[GHC] #13097: Num a => Num (Down a)

[GHC]

#13097: Num a => Num (Down a)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

@@ -1,1 +1,2 @@
- Let's us write
+ There are many missing instances for `Down`, I happened to need `Num (Down
+ _)`. It can be derived with GND:
@@ -4,1 +5,2 @@
- >>> deriving instance Num a => Num (Down a)
+ newtype Down a = Down a deriving (Eq, Show, Read, Num)
+ }}}
@@ -6,0 +8,3 @@
+ which let's us write
+
+ {{{#!hs
@@ -12,0 +17,6 @@
+
+ We can of course add instances for `Functor`, `Applicative`, `Monad`,
+ `MonadFix`, ... I don't know that they would ever get but it wouldn't be
+ much trouble copying `Identity`, `Dual` instances.
+
+ ----

New description:

 There are many missing instances for `Down`, I happened to need `Num (Down
 _)`. It can be derived with GND:

 {{{#!hs
 newtype Down a = Down a deriving (Eq, Show, Read, Num)
 }}}

 which let's us write

 {{{#!hs
 >>> (<) 10 20
 True

 >>> (<) @(Down _) 10 20
 False
 }}}

 We can of course add instances for `Functor`, `Applicative`, `Monad`,
 `MonadFix`, ... I don't know that they would ever get but it wouldn't be
 much trouble copying `Identity`, `Dual` instances.

 ----

 With #12363


 {{{#!hs
 >>> 10 < @(Down _) 20
 False
 }}}

--

Comment (by Iceland_jack):

 Replying to [comment:1 mpickering]:
 > Is this a feature request to add a `Num` instance for `Down`? It would
 be very helpful if you explained this on the ticket!

 Yes, along with other instances — changed description.

 What is the stance of ''base'', do we provide every instance that makes
 sense or limit it to instances we know have a use?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13097#comment:2>
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