Re: [Haskell-cafe] Coercion from a Word32/64 to a Float/Double

2009/9/14 Judah Jacobson :

On Mon, Sep 14, 2009 at 12:24 PM, minh thu wrote:

...

Hi,

I'd like to know if this should work:

-- GHC coercion getFloat :: BHeader -> Get Float getFloat h = case endianness h of LittleEndian -> fmap (coerce . fromIntegral) getWord32le BigEndian -> fmap (coerce . fromIntegral) getWord32be where coerce (I32# x) = F# (unsafeCoerce# x)

-- GHC coercion getDouble :: BHeader -> Get Double getDouble h = case endianness h of LittleEndian -> fmap (coerce . fromIntegral) getWord64le BigEndian -> fmap (coerce . fromIntegral) getWord64be where coerce (I64# x) = D# (unsafeCoerce# x)

Loading this into ghci compiles fine (but I haven't try to parse data), but compiling with ghc (6.10.1 or 6.10.4) reports: /tmp/ghc1967_0/ghc1967_0.s: Assembler messages:

/tmp/ghc1967_0/ghc1967_0.s:287:0: Error: bad register name `%fake0'

/tmp/ghc1967_0/ghc1967_0.s:349:0: Error: bad register name `%fake0'

I think that a more reliable (though not very pretty) way to do the casts is via pointer manipulation. Code like the following has worked for me in the past:

getDouble :: Get Double getDouble = fmap word2Double getWord64le where word2Double w = unsafePerformIO $ with w $ \p -> do d :: CDouble <- peek (castPr p) return (realToFrac d :: Double)

Best, -Judah

Thank you, it compiles fine. Thu