Re: [GHC] #12047: Users Guide: GeneralizedNewtypeDeriving derives “instance Num Int => Num Dollars”

[GHC]

#12047: Users Guide: GeneralizedNewtypeDeriving derives “instance Num Int => Num
Dollars”
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        Reporter:  Iceland_jack      |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Documentation     |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Documentation     |  Unknown/Multiple
  bug                                |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * component:  Compiler => Documentation


Comment:

 That section isn't saying that `newtype Dollars = Dollars Int deriving
 Num` literally derives an instance of the form `instance Num Int => Num
 Dollars`, but rather it generates an `instance Num Dollars` by starting
 with `Num Int` as the context, then simplifying as much as possible. In
 that example, since a `Num Int` instance exists, we can discharge that
 constraint and be left with `()`, giving us the `instance Num Dollars`
 that you actually get when you compile with `-ddump-deriv`.

 I agree that the wording in that section is a bit vague (it uses the
 phrase "Notationally" without going into much detail about what it really
 entails) and could stand to be clarified.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12047#comment:3>
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