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copied!<p>Simon Marlow mentions another approach in <a href="http://hackage.haskell.org/trac/ghc/ticket/2209">GHC bug 2209</a> (also linked to from Bryan O'Sullivan's answer)</p> <blockquote> <p>You can achieve the desired effect using castSTUArray, incidentally (this is the way we do it in GHC).</p> </blockquote> <p>I've used this option in some of my libraries in order to avoid the <code>unsafePerformIO</code> required for the FFI marshalling method.</p> <pre><code>{-# LANGUAGE FlexibleContexts #-} import Data.Word (Word32, Word64) import Data.Array.ST (newArray, castSTUArray, readArray, MArray, STUArray) import GHC.ST (runST, ST) wordToFloat :: Word32 -> Float wordToFloat x = runST (cast x) floatToWord :: Float -> Word32 floatToWord x = runST (cast x) wordToDouble :: Word64 -> Double wordToDouble x = runST (cast x) doubleToWord :: Double -> Word64 doubleToWord x = runST (cast x) {-# INLINE cast #-} cast :: (MArray (STUArray s) a (ST s), MArray (STUArray s) b (ST s)) => a -> ST s b cast x = newArray (0 :: Int, 0) x >>= castSTUArray >>= flip readArray 0 </code></pre> <p>I inlined the cast function because doing so causes GHC to generate much tighter core. After inlining, <code>wordToFloat</code> is translated to a call to <a href="http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-ST.html#runSTRep">runSTRep</a> and three <a href="http://www.haskell.org/ghc/docs/7.0.4/html/libraries/ghc-prim-0.2.0.0/GHC-Prim.html">primops</a> (<code>newByteArray#</code>, <code>writeWord32Array#</code>, <code>readFloatArray#</code>).</p> <p>I'm not sure what performance is like compared to the FFI marshalling method, but just for fun I compared the core <a href="https://gist.github.com/1135110">generated by both options</a>.</p> <p>Doing FFI marshalling is a fair bit more complicated in this regard. It calls <a href="http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-IO.html#unsafeDupablePerformIO">unsafeDupablePerformIO</a> and 7 primops (<code>noDuplicate#</code>, <code>newAlignedPinnedByteArray#</code>, <code>unsafeFreezeByteArray#</code>, <code>byteArrayContents#</code>, <code>writeWord32OffAddr#</code>, <code>readFloatOffAddr#</code>, <code>touch#</code>).</p> <p>I've only just started learning how to analyse core, perhaps someone with more experience can comment on the cost of these operations?</p>

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