business application 
:-) Let me tell you, they can have really selective hearing when it suits 
them :-)

In my world, getting a program "correct" includes effieincy within reason. 
For exmaple, a 5 minute response time for interactive process can be pushed 
on employees with a relativilty minial cost of a small loss in productivity. 
However 5 minutes as a response for customer sitting on your web site is 
going to cost you millions and probably put you out of business. There are 
dozens of senarios like this where a "correct" program is as good as no 
program if it doesnt perform fast enough.

What you say about guessing at performance is correct. I have given lectures 
on performance which start off with slides which say the trick to 
performance tuning is:

"Measure, Measure, Measure"

But I have found through experience (in the iterative world) that certian 
patterns of bad performance are easily avoided at development time because 
we know from experince they are bad at runtime.

Performance has 3 flavours in my mind:

1) Hard Core
2) Design (Algortihms and data representation)
3) Common sense coding

The fiirst, Hard code, invovles assemlber optimizations, branch reductions 
using boolean algebra etc.. No one argues these are mostly not worth the 
effort and so these are mostly ingnored expect for very special 
cercumstances. The phrase "buy more hardware" applies in this case 99% of 
the time.

At the other end of the spectrum you have design. Few people will argue that 
getting your design right up front is very important. Changing Algorithms or 
Data Representation usually invovles changes of a significance that you have 
wasted your time on the "incorrect" version as you are starting mostly from 
scratch. Admitatly this looks a lot easier in haskell than in other 
impertive lanaguages.

The 3rd area is where I differ from most people. Common sense coding is 
simply not calculating or looking something up over and over in an inner 
loop etc... While the programs can be changed afterwards to incorporate 
these, a few seconds thought by the developer avoids the bottle neck in the 
first place. An Example of the impact of such ineffiencies is this:  
Recenetly I "optimized" an interactive job which was taking over 10 minutes 
to run. Without understanding much about it (its highly complex) I was able 
to bring it down to < 1 minute applying simple common sense things. I never 
changed or looked at the "logic" behind it. Happy user base = productive 
company  :-)

That said haskell compiler seem pretty smart from what you have shown me so 
far ... which is a welcome change :-)

Thanks for your reponse. This is the most helpfull community I have 
encountered so far .. :-)

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