That is a good question. Weirdly enough, the current behavior for how a deriving strategy is resolved (without explicit keywords) isn't really documented anywhere, so I attempted to figure out what GHC currently does and documented it here [1]. I'll reproduce the algorithm (including deriving strategies) below: 1. Look for a deriving strategy. If one is present, use that. 2. (a) If deriving an `Eq`, `Ord`, `Ix`, or `Bounded` instance for a newtype, use the `GeneralizedNewtypeDeriving` strategy (even if the language extension isn't enabled). (b) If deriving a `Read`, `Show`, `Data`, `Generic`, `Generic1`, `Typeable`, `Traversable`, or `Lift` instance for a newtype, go to step 3. (c) Otherwise, if deriving a "standard derivable class" (e.g., `Eq`, `Ord`, `Generic`, etc.) instance for a newtype, and `-XGeneralizedNewtypeDeriving` is enabled, derive the class using the `GeneralizedNewtypeDeriving` strategy. (d) Otherwise, if deriving an instance for a newtype and both `-XGeneralizedNewtypeDeriving` and `-XDeriveAnyClass` are enabled, default to `DeriveAnyClass`, but emit a warning stating the ambiguity. (e) Otherwise, if deriving an instance for a newtype, the datatype and typeclass can be successfully used with `GeneralizedNewtypeDeriving`, and `-XGeneralizedNewtypeDeriving` is enabled, do so. 3. (a) If deriving a "standard derivable class" (e.g., `Eq`, `Ord`, `Generic`, etc.) and the corresponding language extension is enabled (if necessary), do so. If the language extension is not enabled, throw an error. (b) Otherwise, if `-XDeriveAnyClass` is enabled, use that. (c) Otherwise, throw an error. This is quite ugly, but to my knowledge this is currently what GHC does (excluding 2(c), which is a bit that fixes a bug reported in Trac #10598 [2]), so there shouldn't be any change in behavior from before. This is another good reason to want deriving strategies: trying to remember all of that is nearly impossible! Ryan S. ----- [1] https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DerivingStrategies... [2] https://ghc.haskell.org/trac/ghc/ticket/10598 On Wed, Aug 3, 2016 at 11:53 AM, Richard Eisenberg wrote:

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On Aug 2, 2016, at 6:59 PM, Ryan Scott wrote: Any other questions or comments?

Yes, a question: What is the behavior if no keyword is given? Any change here from the status quo is quite dangerous from a backward-compatibility standpoint, but I wonder if we could start issuing warnings/errors in a few years.

Richard