[GHC] #3205: Generalized isomorphic deriving

[GHC]

#3205: Generalized isomorphic deriving
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Reporter:  ajd               |          Owner:                  
    Type:  feature request   |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  6.10.2            |       Severity:  normal          
Keywords:                    |       Testcase:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
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 A newtype is isomorphic to the type that it is based on. For example,
 newtype A = A Int is isomorphic to Int, so it is possible to automatically
 derive all of Int's instance for A.

 It would be nice if this could be extended to other datatypes that were
 isomorphic to each other. For example, this would work

 {{{
 data K a = K a a a

 instance C (K a) where
   ...

 data L a = L a a a
   deriving (Eq,C<-K,Show)
 }}}

 where the {{{C<-K}}} notation means "derive an instance of C for this type
 that is 'the same as' the instance of C for K". The compiler would have to
 check that K and L were actually isomorphic.

 I don't know if this is possible with the current internals of GHC, but it
 would be cool if it would be done. My main usage scenario would be
 deriving things for single-argument data constructors based on tuples; for
 example for serialization classes.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3205>
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