[GHC] #9548: Numerical type system problem

[GHC]

#9548: Numerical type system problem
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              Reporter:  vxanica     |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.3
             Component:              |         Keywords:  Number
  libraries/base                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by ekmett):

 All numbers do not have a decidable notion of equality.

 {{{
 instance Num b => Num (a -> b)
 }}}

 is the most common example, but computable reals and purely symbolic
 numeric types are other counter examples.

 GHC 7.4 deliberately made a breaking change away from the Haskell 98
 standard to avoid forcing users to lie about equalities that couldn't
 exist. Going back on that decision would be a mistake.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9548#comment:1>
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