[GHC] #12665: Make Read instances faster, and make them fail fast

[GHC]

#12665: Make Read instances faster, and make them fail fast
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        Reporter:  dfeuer            |                Owner:  dfeuer
            Type:  feature request   |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Core Libraries    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Let me take that back; I think my modified implementation actually does
 what Haskell98 says, whereas the current implementation does something
 else. I don't know for sure what should be considered the correct
 behavior.

 P.S., to match the current behavior, we'd need to accept `"122e-"` as an
 integer, but not `"122e-3"`, because the current implementation lexes
 `"122e-3"` as a fractional number and thus not an integer, whereas it
 lexes `"122e-"` as an integer followed by an identifier followed by a
 symbol.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12665#comment:6>
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