Re: [Haskell-cafe] Bytestring map/zipWith rationale

On Thu, Sep 12, 2013 at 12:44 PM, Nicolas Trangez wrote:

I did use that a couple of times (`xor`ing 2 ByteStrings together), and was surprised by the omission back then, but IIRC (can't validate now), there's a specialised zipWith (as proposed) in the module (with some other name, obviously), which is not exported, but used when you 'pack' the result of 'zipWith' when the result is '[Word8]'... You might want to look into that.

This is correct - there is a RULES pragma that rewrites `pack (zipWith f)` into a more efficient `zipWith'` function of the type desired (see the bytestring package). I was very concerned about this when writing lots of bytestring xor code for crypto-api and was pleased to find that, if the syntactic form matches, things get optimized as you desire. Thomas

Nicolas

On Sep 12, 2013 8:11 PM, "John Lato" wrote:

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Carter: we don't have both. We have one function from each category. My guess is nobody's ever really needed a really fast zipWith :: (Word8->Word8->Word8) -> ByteString -> ByteString -> ByteString; that's the only reason I can think of for its omission.

On Thu, Sep 12, 2013 at 10:45 AM, Carter Schonwald wrote:

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Scott: benchmark the two and you'll see why we have both :-)

On Thursday, September 12, 2013, Scott Lawrence wrote:

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On Thu, 12 Sep 2013, Tom Ellis wrote:

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On Thu, Sep 12, 2013 at 09:21:20AM -0400, Scott Lawrence wrote:

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Something's always bothered me about map and zipWith for ByteString. Why is it

map :: (Word8 -> Word8) -> ByteString -> ByteString

but

zipWith :: (Word8 -> Word8 -> a) -> ByteString -> ByteString -> [a]

Well, what if you wanted to zipWith a function of type "Word8 -> Word8 -> Foo" instead of "Word8 -> Word8 -> Word8"?

Then I would do what I do with map, and call `unpack` first.

Either of the two options is usable:

map :: (Word8 -> Word8) -> ByteString -> ByteString zipWith :: (Word8 -> Word8 -> Word8) -> ByteString -> ByteString -> ByteString (or) map :: (Word8 -> a) -> ByteString -> [a] zipWith :: (Word8 -> Word8 -> a) -> ByteString -> ByteString -> [a]

I just don't understand why we have one from each.

-- Scott Lawrence _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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