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# Derived Instances
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## Minor adjustments
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Haskell 98 allows derived instances of `Bounded` only for enumerations and single constructor types.
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Proposal: allow derived instances of `Bounded` for all algebraic data types, with
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```wiki
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minBound = K1 minBound ... minBound
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maxBound = Kn maxBound ... maxBound
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```
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## Automatic derivation
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Automatic derivation of instances is jolly useful. Current problems are:
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- Automatic derivation is only available for certain classes defined in the Prelude and standard libraries.
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- There is no way to specify how to derive a user-defined class in the general case.
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- There is no way to derive an instance of a class for a data type that is defined elsewhere (in another module).
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- There is no easy way to refer to the definitions that would have been derived when defining an explicit instance.
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- Deriving fails for some Haskell-98 data types. Try
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```wiki
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data Rose f a = Fork a (f (Rose f a)) deriving Show
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```
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as an example.
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- Even in the absence of user-defined deriving, it should be easy for a `newtype` declaration to inherit some of the instances of its parent type, no matter where the class was defined, but this is currently only available as a GHC extension to Haskell 98 (see [NewtypeDeriving](newtype-deriving)).
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## User-specified derived instances
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There are various extant proposals to enable the user-specification of the derivation of class instances:
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- PolyP ([ Jeuring and Jansson](http://www.cs.uu.nl/~johanj/publications/notes.ps), AFP 1996)
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- ghc -fgenerics (Hinze, [ PoPL 2000](http://www.informatik.uni-bonn.de/~ralf/publications/POPL00.ps.gz), [ Haskell Workshop 1999](http://www.informatik.uni-bonn.de/~ralf/publications/HW99.ps.gz), [ Haskell Workshop 2000](http://www.informatik.uni-bonn.de/~ralf/publications/HW00.ps.gz))
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- Lightweight Generics ([ Cheney and Hinze](http://www.informatik.uni-bonn.de/~ralf/publications/HW2002.pdf), Haskell Workshop 2002)
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- Scrap your Boilerplate ([ Lämmel and Peyton Jones](http://www.cs.vu.nl/boilerplate/), TLDI 2003 etc)
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- Generics for the Masses ([ Hinze](http://www.informatik.uni-bonn.de/~ralf/publications/ICFP04.pdf), ICFP 2004)
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- Type-indexed datatypes ([ Hinze, Jeuring, Löh](http://www.cs.uu.nl/~johanj/publications/tidata.pdf), SoCP 2004)
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- TemplateHaskell? is capable of this amongst other things
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- external tools: [ DrIFT](http://repetae.net/~john/computer/haskell/DrIFT/), [ Strafunski](http://www.cs.vu.nl/Strafunski/)
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- [ Generic Haskell](http://www.generic-haskell.org/) preprocessor
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- fill out this list with some more…?
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## Standalone derived instances
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- Even in the absence of user-defined deriving, it should be easy for a 'newtype' decl to inherit some of the instances of its parent type, no matter where the class was defined, but this is currently only available as a ghc extension to haskell'98.
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The development version of GHC adds a top-level declaration on the form:
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```wiki
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'deriving' qtycls 'for' qtycon
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```
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There are various extant proposals to enable the user-specification of the derivation of class instances.
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which produces the same instance as would be produced by a `deriving` clause in a `data` or `newtype` declaration, but allows this derivation to be decoupled from the datatype declaration, e.g. in another module.
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See [ Stand-alone deriving declarations](http://haskell.org/haskellwiki/GHC/StandAloneDeriving) on the GHC Wiki. |
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- fill out a list |
