Network/HTTP/Headers.hs

-----------------------------------------------------------------------------
-- |
-- Module : Network.HTTP.Headers
-- Copyright : See LICENSE file
-- License : BSD
-- 
-- Maintainer : Ganesh Sittampalam <ganesh@earth.li>
-- Stability : experimental
-- Portability : non-portable (not tested)
--
-- This module provides the data types for representing HTTP headers, and
-- operations for looking up header values and working with sequences of
-- header values in 'Request's and 'Response's. To avoid having to provide
-- separate set of operations for doing so, we introduce a type class 'HasHeaders'
-- to facilitate writing such processing using overloading instead.
-- 
-----------------------------------------------------------------------------
module Network.HTTP.Headers
 ( HasHeaders(..) -- type class

 , Header(..)
 , mkHeader -- :: HeaderName -> String -> Header
 , hdrName -- :: Header -> HeaderName
 , hdrValue -- :: Header -> String

 , HeaderName(..)

 , insertHeader -- :: HasHeaders a => HeaderName -> String -> a -> a
 , insertHeaderIfMissing -- :: HasHeaders a => HeaderName -> String -> a -> a
 , insertHeaders -- :: HasHeaders a => [Header] -> a -> a
 , retrieveHeaders -- :: HasHeaders a => HeaderName -> a -> [Header]
 , replaceHeader -- :: HasHeaders a => HeaderName -> String -> a -> a
 , findHeader -- :: HasHeaders a => HeaderName -> a -> Maybe String
 , lookupHeader -- :: HeaderName -> [Header] -> Maybe String

 , parseHeader -- :: parseHeader :: String -> Result Header
 , parseHeaders -- :: [String] -> Result [Header]
 
 , headerMap -- :: [(String, HeaderName)]
 
 , HeaderSetter
 ) where

import Data.Char (toLower)
import Network.Stream (Result, failParse)
import Network.HTTP.Utils ( trim, split, crlf )

-- | The @Header@ data type pairs header names & values.
data Header = Header HeaderName String

hdrName :: Header -> HeaderName
hdrName (Header h _) = h

hdrValue :: Header -> String
hdrValue (Header _ v) = v

-- | Header constructor as a function, hiding above rep.
mkHeader :: HeaderName -> String -> Header
mkHeader = Header

instance Show Header where
 show (Header key value) = shows key (':':' ':value ++ crlf)

-- | HTTP @HeaderName@ type, a Haskell data constructor for each
-- specification-defined header, prefixed with @Hdr@ and CamelCased,
-- (i.e., eliding the @-@ in the process.) Should you require using
-- a custom header, there's the @HdrCustom@ constructor which takes
-- a @String@ argument.
--
-- Encoding HTTP header names differently, as Strings perhaps, is an
-- equally fine choice..no decidedly clear winner, but let's stick
-- with data constructors here.
-- 
data HeaderName 
 -- Generic Headers --
 = HdrCacheControl
 | HdrConnection
 | HdrDate
 | HdrPragma
 | HdrTransferEncoding 
 | HdrUpgrade 
 | HdrVia
 -- Request Headers --
 | HdrAccept
 | HdrAcceptCharset
 | HdrAcceptEncoding
 | HdrAcceptLanguage
 | HdrAuthorization
 | HdrCookie
 | HdrExpect
 | HdrFrom
 | HdrHost
 | HdrIfModifiedSince
 | HdrIfMatch
 | HdrIfNoneMatch
 | HdrIfRange
 | HdrIfUnmodifiedSince
 | HdrMaxForwards
 | HdrProxyAuthorization
 | HdrRange
 | HdrReferer
 | HdrUserAgent
 -- Response Headers
 | HdrAge
 | HdrLocation
 | HdrProxyAuthenticate
 | HdrPublic
 | HdrRetryAfter
 | HdrServer
 | HdrSetCookie
 | HdrTE
 | HdrTrailer
 | HdrVary
 | HdrWarning
 | HdrWWWAuthenticate
 -- Entity Headers
 | HdrAllow
 | HdrContentBase
 | HdrContentEncoding
 | HdrContentLanguage
 | HdrContentLength
 | HdrContentLocation
 | HdrContentMD5
 | HdrContentRange
 | HdrContentType
 | HdrETag
 | HdrExpires
 | HdrLastModified
 -- | MIME entity headers (for sub-parts)
 | HdrContentTransferEncoding
 -- | Allows for unrecognised or experimental headers.
 | HdrCustom String -- not in header map below.
 deriving(Eq)

-- | @headerMap@ is a straight assoc list for translating between header names 
-- and values.
headerMap :: [ (String,HeaderName) ]
headerMap =
 [ p "Cache-Control" HdrCacheControl
 , p "Connection" HdrConnection
 , p "Date" HdrDate
 , p "Pragma" HdrPragma
 , p "Transfer-Encoding" HdrTransferEncoding
 , p "Upgrade" HdrUpgrade
 , p "Via" HdrVia
 , p "Accept" HdrAccept
 , p "Accept-Charset" HdrAcceptCharset
 , p "Accept-Encoding" HdrAcceptEncoding
 , p "Accept-Language" HdrAcceptLanguage
 , p "Authorization" HdrAuthorization
 , p "Cookie" HdrCookie
 , p "Expect" HdrExpect
 , p "From" HdrFrom
 , p "Host" HdrHost
 , p "If-Modified-Since" HdrIfModifiedSince
 , p "If-Match" HdrIfMatch
 , p "If-None-Match" HdrIfNoneMatch
 , p "If-Range" HdrIfRange
 , p "If-Unmodified-Since" HdrIfUnmodifiedSince
 , p "Max-Forwards" HdrMaxForwards
 , p "Proxy-Authorization" HdrProxyAuthorization
 , p "Range" HdrRange
 , p "Referer" HdrReferer
 , p "User-Agent" HdrUserAgent
 , p "Age" HdrAge
 , p "Location" HdrLocation
 , p "Proxy-Authenticate" HdrProxyAuthenticate
 , p "Public" HdrPublic
 , p "Retry-After" HdrRetryAfter
 , p "Server" HdrServer
 , p "Set-Cookie" HdrSetCookie
 , p "TE" HdrTE
 , p "Trailer" HdrTrailer
 , p "Vary" HdrVary
 , p "Warning" HdrWarning
 , p "WWW-Authenticate" HdrWWWAuthenticate
 , p "Allow" HdrAllow
 , p "Content-Base" HdrContentBase
 , p "Content-Encoding" HdrContentEncoding
 , p "Content-Language" HdrContentLanguage
 , p "Content-Length" HdrContentLength
 , p "Content-Location" HdrContentLocation
 , p "Content-MD5" HdrContentMD5
 , p "Content-Range" HdrContentRange
 , p "Content-Type" HdrContentType
 , p "ETag" HdrETag
 , p "Expires" HdrExpires
 , p "Last-Modified" HdrLastModified
 , p "Content-Transfer-Encoding" HdrContentTransferEncoding
 ]
 where
 p a b = (a,b)

instance Show HeaderName where
 show (HdrCustom s) = s
 show x = case filter ((==x).snd) headerMap of
 [] -> error "headerMap incomplete"
 (h:_) -> fst h

-- | @HasHeaders@ is a type class for types containing HTTP headers, allowing
-- you to write overloaded header manipulation functions
-- for both 'Request' and 'Response' data types, for instance.
class HasHeaders x where
 getHeaders :: x -> [Header]
 setHeaders :: x -> [Header] -> x

-- Header manipulation functions

type HeaderSetter a = HeaderName -> String -> a -> a

-- | @insertHeader hdr val x@ inserts a header with the given header name
-- and value. Does not check for existing headers with same name, allowing
-- duplicates to be introduce (use 'replaceHeader' if you want to avoid this.)
insertHeader :: HasHeaders a => HeaderSetter a
insertHeader name value x = setHeaders x newHeaders
 where
 newHeaders = (Header name value) : getHeaders x

-- | @insertHeaderIfMissing hdr val x@ adds the new header only if no previous
-- header with name @hdr@ exists in @x@.
insertHeaderIfMissing :: HasHeaders a => HeaderSetter a
insertHeaderIfMissing name value x = setHeaders x (newHeaders $ getHeaders x)
 where
 newHeaders list@(h@(Header n _): rest)
 | n == name = list
 | otherwise = h : newHeaders rest
 newHeaders [] = [Header name value]

-- | @replaceHeader hdr val o@ replaces the header @hdr@ with the
-- value @val@, dropping any existing 
replaceHeader :: HasHeaders a => HeaderSetter a
replaceHeader name value h = setHeaders h newHeaders
 where
 newHeaders = Header name value : [ x | x@(Header n _) <- getHeaders h, name /= n ]
 
-- | @insertHeaders hdrs x@ appends multiple headers to @x@'s existing
-- set.
insertHeaders :: HasHeaders a => [Header] -> a -> a
insertHeaders hdrs x = setHeaders x (getHeaders x ++ hdrs)

-- | @retrieveHeaders hdrNm x@ gets a list of headers with 'HeaderName' @hdrNm@.
retrieveHeaders :: HasHeaders a => HeaderName -> a -> [Header]
retrieveHeaders name x = filter matchname (getHeaders x)
 where
 matchname (Header n _) = n == name 

-- | @findHeader hdrNm x@ looks up @hdrNm@ in @x@, returning the first
-- header that matches, if any.
findHeader :: HasHeaders a => HeaderName -> a -> Maybe String
findHeader n x = lookupHeader n (getHeaders x)

-- | @lookupHeader hdr hdrs@ locates the first header matching @hdr@ in the
-- list @hdrs@.
lookupHeader :: HeaderName -> [Header] -> Maybe String
lookupHeader _ [] = Nothing
lookupHeader v (Header n s:t) 
 | v == n = Just s
 | otherwise = lookupHeader v t

-- | @parseHeader headerNameAndValueString@ tries to unscramble a
-- @header: value@ pairing and returning it as a 'Header'.
parseHeader :: String -> Result Header
parseHeader str =
 case split ':' str of
 Nothing -> failParse ("Unable to parse header: " ++ str)
 Just (k,v) -> return $ Header (fn k) (trim $ drop 1 v)
 where
 fn k = case map snd $ filter (match k . fst) headerMap of
 [] -> (HdrCustom k)
 (h:_) -> h

 match :: String -> String -> Bool
 match s1 s2 = map toLower s1 == map toLower s2
 
-- | @parseHeaders hdrs@ takes a sequence of strings holding header
-- information and parses them into a set of headers (preserving their
-- order in the input argument.) Handles header values split up over
-- multiple lines.
parseHeaders :: [String] -> Result [Header]
parseHeaders = catRslts [] . 
 map (parseHeader . clean) . 
 joinExtended ""
 where
 -- Joins consecutive lines where the second line
 -- begins with ' ' or '\t'.
 joinExtended old [] = [old]
 joinExtended old (h : t)
 | isLineExtension h = joinExtended (old ++ ' ' : tail h) t
 | otherwise = old : joinExtended h t

 isLineExtension (x:_) = x == ' ' || x == '\t'
 isLineExtension _ = False

 clean [] = []
 clean (h:t) | h `elem` "\t\r\n" = ' ' : clean t
 | otherwise = h : clean t

 -- tolerant of errors? should parse
 -- errors here be reported or ignored?
 -- currently ignored.
 catRslts :: [a] -> [Result a] -> Result [a]
 catRslts list (h:t) = 
 case h of
 Left _ -> catRslts list t
 Right v -> catRslts (v:list) t
 catRslts list [] = Right $ reverse list