[Haskell-cafe] Improving *> and >> for Data.Sequence

[David Feuer]

On Thu, Nov 20, 2014 at 11:00 AM, Ross Paterson <R.Paterson at city.ac.uk>
wrote:

> On Thu, Nov 20, 2014 at 12:37:49AM +0000, Ross Paterson wrote:
> > On Wed, Nov 19, 2014 at 02:58:46PM -0500, David Feuer wrote:
> > > I got to looking at <*> just now, and it suggests the
> > > following question: is there a particularly efficient way to build a
> Seq when
> > > its ultimate size is known in advance, avoiding the usual incremental
> > > rebuilding?
> >
> > The following avoids the rebuilding, but I haven't tweaked or timed it:
> >
> > [...]
>
> Actually this is pretty much what the existing fromList2 does.
>

I think the technique fromList2 uses is probably sub-optimal for <*>,
because it steps through things in order. The ends of fs <*> xs don't
depend on the middle of f. It should be better, I think, to delay actually
touching that middle until it's actually demanded.
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