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