Ord methods are surprisingly strict

[David Feuer]

I believe this is all about strictness analysis. If these were lazy, then
users would have to be very careful to force the lazy arguments when they
don't need that laziness to avoid building unnecessary thunks.


On Thu, May 7, 2020, 9:03 AM Simon Jakobi via Libraries <
libraries at haskell.org> wrote:

> Hi!
>
> Generally, when using libraries, I expect functions to be as lazy as
> possible, unless they are documented to have different strictness
> properties. My impression was that this rule of thumb is fairly widely
> accepted. If this is not a good rule to work with, please do correct
> me!
>
> In any case, I noticed that many instances of Ord in base are an
> exception to that rule: (>=), max, etc. tend to evaluate both
> arguments, although a result could often be produced based on the
> value of only one argument.
>
> For example (True >= x) could return True without evaluating x, but it
> doesn't.
>
> The most blatant and trivial example would be () where the ordering
> can be determined without looking at any argument. However:
>
> $ ghci
> GHCi, version 8.10.1: https://www.haskell.org/ghc/  :? for help
> > () >= undefined
> *** Exception: Prelude.undefined
> CallStack (from HasCallStack):
>   error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
>   undefined, called at <interactive>:13:7 in interactive:Ghci1
> > undefined >= ()
> *** Exception: Prelude.undefined
> CallStack (from HasCallStack):
>   error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
>   undefined, called at <interactive>:14:1 in interactive:Ghci1
>
> The code where I first noticed the issue looked a bit like this:
>
> data Const = Type | Kind | Sort deriving (Eq, Ord, Show)
>
> f :: NonEmpty Const -> Const
> f = maximum
>
> Ideally f would stop traversing the list once it encounters the first
> Sort. Yet it doesn't:
>
> > f (Sort :| [undefined])
> *** Exception: Prelude.undefined
>
> Redefining the Ord instance is trickier than expected too:
>
> instance Ord Const where
>   Type <= _    = True
>   Kind <= Kind = True
>   Kind <= Sort = True
>   Sort <= Sort = True
>   _    <= _    = False
>
> This is insufficient to fix max, since its default implementation is
> biased towards the second argument:
>
> max x y = if x <= y then y else x
>
> > max Sort undefined
> *** Exception: Prelude.undefined
>
> So I customize max:
>
>   max Type x    = x
>   max Kind Sort = Kind
>   max Kind _    = Kind
>   max Sort _    = Sort
>
> (f (Sort :| [undefined])) still fails!
>
> This turns out be due to NonEmpty's Foldable instance relying on the
> default definition for maximum:
>
>     maximum :: forall a . Ord a => t a -> a
>     maximum = fromMaybe (errorWithoutStackTrace "maximum: empty
> structure") .
>        getMax . foldMap (Max #. (Just :: a -> Maybe a))
>
> The problem here is that Maybe's Semigroup instance is strict in both
> arguments!
>
> So I have to define
>
> f = foldr1 max
>
> …to finally get
>
> > f (Sort :| [undefined])
> Sort
>
> So my questions are:
>
> Why are derived Ord instances and most Ord instances in base so
> surprisingly strict?
>
> How come the entire tooling around Ord seems so biased towards strict
> Ord implementations?
>
> Cheers,
> Simon
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20200507/53451fcc/attachment.html>