[Haskell-cafe] How to speed things up

[Greg Buchholz]

Dmitry Vyal wrote:
> Hello everybody.
> 
> I have a long list consisted of a small number (about a dozen) of 
> elements repeating in random pattern. I know all the possible elements.
> I need to count number of occurences of each particular element and to 
> do i quickly.
> 
>    For example
> quick_func Eq a => [a] -> [(a,Int)]
> quick_func [1,2,3,1,2,9,1,9] == [(1,3),(2,2),(3,1),(9,2)]
> 
> According to profiler this function is the bottle-neck in my sluggish 
> program so I really need to speed it up.

    What's been tried so far?  Below is a snippet using arrays.  You'd
probably get a faster program with Unboxed arrays and unsafeAccumArray.
 

> import Data.Array
> 
> main = print $ quick_func [1,2,3,1,2,9,1,9]
> 
> quick_func is = assocs $ accumArray (+) 0 (1,12) [(i, 1) | i<-is]