inits

[Chris Kuklewicz]

Ross Paterson wrote:
> On Mon, Apr 10, 2006 at 03:54:09PM +0100, Chris Kuklewicz wrote:
>> If the goal is speed, then this definition is running over 10% faster with ghc
>> -O2 on my powerbook for (sum $ map length $ inits [1..10000])
>>
>> inits' = helper id
>>     where helper f [] = (f []):[]
>>           helper f (x:xs) = (f []):helper (f.(x:)) xs
> 
> I rather like
> 
> 	inits = map ($ []) . scanl (.) id . map (:)
> 

That takes 3 times longer than the helper function definition.  Barring fusion,
it is creating an extra list.  The "scanl" makes a list, the "map" makes a list
and the "(f [])" makes a list.

The helper function makes a list with "(f[])" and with "(...):helper...".

> but this is also competitive:
> 
> 	inits = map reverse . scanl (flip (:)) []
> 

I would never try "reverse" when looking for performance, but that runs at the
same speed as the helper and allocates the same amount of space.

-- 
Chris