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+{-# LANGUAGE
+    FlexibleInstances
+  , MultiParamTypeClasses
+  , OverloadedStrings
+  , TemplateHaskell
+  , UnicodeSyntax
+  #-}
+-- |This module provides functions to parse and format HTTP\/1.1 date
+-- and time strings
+-- (<http://tools.ietf.org/html/rfc2616#section-3.3>).
+--
+-- The HTTP\/1.1 specification (RFC 2616) says that HTTP\/1.1 clients
+-- and servers which parse the date value MUST accept all the
+-- following formats, though they MUST only generate the RFC 1123
+-- format for representing HTTP-date values in header fields:
+--
+-- > Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
+-- > Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
+-- > Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
+--
+-- It also says that all HTTP date\/time stamps MUST be represented in
+-- Greenwich Mean Time (GMT), without exception. For the purposes of
+-- HTTP, GMT is exactly equal to UTC (Coordinated Universal
+-- Time). This is indicated in the first two formats by the inclusion
+-- of @\"GMT\"@ as the three-letter abbreviation for time zone, and
+-- MUST be assumed when reading the asctime format.
+--
+-- > HTTP-date    = rfc1123-date | rfc850-date | asctime-date
+-- > rfc1123-date = wkday "," SP date1 SP time SP "GMT"
+-- > rfc850-date  = weekday "," SP date2 SP time SP "GMT"
+-- > asctime-date = wkday SP date3 SP time SP 4DIGIT
+-- > date1        = 2DIGIT SP month SP 4DIGIT
+-- >                ; day month year (e.g., 02 Jun 1982)
+-- > date2        = 2DIGIT "-" month "-" 2DIGIT
+-- >                ; day-month-year (e.g., 02-Jun-82)
+-- > date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
+-- >                ; month day (e.g., Jun  2)
+-- > time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
+-- >                ; 00:00:00 - 23:59:59
+-- > wkday        = "Mon" | "Tue" | "Wed"
+-- >              | "Thu" | "Fri" | "Sat" | "Sun"
+-- > weekday      = "Monday" | "Tuesday" | "Wednesday"
+-- >              | "Thursday" | "Friday" | "Saturday" | "Sunday"
+-- > month        = "Jan" | "Feb" | "Mar" | "Apr"
+-- >              | "May" | "Jun" | "Jul" | "Aug"
+-- >              | "Sep" | "Oct" | "Nov" | "Dec"
+module Data.Time.Format.HTTP
+    ( HTTP
+    , httpDateAndTime
+    )
+    where
+import Control.Applicative
+import Data.Ascii (Ascii, AsciiBuilder)
+import qualified Data.Ascii as A
+import Data.Attoparsec.Char8
+import Data.Convertible.Base
+import Data.Tagged
+import Data.Time
+import Data.Time.Format.Asctime
+import Data.Time.Format.HTTP.Common
+import Data.Time.Format.RFC733
+import Data.Time.Format.RFC822
+import Data.Time.Format.RFC1123
+import Prelude.Unicode
+
+-- |The phantom type for conversions between HTTP/1.1 date and time
+-- strings and 'UTCTime'.
+--
+-- >>> convertSuccess (UTCTime (ModifiedJulianDay 49662) 31777)
+-- Tagged "Sun, 06 Nov 1994 08:49:37 GMT"
+data HTTP
+
+instance ConvertSuccess UTCTime (Tagged HTTP Ascii) where
+    {-# INLINE convertSuccess #-}
+    convertSuccess = (A.fromAsciiBuilder <$>) ∘ cs
+
+instance ConvertSuccess UTCTime (Tagged HTTP AsciiBuilder) where
+    {-# INLINE convertSuccess #-}
+    convertSuccess = Tagged ∘ toAsciiBuilder
+
+instance ConvertAttempt (Tagged HTTP Ascii) UTCTime where
+    {-# INLINE convertAttempt #-}
+    convertAttempt = parseAttempt' httpDateAndTime ∘ untag
+
+-- |Parse a date and time string in any of RFC 822, RFC 1123, RFC 850
+-- and ANSI C's asctime() formats.
+--
+-- This function is even more permissive than what HTTP\/1.1 (RFC
+-- 2616) specifies. That is, it accepts 2-digit years in RFC 822,
+-- omitted separator symbols in RFC 850, omitted sec fields, and
+-- non-GMT time zones. I believe this behavior will not cause a
+-- problem though.
+httpDateAndTime ∷ Parser UTCTime
+httpDateAndTime
+    = choice [ zonedTimeToUTC     <$> try rfc1123DateAndTime
+             , zonedTimeToUTC     <$> try rfc733DateAndTime
+             , zonedTimeToUTC     <$> try rfc822DateAndTime
+             , localTimeToUTC utc <$> asctime
+             ]
+
+toAsciiBuilder ∷ UTCTime → AsciiBuilder
+toAsciiBuilder = untag' ∘ cs ∘ ut2zt
+    where
+      untag' ∷ Tagged RFC1123 AsciiBuilder → AsciiBuilder
+      {-# INLINE CONLIKE untag' #-}
+      untag' = untag
+
+      ut2zt ∷ UTCTime → ZonedTime
+      {-# INLINE ut2zt #-}
+      ut2zt = utcToZonedTime gmt
+
+      gmt ∷ TimeZone
+      {-# INLINE CONLIKE gmt #-}
+      gmt = TimeZone 0 False "GMT"
+
+deriveAttempts [ ([t| UTCTime |], [t| Tagged HTTP Ascii        |])
+               , ([t| UTCTime |], [t| Tagged HTTP AsciiBuilder |])
+               ]