[Haskell-cafe] Re: Why?

Richard O'Keefe ok at cs.otago.ac.nz
Thu Dec 10 20:43:47 EST 2009

On Dec 11, 2009, at 3:50 AM, John D. Earle wrote:

> David, think of the machine as being the earth and laziness is in  
> the clouds.

It reads so much better as "laziness is in the heavens".

> Strict evaluation is closer to the machine.

It doesn't have to be.  Graph reduction hardware has been built in the
past and could be again.

> The relationship between a lazy algorithm and the earth is abstract;  
> hence, it will make creating algorithms especially efficient ones  
> difficult.

All programming (except possibly assembly coding, and given the  
of optimising assemblers, even that) is abstract.  In fact it had better
be abstract, because I don't want to write different programs for  
PowerPCs, x86s, &c.  I'd really rather not even write different programs
for 32-bit and 64-bit environments.  Given the massive amounts of stuff
happening in modern processors (like the dynamic register renaming &c in
x86s, deep write buffers, out-of-order execution, short SIMD vector
instructions), even simple C code is a *long* way from what is really
happening.  To use your analogy, if the machine is the earth, C  
is floating around in a balloon _just under_ the clouds.

Paradoxically, working at an abstract level can make creating efficient
algorithms MUCH EASIER.

Recently, I was looking at some Java code from a new book about
numerical algorithms for scientists and engineers in Java.  Because I
have lots of uses for the Singular Value Decomposition, I chose to
look at the SVD code.

Version			speed		reason
Java from book		1
rewritten in C		2		a[i][j] doesn't indirect twice
transpose the matrices	4		goes with the grain of memory
use LAPACK		7		algorithm, blocking for cache?

Working at an abstract level ("I want (u,s,v) = svd a") instead of
rolling my own low level code means that I can get _faster_ code.
(There's no reason why Java couldn't call LAPACK through JNI, and if
I had much numeric code to do in Java, that's what I'd do.)  With the
SVD (and similar things) as basic building blocks, it's much easier
to develop interesting algorithms.  For example, if I wanted to write
my own code for Correspondence Analysis, it would be far more sensible
to develop the algorithm in R (the free S) or Matlab or Octave, and
_not_ write my own array loops, than to write in C.  I'd probably get
something much faster.

Oh, and there's a new SVD algorithm being worked on for LAPACK, which
is available for real numbers only in the current LAPACK release.
Supposedly it's not only more accurate but faster.  Change one line in
my code to call that, and I get a very nice benefit from abstraction!

> All of this is still a work in progress. The Haskell creed appears  
> to be, This is the way so stick to it!

There is no Haskell "creed".  People use Haskell for different reasons.
Many Haskell programmers are totally pragmatic about it.
Remember, PROGRAMMING IS HARD.  If Haskell lets people _write_ correct
programs faster, that's an enormous advantage, even if they don't _run_
faster.  Once you have something actually working, you can worry about

The Java code I mentioned above came from a *recently* published book.
Here's someone willing to put up with a factor of 7 loss of performance
in order to get the other benefits of Java and persuading a publisher
that a lot of other people will be happy with it too.

> The idea appears to be that by sticking to the program the problems  
> will be overcome in time and we will be left with all the glorious  
> goodness. At one time it was not possible to achieve the sort of  
> efficiency that Haskell achieves as a matter of course.

Abstraction makes it comparatively easy to switch over from String to
ByteString.  Purity means that taking advantage of multicore machines
is already much easier than in C (with pthreads), C++ (with TBB), or
Java (with itself).

To be honest, I don't expect getting high performance out of Haskell
to ever be easy.  But then, getting high performance out of C or
Fortran isn't easy either.

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