[GHC] #9206: OverloadedStrings breaks show?

[GHC]

#9206: OverloadedStrings breaks show?
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        Reporter:  j80JjBjVNRMajmA   |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      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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Comment (by simonpj):

 This behaviour is mostly documented in the
 [http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-
 extensions.html#overloaded-strings overloaded strings section of the user
 manual], although there is an important omission: with
 `-XOverloadedStrings`, the default types (if none are specified by the
 user) are `Integer, Double, String`, not just `Integer, Double`.

 Reminder: `fromString :: IsString a => String -> a`, exported by
 `GHC.Exts`.

 So

  * Without `-XOverloadedStrings`, `test.hs` is rightly rejected.

  * With `-XOverloadedStrings`, defaulting kicks into play and chooses the
 first type from `Integer, Double, String` that satisfies `(Show a0,
 IsString a0)`.  The only satisfying type is `String` so that is chosen.

  * In `test2.hs`, `Double` is made an instance of `IsString`, so that is
 picked first, before `String`.

 So it all looks right except that the user manual is deficient. I'll fix
 that.

 Of course, you could argue that the specification is wrong.  If you want
 to do that, we should go back to the original discussion threads on
 overloaded strings.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9206#comment:2>
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