Proposal: Make intersperse lazier

[Christian Maeder]

Am 24.09.2010 16:53, schrieb Daniel Fischer:
[...]
> However, I benchmarked some more today, and the results suggest that
> 
> intersperse :: a -> [a] -> [a]
> intersperse _ [] = []
> intersperse s (x:xs) = x : foo s xs
> 
> foo :: a -> [a] -> [a]
> foo s (x:xs) = s : x : foo s xs
> foo _ []  = []
> 
> is a little faster than the SATed isgo (about 4-5% in the benchmark).
> Compiled with optimisations, it makes no difference whether foo (of course 
> we need a better name) is a top-level function or local to intersperse, but 
> without optimisations, a top-level function is better.
> Also nice is that that gives identical core for all optimisation levels 
> (0,1,2) [apart from the strictness/unfolding information, which is absent 
> with -O0].
> (Tested with 6.12.3 and HEAD.)
> 
> So far, the above is IMO the best implementation.
> A good name for foo remains to be found.

 prependToAll

> I don't like 'prepend' because prepend suggests only putting something 
> before a list (with the given type, it should be (:)) and not changing 
> anything inside.
> 
> If it's not to be exported from Data.List (and I don't consider it useful 
> enough to be), maybe intersperseLoop wouldn't be too daft.

I consider it useful, because the "natural" implementation:

  prependToAll s = foldr (\ x r -> s : x : r) []

seems to leak without optimization.

Cheers Christian

> 
> Cheers,
> Daniel
>