[GHC] #8171: Extending ExtendedDefaultRules

[GHC]

#8171: Extending ExtendedDefaultRules
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        Reporter:  ekmett            |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Description changed by simonpj:

Old description:

> The current `ExtendedDefaultRules` is set up in a way that largely
> prevents it from being used for scenarios beyond what they get used for
> in GHCi today.
>
> It would be great if there was an extra class that could be included in
> the list of acceptable classes for Rule 3.
>
> {{{
> class Defaulting a
> }}}
>
> The proposed change would be that Rule 3 under EDR can be be relaxed to:
> At least one of the classes Ci is numeric, or is `Show`, `Eq`, `Ord` or
> `Defaulting`.
>
> I went with `Defaulting` rather than `Default`, because `Default` is a
> common class in use across the ecosystem from the `data-default` package,
> and I wanted to avoid gratuitous fallout.
>
> This would permit projects like Shae Erisson's `ghclive` from last year
> that need to use defaulting to not have to shoe-horn unnecessary `Show`
> constraints onto functions.
>
> Even better would be for that class to be polykinded!
>
> That opens up possibilities for expert users to get defaulting to fire on
> arguments of other kinds, which is impossible under the current EDR.

New description:

 The current `ExtendedDefaultRules` (see
 [http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-
 evaluation.html#extended-default-rules manual page]) is set up in a way
 that largely prevents it from being used for scenarios beyond what they
 get used for in GHCi today.

 It would be great if there was an extra class that could be included in
 the list of acceptable classes for Rule 3.

 {{{
 class Defaulting a
 }}}

 The proposed change would be that Rule 3 under EDR can be be relaxed to:
 At least one of the classes Ci is numeric, or is `Show`, `Eq`, `Ord` or
 `Defaulting`.

 I went with `Defaulting` rather than `Default`, because `Default` is a
 common class in use across the ecosystem from the `data-default` package,
 and I wanted to avoid gratuitous fallout.

 This would permit projects like Shae Erisson's `ghclive` from last year
 that need to use defaulting to not have to shoe-horn unnecessary `Show`
 constraints onto functions.

 Even better would be for that class to be polykinded!

 That opens up possibilities for expert users to get defaulting to fire on
 arguments of other kinds, which is impossible under the current EDR.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8171#comment:2>
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