#3676: realToFrac doesn't sanely convert between floating types
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Reporter: draconx | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: libraries (other) | Version: 6.10.4
Resolution: | Keywords:
Difficulty: | Os: Unknown/Multiple
Testcase: | Architecture: x86_64 (amd64)
Failure: None/Unknown |
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Changes (by igloo):
* difficulty: =>
Old description:
> As far as I can tell, the only way to convert between floating types in
> Haskell is to use realToFrac. Unfortunately, this function does some
> insane mangling of values when converting. Some examples:
>
> realToFrac (0/0 :: Double) :: Double
> --> -Infinity
> realToFrac (-1/0 :: Float) :: Double
> --> -3.402823669209385e38
> realToFrac (-0 :: Double) :: CDouble
> --> 0.0
>
> The last item illustrates an important point: it is impossible to
> convert a value from Double to CDouble without potentially changing it.
> This makes it difficult or impossible to use the FFI to call any
> functions with floating point parameters/return values.
>
> Using a combination of unsafeCoerce (to shoehorn Double values in/out of
> CDoubles) and some functions written in C (to perform float<=>double), I
> was able to work around these problems, but that hardly seems like a nice
> solution.
New description:
As far as I can tell, the only way to convert between floating types in
Haskell is to use realToFrac. Unfortunately, this function does some
insane mangling of values when converting. Some examples:
{{{
realToFrac (0/0 :: Double) :: Double
--> -Infinity
realToFrac (-1/0 :: Float) :: Double
--> -3.402823669209385e38
realToFrac (-0 :: Double) :: CDouble
--> 0.0
}}}
The last item illustrates an important point: it is impossible to convert
a value from Double to CDouble without potentially changing it. This
makes it difficult or impossible to use the FFI to call any functions with
floating point parameters/return values.
Using a combination of unsafeCoerce (to shoehorn Double values in/out of
CDoubles) and some functions written in C (to perform float<=>double), I
was able to work around these problems, but that hardly seems like a nice
solution.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3676#comment:1>
GHC <http://www.haskell.org/ghc/>
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