[Haskell-cafe] "sum" in hmatrix and blas?

[Don Stewart]

xj2106:
> Tomas Andersson <toman144 at student.liu.se> writes:
> 
> >  You can never go wrong with a good old fashioned hand written tail recursion 
> > when you're in doubt, they are pretty much the closest thing to for-loops 
> > there is in haskell and should be easy to grok for Imperative programmers and 
> > usually produce really fast code.
> >
> > sum vect = let d = dim vect
> >            in sum' (d - 1) 0
> >   where sum' 0 s = s + (vect @> 0)
> >         sum' index s = sum' (index - 1) (s + (vect @> index))
> >
> 
> Do I need the strict version of sum'?  Put something like
> 
> sum' a b | a `seq` b `seq` False = undefined
> 
> Or ghc will optimize it automatically?  I always don't know
> when such optimization is useful.

If you give a type annotation, so GHC can tell its an atomic type, like
Int or Double, 's' will be inferred as strict. But if in doubt, use bang
patterns:

    {-# LANGUAGE BangPatterns #-}

    sum vect = sum' (d-1) 0
        where
            d = dim vect
            
            go :: Int -> Double -> Double           -- for example
            go 0 s = s + (vect @> 0)
            go !i !s = go (i-1) (s + (vect @> i))


See this recent post on understanding how these kind of loops are compiled,

    http://cgi.cse.unsw.edu.au/~dons/blog/2008/05/16

The core for your loop should look very good, assuming @> is efficient.

-- Don