[GHC] #5462: Deriving clause for arbitrary classes

[GHC]

#5462: Deriving clause for arbitrary classes
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    Reporter:  simonpj           |        Owner:              
        Type:  feature request   |       Status:  new         
    Priority:  normal            |    Milestone:              
   Component:  Compiler          |      Version:  7.2.1       
    Keywords:                    |     Testcase:              
   Blockedby:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Blocking:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
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Comment(by dreixel):

 One extra issue to consider is that the instance head is not trivial to
 generate. Say we have an arbitrary deriving clause:

 {{{ data MyDatatype a = C1 | C2 a (MyDatatype a) deriving MyClass }}}

 We want to convert it into an empty instance:

 {{{ instance (...) => MyClass (MyDatatype a) }}}

 But how do we fill in the constraints? This depends on both `MyClass` and
 `MyDatatype`, and in a non-trivial way, as far as I can see. E.g., if
 `MyClass` implements a `show` function, we probably want the `MyClass a`
 constraint. But if `a` was actually a phantom type, then we don't. Nor if
 `MyClass` computes shallow equality, for instance. Things get even worse
 when `MyDatatype` is not a regular type (think of nested datatypes, or
 types with higher-order arguments).

 This problem does not occur when standalone deriving is used, because then
 the user specifies the constraints directly. But I guess that limiting
 this new feature to standalone deriving would make it a bit inconsistent,
 and also defeat the purpose of extra compactness/simplicity.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5462#comment:8>
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