[GHC] #5462: Deriving clause for arbitrary classes

[GHC]

#5462: Deriving clause for arbitrary classes
---------------------------------+------------------------------------------
    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 simonpj):

 Standalone deriving is, in effect, ''already'' allowed for any class.  You
 just write
 {{{
 instance C a => C (T a)  -- No 'where' clause
 }}}
 (not `deriving instance`, just plain `instance`).  Like any instance decl,
 this will fill in with the default methods; and if the default methods use
 Pedro's new generic-programming infrastructure, the default methods will
 do the right thing.

 The question for this ticket is: should be able to generate this (where-
 clause-less) instance decl by saying `data T a = ... deriving( Generic, C
 )`.

 You ask "Why would I want to write a class and all the supporting code to
 allow it to be derived but not add the DERIVABLE pragma?"  By "all the
 supporting code" I assume you mean default methods a la Pedro (see
 http://www.haskell.org/ghc/dist/current/docs/html/users_guide/type-class-
 extensions.html#class-default-signatures).  Well, you wouldn't.  But what
 counts as "you have written all the supporting code"?

 Perhaps your proposal is this? A class can appear in a `deriving` clause
 if (and only if)
    * the class has at least one `default foo :: type` signature, and
    * the class has a default method definition for every method

 Thus
 {{{
 class C1 a where  -- NO (no default method)
   op1 :: a -> a

 class C2 a where  -- NO (non default method signature)
   op2 :: a -> a
   op2 x = x

 class C3 a where  -- YES (both are present)
   op3 :: a -> a
   default op3 :: Ord a => a -> a
   op3 x = x>x
 }}}
 But I'm not sure whether that is what you meant.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5462#comment:9>
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