> >- return $ InstanceCpuLoad withinRange
> >+ withoutOld = Map.filter
> >+ (liftA2 (&&) (not . Seq.null)
> >+ $ (>) (fromIntegral $ C.xentopAverageThreshold *
> >1000000)
> >+ . (clockTimeToUSec now -) . clockTimeToUSec
> >+ . fst . flip Seq.index 0)
> >+ withinRange
> >+ return $ InstanceCpuLoad withoutOld
>
> While correct, this part somewhat worries me from the reability
> point of view, as this relies on `False && undefined == False`. I
> was surprised to see that there is no such thing as `headMaybe ::
> Seq a -> Maybe a` (or its generalization to Foldable), nor a safe
> version of `index` in the standard libraries. Also there is no
> folding function on `ViewL` which would allow to write point-free
> expressions targetting the head of a sequence. That looks like an
> omission in the standard libraries.
Yes, the interface of the Seq library seems to be designed around
the idea of checking for length and then using index knowing it
will work. That's why I wrote it the way the library interfaces
suggests---believing that the "liftA2 (&&) (not . Seq.null)"
guard is visible enough.
Note that you didn't have the same objection when reviewing the
actual collector in an earlier patch series, where the
combineWithRollover new old | Seq.null new || Seq.null old = new Seq.>< old
combineWithRollover new old =
let (t2, x2) = Seq.index new $ Seq.length new - 1
(t1, x1) = Seq.index old 0
...
is also used a lot---simply because this is the usage the
library interface suggests.
Regards,
Klaus
--
Klaus Aehlig
Google Germany GmbH, Dienerstr. 12, 80331 Muenchen
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