Re: [Hs-Generics] RE: Generics

Hi José, You may also want to weigh in on this issue: http://hackage.haskell.org/trac/ghc/ticket/2782 The instance of Ratio changed between 6.10 and 6.8 in a way that means a program can't reflect on the fields contained within the :% constructor using the value undefined. The reason is that :% is marked as strict in both arguments, so _|_ :% _|_ == _|_. This breaks Uniplate, and I've had to explicitly make a test for a type of Rational in the Uniplate code - which is ugly. I'm not sure how many other programs this might break - or if the impact was well understood when the change was made. This change is made massively worse by giving the error message "undefined" when it fails! If an SYB using program goes from working in GHC 6.8 to failing in GHC 6.10 with "undefined", this may be a culprit. I'm not sure there is a nice solution - reflection at the type level (using _|_ at the value level), combined with strictness at the value level, has limitations. It may be that the reflection machinery in SYB can be tweaked to either alert the user in advance (i.e. by getting the strictness of various fields), or providing some operation combining gmapQ and fromConstr which isn't strict. To see my use case take a look at "contains" in: http://www.cs.york.ac.uk/fp/darcs/uniplate/Data/Generics/PlateData.hs Thanks Neil On Fri, Nov 21, 2008 at 5:28 PM, Simon Peyton-Jones wrote:

Thanks. And here is one more

http://hackage.haskell.org/trac/ghc/ticket/2750

For potentially-breaking changes the right thing to do is to announce (and justify) the proposed change using the usual library-change protocol, with a discussion period. The important thing is that someone is driving the discussion -- thank you very much for that. I'll assign these tickets to you.

Simon

From: josepedromagalhaes@gmail.com [mailto:josepedromagalhaes@gmail.com] On Behalf Of José Pedro Magalhães Sent: 21 November 2008 15:30 To: Simon Peyton-Jones Cc: libraries@haskell.org; Generics Mailing List Subject: Re: Generics

Hello Simon,

Thanks for pointing these out to me. I won't be able to look into this for the next week, but from what I could see so far, #2760 seems easy: copy mkNorepType to mkNoRepType, mark mkNorepType as deprecated and replace usage everywhere to avoid warnings and pass validation. I can submit the patch for this.

#2759 has some deeper implications, though. Replacing the FloatConstr Double by FloatConstr Rational might break existing client code (even though it probably wouldn't be too hard to fix it). Additionally, I see that the serializer code in compiler/utils/Serialized.hs would also need changes (it's no longer serializing a Double). But I guess this is also not too problematic...

Cheers, Pedro

On Fri, Nov 21, 2008 at 13:34, Simon Peyton-Jones wrote:

Jose

As part of your work on the SYB library, could you spare the time to look into these two library bug reports?

http://hackage.haskell.org/trac/ghc/ticket/2759 http://hackage.haskell.org/trac/ghc/ticket/2760

Many thanks

Simon

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