[GHC] #9564: Floating point subnormals overrounded on output

[GHC]

#9564: Floating point subnormals overrounded on output
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              Reporter:  jrp         |            Owner:  simonmar
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Runtime     |          Version:  7.8.3
  System                             |         Keywords:
            Resolution:              |     Architecture:  x86_64 (amd64)
      Operating System:  MacOS X     |       Difficulty:  Unknown
       Type of failure:  Incorrect   |       Blocked By:
  result at runtime                  |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by rwbarton):

 These extra digits past the decimal point suggest a degree of precision
 that doesn't exist in a Float: for instance 1.4013e-45 is the same Float
 as 1.4014e-45. In fact all the numbers 1.0e-45 through 2.0e-45 (and a bit
 farther in each direction) round to the same Float, so why not output the
 simplest one, 1.0e-45? This is the same logic by which we display the
 Float 1/10 as 0.1 and not, say, 0.10000000149 even though the latter is
 closer to the true value.

 printf's %g is not a format you can round-trip through: by default it
 always prints 6 digits (in this case the last digit would be 0 and is
 dropped). So it's not a good analogue of `show`. (Also, %g is printing a
 double, not a float, but it has the same behavior with 2^-1074^, the
 smallest positive double.)

 Python has similar behavior to GHC: it doesn't have single-precision
 floating point AFAIK, but for doubles, it displays 2^-1074^ as 5e-324,
 just like GHC displays 5.0e-324. Arguably we could drop the ".0" part, but
 I don't think that's worth changing.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9564#comment:5>
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