[Haskell-cafe] #haskell works

Andrew Coppin andrewcoppin at btinternet.com
Sat Dec 15 14:48:32 EST 2007

Neil Mitchell wrote:
> Hi
> I agree with Bulat that Haskell has, if anything, even better
> optimisation potential than something like C. With Haskell you can do
> the crazy high-level optimisations that things like C would demand
> really advanced alias-analysis. Compare this to low-level
> optimisations which in Haskell require strictness analysis but in C
> are easy. At some point high-level will become more important than
> low-level, when it does, we win :-)

I had this conversation where Mr C++ basically said that my code 
implements 3 loops and it's "not possible" to optimise that into just 1 
loop like the C program is doing. Then I (or rather, Don) demonstrated 
that the stream fusion library *has* optimised it into just 2 loops. 
(Apparently the library isn't 100% complete as yet.)

I liken transformations like this to the sort of high-level 
optimisations that a database engine might do given an SQL statement. An 
SQL SELECT certainly *looks* just like a loop construct. But it isn't; 
it declares the result, not the algorithm, freeing the database to use 
*any* algorithm that produces the right answer. The result is that, as 
is well known, databases are supremely good at executing SQL queries 
blisteringly fast. When I see the compiler turn 3 loops into 2 by using 
algebraic properties of the program source code, that's how I think of it.

Hmm, perhaps to really show this off, we need a more complicated 
program. Something that would be just too hard to implement as 1 loop in 
C, but is easy for the GHC optimiser to build. I shall meditate on this 

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