#4383: Uncanonical display of Double
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Reporter: daniel.is.fischer | Owner:
Type: bug | Status: patch
Priority: normal | Component: libraries/base
Version: 6.12.3 | Keywords: Double, show
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
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Changes (by daniel.is.fischer):
* status: new => patch
Comment:
1. fix4383.dpatch fixes the issue, we no longer overestimate the base 10
logarithm
2. There was a typo (b instead of be) in the distance to the next smaller
FP number for powers of floatRadix. I thought it ought to be `b^e` and
verified it in the paper. This makes numrun012 fail because now `2 ^ 31 ::
Float` shows as "2.1474836e9" instead of "2.1474837e9" (and some other
show values change too, e.g. `2.0 ^ 852` was "3.0030067315218801e256" is
now "3.00300673152188e256").
3. Performance enhancement by using quot/quotRem instead of div/divMod
and using the cache of powers of 2 instead of recomputing.
4. Further performance enhancement by adding a cache for powers of 10.
5. All of the above in one patch.
1. and 2. should be applied since they correct errors.[[BR]]
The performance enhancement isn't too bad, I think a few more percent
could be squeezed out, but it doesn't solve the fundamental problem of the
slowness of show for !RealFloat values (cf.
http://www.haskell.org/pipermail/haskell-cafe/2009-June/062904.html). I
plan to look into it for 7.2, not now.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4383#comment:3>
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