{-# LANGUAGE CPP, FlexibleContexts #-}
{-
	Copyright (C) 2013-2014 Dr. Alistair Ward

	This file is part of WeekDaze.

	WeekDaze is free software: you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation, either version 3 of the License, or
	(at your option) any later version.

	WeekDaze is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU General Public License for more details.

	You should have received a copy of the GNU General Public License
	along with WeekDaze.  If not, see <http://www.gnu.org/licenses/>.
-}
{- |
 [@AUTHOR@]	Dr. Alistair Ward

 [@DESCRIPTION@]

	* Describes a /subject/, in terms of the /topic/, & the /level/ at which it is being taught.

	* The /level/ may correspond to an academic year, but may be a finer-grain value as required to model /topic/-specific streaming.

	* A non-academic /subject/ like cross-country running, may not have a concept of multiple /level/s,
	but since it doesn't have zero but rather one /level/, the concept exists for all /subject/s.
-}

module WeekDaze.Data.Subject(
-- * Types
-- ** Type-synonyms
        Topic,
        Knowledge,
-- ** Data-types
        Subject(
--		MkSubject,
                getTopic,
                getLevel
        ),
-- * Constants
        tag,
        levelTag,
        topicTag,
-- * Functions
-- ** Constructors
#ifdef USE_HDBC
        mkSubjectFromSql,
#endif
        mkSubject
) where

import qualified        Control.Arrow
import                  Control.Arrow((&&&))
import qualified        Control.DeepSeq
import qualified        Data.Set
import qualified        Text.XHtml.Strict
import                  Text.XHtml.Strict((+++), (<<))
import qualified        Text.XML.HXT.Arrow.Pickle       as HXT
import qualified        ToolShed.SelfValidate
import qualified        WeekDaze.Text.XHTML             as Text.XHTML

#ifdef USE_HDBC
import qualified        Data.Convertible
import qualified        Data.Maybe
import qualified        Database.HDBC

-- | Construct from two database-values.
mkSubjectFromSql
        :: (Data.Convertible.Convertible Database.HDBC.SqlValue level, Show level)      -- Flexible context.
        => Database.HDBC.SqlValue       -- ^ Topic.
        -> Database.HDBC.SqlValue       -- ^ Level.
        -> Subject level
mkSubjectFromSql topicSql       = mkSubject (
        Data.Maybe.fromMaybe (
                error $ "WeekDaze.Data.Subject.mkSubjectFromSql:\tnull " ++ show topicTag ++ "."
        ) $ Database.HDBC.fromSql topicSql
 ) . Data.Maybe.fromMaybe (
        error $ "WeekDaze.Data.Subject.mkSubjectFromSql:\tnull " ++ show levelTag ++ "."
 ) . Database.HDBC.fromSql
#endif /* USE_HDBC */

-- | Used to qualify CSS & XML.
tag :: String
tag             = "subject"

-- | Used to qualify CSS, SQL & XML.
levelTag :: String
levelTag        = "level"

-- | Used to qualify CSS, SQL & XML.
topicTag :: String
topicTag        = "topic"

-- | The type of the /topic/ of study.
type Topic      = String

-- | The subject which is being either taught or learned.
data Subject level      = MkSubject {
        getTopic        :: Topic,       -- ^ The /topic/ of study, which would typically be something like /Maths/ or /English/, but it could be something non-academic, like /Games/; but it shouldn't be either /Assembly/ or /Lunch/, since these aren't required to have exactly one /teacher/ in attendance.
        getLevel        :: level        -- ^ The /level/ at which this /topic/ is being taught; which may be merely the academic year, or a finer-grain concept representing a /topic/-specific stream.
} deriving (Eq, Ord)

instance (Read level, Show level) => Read (Subject level) where
        readsPrec _     = map (Control.Arrow.first $ uncurry mkSubject) . reads

instance Show level => Show (Subject level) where
        showsPrec _     = shows . (getTopic &&& getLevel)       -- Hides the constructor & accessors.

instance Text.XHtml.Strict.HTML level => Text.XHtml.Strict.HTML (Subject level) where
        toHtml subject  = Text.XHTML.mkXHTMLSpan tag << (
                Text.XHTML.mkXHTMLSpan topicTag << getTopic subject +++ Text.XHtml.Strict.spaceHtml {-level may be null, so make separator invisible-} +++ getLevel subject
         )

instance (HXT.XmlPickler level, Show level) => HXT.XmlPickler (Subject level) where
        xpickle = HXT.xpElem tag . HXT.xpWrap (
                uncurry mkSubject,      -- Construct from a Pair.
                getTopic &&& getLevel   -- Deconstruct to a Pair.
         ) $ HXT.xpTextAttr topicTag {-can't be null-} `HXT.xpPair` HXT.xpickle {-Level-}

instance Control.DeepSeq.NFData level => Control.DeepSeq.NFData (Subject level) where
        rnf     = Control.DeepSeq.rnf . (getTopic &&& getLevel)

instance Show level => ToolShed.SelfValidate.SelfValidator (Subject level) where
        getErrors subject       = ToolShed.SelfValidate.extractErrors [(null $ getTopic subject, "null " ++ show topicTag ++ "; " ++ show subject)]

-- | Smart constructor.
mkSubject :: Show level => Topic -> level -> Subject level
mkSubject topic level
        | ToolShed.SelfValidate.isValid subject = subject
        | otherwise                             = error $ "WeekDaze.Data.Subject.mkSubject:\t" ++ ToolShed.SelfValidate.getFirstError subject ++ "."
        where
                subject = MkSubject topic level

-- | A collection of distinct 'Subject's.
type Knowledge level    = Data.Set.Set (Subject level)