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|
{-# LANGUAGE ScopedTypeVariables
#-}
{-|
Description: Deterministic finite state transducers
-}
module Data.String.DFST
( DFST(..)
, runDFST, runDFST'
) where
import Data.Map.Strict (Map, (!?))
import qualified Data.Map.Strict as Map
import Data.Set (Set)
import qualified Data.Set as Set
import Control.Monad
data DFST state = DFST
{ stInitial :: state
, stTransition :: Map (state, Char) (state, String)
-- ^ All @(s, c)@-combinations not mapped are assumed to map to @(s, Nothing)@
, stAccept :: Set state
}
runDFST :: forall state. Ord state => DFST state -> String -> Maybe String
runDFST dfst@DFST{..} str = let (finalState, str') = runDFST' dfst stInitial str id
in str' "" <$ guard (finalState `Set.member` stAccept)
runDFST' :: forall state. Ord state
=> DFST state
-> state -- ^ Current state
-> String -- ^ Remaining input
-> (String -> String) -- ^ Output as difference list
-> (state, (String -> String)) -- ^ Next state, altered output
runDFST' _ st [] acc = (st, acc)
runDFST' dfst@DFST{..} st (c:cs) acc
| Just (st', mc') <- stTransition !? (st, c)
= runDFST' dfst st' cs $ acc . (mc' ++)
| otherwise
= runDFST' dfst st cs acc
|
