I was going to propose this as well! Would probably provide valuable practicability feedback to the proposed proposal process.
- Moritz > On Jul 17, 2016, at 5:10 PM, Oleg Grenrus <oleg.gren...@iki.fi> wrote: > > Should we test drive https://github.com/ghc-proposals/ghc-proposals > on this proposal? > > - Oleg > >> On 17 Jul 2016, at 05:02, Ryan Scott <ryan.gl.sc...@gmail.com> wrote: >> >> I'm pursuing a fix to Trac #10598 [1], an issue in which GHC users do >> not have fine-grained control over which strategy to use when deriving >> an instance, especially when multiple extensions like >> -XGeneralizedNewtypeDeriving and -XDeriveAnyClass are enabled >> simultaneously. I have a working patch up at [2] which would fix the >> issue, but there's still a lingering question of what the right syntax >> is to use here. I want to make sure I get this right, so I'm >> requesting input from the community. >> >> To condense the conversation in [1], there are three means by which >> you can derive an instance in GHC today: >> >> 1. -XGeneralizedNewtypeDeriving >> 2. -XDeriveAnyClass >> 3. GHC's builtin algorithms (which are used for deriving Eq, Show, >> Functor, Generic, Data, etc.) >> >> The problem is that it's sometimes hard to know which of the three >> will kick in when you say `deriving C`. To resolve this ambiguity, I >> want to introduce the -XDerivingStrategies extension, where a user can >> explicitly request which of the above ways to derive an instance. >> >> Here are some of the previously proposed syntaxes for this feature, >> with their perceived pros and cons: >> >> ----- Pragmas >> * Examples: >> - newtype T a = T a deriving ({-# BUILTIN #-} Eq, {-# GND #-} >> Ord, {-# DAC #-} Read, Show) >> - deriving {-# BUILTIN #-} instance Functor T >> * Pros: >> - Backwards compatible >> - Requires no changes to Template Haskell >> * Cons: >> - Unlike other pragmas, these ones can affect the semantics of a program >> ----- Type synonyms >> * Examples: >> - newtype T a = T a deriving (Builtin Eq, GND Ord, DAC Read, Show) >> - deriving instance Builtin (Functor T) >> * Pros: >> - Requires no Template Haskell or parser changes, just some >> magic in the typechecker >> - Backwards compatible (back to GHC 7.6) >> * Cons: >> - Some developers objected to the idea of imbuing type synonyms >> with magical properties >> ----- Multiple deriving clauses, plus new keywords >> * Examples: >> - newtype T a = T a >> deriving Show >> deriving builtin instance (Eq, Foldable) >> deriving newtype instance Ord >> deriving anyclass instance Read >> - deriving builtin instance Functor T >> * Pros: >> - Doesn't suffer from the same semantic issues as the other suggestions >> - (Arguably) the most straightforward-looking syntax >> * Cons: >> - Requires breaking changes to Template Haskell >> - Changes the parser and syntax significantly >> >> Several GHC devs objected to the first two of the above suggestions in >> [1], so I chose to implement the "Multiple deriving clauses, plus new >> keywords" option in [2]. However, I'd appreciate further discussion on >> the above options, which one you prefer, and if you have other >> suggestions for syntax to use. >> >> Ryan S. >> ----- >> [1] https://ghc.haskell.org/trac/ghc/ticket/10598 >> [2] https://phabricator.haskell.org/D2280 >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs ————————————————— Moritz Angermann +49 170 54 33 0 74 mor...@lichtzwerge.de lichtzwerge GmbH Raiffeisenstr. 8 93185 Michelsneukirchen Amtsgericht Regensburg HRB 14723 Geschäftsführung: Moritz Angermann, Ralf Sangl USt-Id: DE291948767 Diese E-Mail enthält vertrauliche und/oder rechtlich geschützte Informationen. Wenn Sie nicht der richtige Adressat sind oder diese E-Mail irrtümlich erhalten haben, informieren Sie bitte sofort den Absender und vernichten Sie diese Mail. Das unerlaubte Kopieren sowie die unbefugte Weitergabe dieser Mail ist nicht gestattet. This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden. _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
