[Haskell-cafe] How to write fast for loops

John Lato jwlato at gmail.com
Sun Apr 27 02:28:30 UTC 2014

Hi Niklas,

No, I mean this:

> V.forM_ (V.enumFromN 1 n)

I agree it's less elegant.  And like Daniel points out with lists, you
definitely don't want the "enumFromN" to be shared.  But one of the biggest
warts of Haskell IMHO is that it's still very difficult to get really
high-performance code without an intimate understanding of the optimizer.

John L.

On Sat, Apr 26, 2014 at 6:16 PM, Niklas Hambüchen <mail at nh2.me> wrote:

> Hey John,
> do you mean something along the lines of `V.forM_ (fromList [1..n])`?
> I guess GHC wouldn't mash away the list in this case and I'd have to
> use the new OverloadedLists with `V.forM_ [1..n]` (indeed that looks
> quite sweet).
> However, would that actually be fast? Judging from how list syntax is
> desugared, I guess that would use `V.enumFromTo` then, whose docs claim
> "WARNING: This operation can be very inefficient. If at all possible,
> use enumFromN instead".
> Does anybody know  wonder if "vector literals" desugar enumFromN, or
> what "can be very inefficient" means?
> Niklas
> On Sun 27 Apr 2014 01:13:52 BST, John Lato wrote:
> > I usually use a manually written loop, but you can use Data.Vector for
> > this and it should fuse.
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