{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Text.LaTeX.Base.Writer
 ( -- * @LaTeXT@ writer
   LaTeXT
 , LaTeXT_
 , runLaTeXT
 , execLaTeXT
 , execLaTeXTWarn
 , extractLaTeX
 , extractLaTeX_
 , textell
 , liftFun
 , liftOp
   -- * Re-export
 , lift
   ) where

import Control.Monad.Trans.Writer
import Control.Monad.Trans.State
import Control.Monad.IO.Class
import Control.Monad.Trans.Class
import Control.Applicative
import Control.Arrow
import Data.String
--
import Text.LaTeX.Base.Syntax
import Text.LaTeX.Base.Render
import Text.LaTeX.Base.Warnings (Warning,checkAll,check)
--
import Control.Monad (liftM)

newtype LaTeXT m a =
 LaTeXT { unwrapLaTeXT :: WriterT LaTeX m a }
   deriving (Functor,Applicative,Monad,MonadIO)

instance MonadTrans LaTeXT where
 lift = LaTeXT . lift

type LaTeXT_ m = LaTeXT m ()

runLaTeXT :: LaTeXT m a -> m (a,LaTeX)
runLaTeXT = runWriterT . unwrapLaTeXT

-- | This is the usual way to run the 'LaTeXT' monad
--   and obtain a 'LaTeX' value.
execLaTeXT :: Monad m => LaTeXT m a -> m LaTeX
execLaTeXT = liftM snd . runLaTeXT

-- | Version of 'execLaTeXT' with possible warning messages.
--   This function applies 'checkAll' to the 'LaTeX' output.
execLaTeXTWarn :: Monad m => LaTeXT m a -> m (LaTeX,[Warning])
execLaTeXTWarn = liftM (id &&& check checkAll) . execLaTeXT

-- | This function run a 'LaTeXT' computation,
--   lifting the result again in the monad.
extractLaTeX :: Monad m => LaTeXT m a -> LaTeXT m (a,LaTeX)
extractLaTeX = LaTeXT . lift . runLaTeXT

extractLaTeX_ :: Monad m => LaTeXT m a -> LaTeXT m LaTeX
extractLaTeX_ = liftM snd . extractLaTeX

-- | With 'textell' you can append 'LaTeX' values to the
--   state of the 'LaTeXT' monad.
textell :: Monad m => LaTeX -> LaTeXT m ()
textell = LaTeXT . tell

-- | Lift a function over 'LaTeX' values to a function
--   acting over the state of a 'LaTeXT' computation.
liftFun :: Monad m
        => (LaTeX -> LaTeX)
        -> (LaTeXT m a -> LaTeXT m a)
liftFun f ml = do
 (a,l') <- lift $ do
            (a,l) <- runLaTeXT ml
            let l' = f l
            return (a,l')
 textell l'
 return a

-- | Lift an operator over 'LaTeX' values to an operator
--   acting over the state of two 'LaTeXT' computations.
--
-- /Note: The returned value is the one returned by the/
-- /second argument of the lifted operator./
liftOp :: Monad m
       => (LaTeX -> LaTeX -> LaTeX)
       -> (LaTeXT m a -> LaTeXT m a -> LaTeXT m a)
liftOp op ml1 ml2 = do
 (a,l') <- lift $ do
            (_,l1) <- runLaTeXT ml1
            (a,l2) <- runLaTeXT ml2
            let l' = l1 `op` l2
            return (a,l')
 textell l'
 return a

-- Overloaded Strings

-- | Be careful when using 'fromString' over a 'LaTeXT' value,
--   the returned value of the computation is bottom (i.e. 'undefined').
instance Monad m => IsString (LaTeXT m a) where
 fromString = (>> return undefined) . textell . fromString