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

Xiao-Yong Jin xj2106 at columbia.edu
Sun Jun 8 10:15:01 EDT 2008

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.

>  I don't know the hmatrix api very well, but if there is a function for 
> computing the inner product between two vectors you could always do something 
> like the following meta code:
> sum v = innerProduct v <1,1,1,1,1,1>

I just doubt the efficiency here.  If v is a very large
vector, I guess the allocation time of that intermediate
vector [1,1..] is not small.  But it is just my guess.

    c/*    __o/*
    <\     * (__
    */\      <

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