[GHC] #9948: Recommend class constraint instead of instance constraint

[GHC]

#9948: Recommend class constraint instead of instance constraint
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        Reporter:  crockeea          |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by crockeea):

 Replying to [comment:7 rwbarton]:
 > I can see why you might want this behavior in some cases, but would you
 really want
 >
 > {{{
 > isPalindrome :: [a] -> Bool
 > isPalindrome xs = xs == reverse xs
 > }}}
 >
 > to suggest adding an `Eq [a]` constraint (that's not even valid Haskell
 98!), or some function to dump a data structure to suggest adding a `Show
 [(Int, Maybe a)]` constraint?

 I see your point on these examples. How's the feature request for GHC
 mind-reading coming along?

 > I'm also a bit confused by the workaround that you mentioned; if you
 know that `Foo` is the class containing `foo` (which you must to comment
 out its instances) then don't you already know what constraint to add? Is
 it very difficult in your use cases to work out what the parameter to the
 class should be?

 Yes, it should be pretty obvious which constraint I need. The only minor
 point I could make is that it isn't always immediately clear if the
 function you are using is top-level or a method. The only way to resolve
 that is to look at some documentation.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9948#comment:8>
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