[Haskell-cafe] saner shootout programs

[Don Stewart]

dons:
> jhc0033:
> > I don't know Haskell very well, but even I can tell, looking at, for
> > example, the N-body benchmark, that the Haskell code is probably not
> > type-safe, and the tricks used in it would not be usable in a larger
> > program (see below).
> > 
> > The task is essentially a pure computation: take a list of bodies
> > having mass, position and velocity; apply Newton laws at discrete
> > intervals for a large number of times; return new positions and
> > velocities.
> > 
> > I could write a C++ procedure that performs this task and have some
> > piece of mind regarding its type correctness, exception safety and
> > functional purity: side effects would be local to the procedure,
> > arguments passed as const or by value, the result returned by value,
> > no type casts or new/delete operators used.
> > 
> > On the other hand, the Haskell code makes assumptions about the size
> > of double-precision floats (obviously not type-safe). Further, the
> > simulation is not a pure function.
> > 
> > It is often argued that one only needs these dirty tricks in the most
> > time-consuming functions. However, if using imperative programming in
> > these "inner loop" procedures places them in IO monad, the "outer
> > loops" (the rest of the program - procedures that call it) will have
> > to go there as well. This makes me doubt the Haskell approach to
> > functional programming.
> > 
> > If anyone has a version of the N-body benchmark, where the simulation
> > is a type-safe pure function, I would very much like to see and time
> > it.
> 
> n-body requires updating a global array of double values to be
> competitive performance-wise, though we haven't really nailed this
> benchmark yet. The current entry should be considered an older approach
> to raw performance -- typically we can get good (or better) results in
> using the ST monad.

Oh, for those following, the program we're talking about is this one:

    http://shootout.alioth.debian.org/gp4/benchmark.php?test=nbody&lang=ghc&id=0

Which I have to argue doesn't /look/ to bad, though the use of Ptr
Double to represent mutable arrays of vectors -- following C -- is a bit
lower level than we'd like.

-- Don