Proposal: (breaking change to avoid fragile breakage) change the definition of foldr2, zip, and zipWith

[Simon Peyton Jones]

|  David, I think you can go ahead and prepare a patch. People can still
|  speak up before (and even after) it is applied if they disagree.

I'm way behind the curve here, but this is really a matter for the Core Libraries Committee, and I gather you are already in good communication with Edward.  You can use GHC's Trac for Core Libraries tickets; just use "Core Libraries" for the "Component" field.

Simon

|  -----Original Message-----
|  From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of
|  Joachim Breitner
|  Sent: 08 September 2014 09:25
|  To: David Feuer
|  Cc: Haskell Libraries
|  Subject: Re: Proposal: (breaking change to avoid fragile breakage)
|  change the definition of foldr2, zip, and zipWith
|  
|  Dear David,
|  
|  
|  Am Montag, den 08.09.2014, 02:58 -0400 schrieb David Feuer:
|  > It's been a couple weeks now, and no one's responded.
|  
|  that can happen. Usually not because noone cares, but because noone
|  knows what’s best. And also there was ICFP last week, which probably
|  kept people busy ... so don’t be discouraged by silence, and keep
|  following up on it.
|  
|  > On Sun, Aug 24, 2014 at 3:22 PM, David Feuer <david.feuer at gmail.com>
|  wrote:
|  > > BACKGROUND
|  > >
|  > > TRAC: #9495
|  
|  > > SOLUTIONS
|  > >
|  > > 1. One solution, of course, is to eliminate unfoldr2/right,
|  bringing
|  > > GHC into compliance with the Report. I really like this idea, but
|  > > Joachim thinks it is not worth the potential performance impact on
|  > > code written before or without regard for the change. We both
|  agree
|  > > that a reasonable alternative is
|  > >
|  > > 3. Modify the baseline definition of unfoldr2 as follows:
|  > >
|  > > foldr2 :: (a -> b -> c -> c) -> c -> [a] -> [b] -> c
|  > > foldr2 k z = go
|  > >   where
|  > >     go [] ys = ys `seq` z
|  > >     go _xs [] = z
|  > >     go (x:xs) (y:ys) = k x y (go xs ys)
|  > >
|  > > This should, we believe, make the baseline definition fail where
|  the
|  > > one fused by foldr2/right would fail, giving consistent semantics.
|  
|  You already said that I agree with that solution, but I can re-state
|  it here :-)
|  
|  > > WHAT MIGHT BREAK
|  > >
|  > > Code that currently works but will break with this change has two
|  > > properties:
|  > >
|  > > 1. It relies on the asymmetry in foldr, zipWith, or zip to avoid
|  > > running into bottom.
|  > > 2. foldr2/right does not fire, either because the second list is
|  not
|  > > a good producer or because GHC applies the foldr2/left rule
|  instead.
|  > >
|  > > That is, most of the code that this change will break is fragile
|  > > under the current scheme.
|  > >
|  > > DISCUSSION PERIOD
|  > >
|  > > Standard two weeks.
|  
|  Given that nobody complained about the standard-non-conformance so far
|  I think having a symmetric zip where the RULES are semantics-
|  preserving is more useful than strictly following the report. But I
|  could be convinced otherwise.
|  
|  
|  David, I think you can go ahead and prepare a patch. People can still
|  speak up before (and even after) it is applied if they disagree.
|  
|  Greetings,
|  Joachim
|  
|  --
|  Joachim “nomeata” Breitner
|    mail at joachim-breitner.de • http://www.joachim-breitner.de/
|    Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0xF0FBF51F
|    Debian Developer: nomeata at debian.org