good fusion (was Re: inits)

"Josef Svenningsson" wrote:

On 4/10/06, Simon Marlow wrote:

...

I propose to replace inits in Data.List with this one.

Fair enough. But how does it play with list fusion?

I don't know the answer to your question, but it does prompt another one of my own.

I know that both (:) and zipWith are good producers but how does it work when they are composed like this? And how does it compare with the strict version which is just a call to zipWith (and must therefore be a good producer I assume).

Are the larger sizes of zipWith (zipWith3, zipWith4, ...) also good producers? I have some evidence from a recent application I have been writing that they may not be. At least, reading the -ddump-simpl output, it does not seem clear-cut that a pipeline e.g. (concat . f . map g . zipWith3 h as bs) cs is turned into a simple loop, although perhaps I am not reading it right. Is there any received wisdom on what should be expected here? Regards, Malcolm