# Where to find/put HTTP header utility functions

**URL:** <https://discourse.haskell.org/t/where-to-find-put-http-header-utility-functions/7671>\
**Category:** Uncategorized\
**Created:** [September 24, 2023, 1:52pm UTC](https://discourse.haskell.org/t/where-to-find-put-http-header-utility-functions/7671 "2023-09-24T13:52:32Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Vlix](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/vlix/32/1551_2.png) [@Vlix](https://discourse.haskell.org/u/Vlix)\
**Post date:** [September 24, 2023, 1:52pm UTC](https://discourse.haskell.org/t/where-to-find-put-http-header-utility-functions/7671/1 "2023-09-24T13:52:32Z")

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I’ve been working on `warp / wai-extra` internals again and I find I’m writing specific HTTP header parsing functions that feel like they should be in a more generic library for everyone to use. Here’s some examples:

> **Parsing ETags**
>
> ```haskell
> import Data.ByteString as BS
> import Data.Word8 (_quotedbl, _W)
> 
> data ETags = AnyResource | ETags [ETag]
> 
> -- ETag without the double quotes.
> -- @"\"e-tag\""@ -> StrongTag "e-tag"
> -- @"W/\"e-tag\""@ -> WeakTag "e-tag"
> data ETag = StrongTag ByteString | WeakTag ByteString
> 
> getTag :: ETag -> ByteString
> getTag (StrongTag bs) = bs
> getTag (WeakTag bs) = bs
> 
> weakEQ :: ETag -> ETag -> Bool
> weakEQ tag1 tag2 = getTag tag1 == getTag tag2
> 
> strongEQ :: ETag -> ETag -> Bool
> strongEQ (StrongTag tag1) (StrongTag tag2) = tag1 == tag2
> strongEQ _ _ = False
> 
> parseETags :: HeaderValue -> Maybe ETags
> parseETags "*" = Just AnyResource
> parseETags val = ETags <$> traverse parseETag tags
> where
> -- 'splitCommas' is also a generic function to split header values
> -- on commas while trimming the allowed whitespaces
> tags = splitCommas val
> 
> parseETag :: ByteString -> Maybe ETag
> parseETag bs =
> case BS.uncons bs of
> Just (w8, rest)
> -- Regular opaque tag in double quotes
> | w8 == _quotedbl -> StrongTag <$> endIsQuote rest
> -- Weak tag in double quotes prefixed with literal "W/"
> | w8 == _W -> case BS.splitAt 2 rest of
> (start, rest')
> | start == "/\"" -> WeakTag <$> endIsQuote rest'
> | otherwise -> Nothing
> _ -> Nothing
> where
> endIsQuote x =
> BS.unsnoc x >>= \(rest, w8) -> rest <$ guard (w8 == _quotedbl)
> 
> ```

> **Parsing Q-values**
>
> ```haskell
> import Data.ByteString as BS
> import Data.ByteString.Char8 as BS8
> import Data.Word8 (_0, _1, _period, _semicolon, _space)
> 
> -- | Only to be used on header's values which support quality value syntax
> --
> -- A few things to keep in mind when using this function:
> -- * The resulting 'Int' will be anywhere from 1000 to 0 ("1" = 1000, "0.6" = 600, "0.025" = 25)
> -- * The absence of a Q value will result in 'Just 1000'
> -- * A bad parse of the Q value will result in a 'Nothing', e.g.
> -- * Q value has more than 3 digits behind the dot
> -- * Q value is missing
> -- * Q value is higher than 1
> -- * Q value is not a number
> parseQValueList :: BS.ByteString -> [(BS.ByteString, Maybe Int)]
> parseQValueList = fmap go . splitCommas
> where
> go = checkQ . S.break (== _semicolon)
> checkQ :: (BS.ByteString, BS.ByteString) -> (BS.ByteString, Maybe Int)
> checkQ (val, "") = (val, Just 1000)
> checkQ (val, bs) =
> -- RFC 7231 says optional whitespace can be around the semicolon.
> -- So drop any before it , . and any behind it $ and drop the semicolon
> (dropWhileEnd (== _space) val, parseQval . BS.dropWhile (== _space) $ BS.drop 1 bs)
> where
> parseQval qVal = do
> q <- BS.stripPrefix "q=" qVal
> (i, rest) <- BS.uncons q
> guard $
> i `elem` [_0, _1]
> && BS.length rest <= 4
> case BS.uncons rest of
> Nothing
> -- q = "0" or "1"
> | i == _0 -> Just 0
> | i == _1 -> Just 1000
> | otherwise -> Nothing
> Just (dot, trail)
> | dot == _period && not (i == _1 && BS.any (/= _0) trail) -> do
> let len = BS.length trail
> extraZeroes = replicate (3 - len) '0'
> guard $ len > 0
> readMaybe $ w2c i : BS8.unpack trail ++ extraZeroes
> | otherwise -> Nothing
> 
> ```

Now I have looked around a bit, but haven’t found a library/package that concerns themselves with these types of functions, so if you know of one, I’d love to hear about it. But in the case where this just isn’t found anywhere yet, here is my question for the general public:

## Where should these types of functions go?

- Into the `http-types` package? Since it already has utility functions for byte ranges.
- Into a new package called `http-header` (or the like)
- Just keep them in the internals of `wai / warp`
- (Some other option?)

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**Author:** ![Kleidukos](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/kleidukos/32/1213_2.png) [@Kleidukos](https://discourse.haskell.org/u/Kleidukos)\
**Post date:** [September 24, 2023, 10:02pm UTC](https://discourse.haskell.org/t/where-to-find-put-http-header-utility-functions/7671/2 "2023-09-24T22:02:21Z")

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Yes please centralise into http-types or expose them in Warp 🙂

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**Author:** ![Vlix](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/vlix/32/1551_2.png) [@Vlix](https://discourse.haskell.org/u/Vlix)\
**Post date:** [September 25, 2023, 8:28am UTC](https://discourse.haskell.org/t/where-to-find-put-http-header-utility-functions/7671/3 "2023-09-25T08:28:19Z")

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I checked out the repo for `http-types` and it seems to not be actively maintained, so I’ve e-mailed the owner to ask for full or co-maintainership.

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**Author:** ![Vlix](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/vlix/32/1551_2.png) [@Vlix](https://discourse.haskell.org/u/Vlix)\
**Post date:** [October 16, 2023, 3:29pm UTC](https://discourse.haskell.org/t/where-to-find-put-http-header-utility-functions/7671/4 "2023-10-16T15:29:32Z")

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It’s been 3 weeks now, and I haven’t heard anything from the maintainer of `http-types`.

I tried via the email address that is registered on hackage and tried via LinkedIn. I’ve now put an issue in the Github, mostly for form since the Github repo has been inactive for a long while now.

I’ll wait for another 3 weeks until I’m back home and will then proceed with step 4 of [Taking over a package - HaskellWiki](https://wiki.haskell.org/Taking_over_a_package)

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<div class="post-metadata">

**Author:** ![Vlix](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/vlix/32/1551_2.png) [@Vlix](https://discourse.haskell.org/u/Vlix)\
**Post date:** [November 10, 2023, 11:25pm UTC](https://discourse.haskell.org/t/where-to-find-put-http-header-utility-functions/7671/5 "2023-11-10T23:25:39Z")

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I’ve been given maintainership of `http-types`.

I’m planning to first revisit the documentation and make it more obvious what everything does (add some examples too where it’d be helpful) and after that check the tests (and add to them if necessary) and maybe add a benchmarking component to test any changes made to the code.