Writer.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Text.LaTeX.Base.Writer
 ( -- * @LaTeXT@ writer
   LaTeXT
 , LaTeXT_
 , runLaTeXT
 , execLaTeXT
 , extractLaTeX
 , extractLaTeX_
 , textell
   ) where

import Control.Monad.Writer
import Control.Applicative
import Data.String
--
import Text.LaTeX.Base.Syntax
import Text.LaTeX.Base.Render

-- | Newtype wrapper over the 'WriterT' monad transformer, with 'LaTeX'
--   as writer state.
newtype LaTeXT m a =
 LaTeXT { unwrapLaTeXT :: WriterT LaTeX m a }
   deriving (Functor,Applicative,Monad,MonadIO)

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 :: Functor m => LaTeXT m a -> m LaTeX
execLaTeXT = fmap snd . runLaTeXT

-- | 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_ = (>>= return . snd) . LaTeXT . lift . runLaTeXT

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

-- Overloaded Strings

instance Monad m => IsString (LaTeXT m a) where
 fromString = (>> return undefined) . textell . fromString