{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

-- | The free 'Cfg'
module Data.Cfg.FreeCfg
  ( FreeCfg(..)
  , toFreeCfg
  ) where

import Data.Cfg.Cfg (Cfg(..), Vs)
import qualified Data.Set as S

-- | Represents a context-free grammar with its nonterminal and
-- terminal types.  The canonical instance of 'Cfg': a record that
-- collects up implementations of each class method.
data FreeCfg t nt = FreeCfg
  { nonterminals' :: S.Set nt
    -- ^ the nonterminals of the grammar
  , terminals' :: S.Set t
    -- ^ the terminals of the grammar
  , productionRules' :: nt -> S.Set (Vs t nt)
    -- ^ the productions of the grammar
  , startSymbol' :: nt
    -- ^ the start symbol of the grammar; must be an element of
    -- 'nonterminals' 'cfg'
  }

instance Cfg FreeCfg t nt where
  nonterminals = nonterminals'
  terminals = terminals'
  productionRules = productionRules'
  startSymbol = startSymbol'

-- | Converts any 'Cfg' into a 'FreeCfg'.
toFreeCfg :: Cfg cfg t nt => cfg t nt -> FreeCfg t nt
toFreeCfg cfg =
  FreeCfg
    { nonterminals' = nonterminals cfg
    , terminals' = terminals cfg
    , productionRules' = productionRules cfg
    , startSymbol' = startSymbol cfg
    }