1 files changed, 38 insertions, 0 deletions

diff --git a/edit-lens/src/Data/String/DFST.hs b/edit-lens/src/Data/String/DFST.hs
new file mode 100644
index 0000000..8a22dd3
--- /dev/null
+++ b/edit-lens/src/Data/String/DFST.hs
@@ -0,0 +1,38 @@
+{-# LANGUAGE RecordWildCards
+           , PatternGuards
+           , ScopedTypeVariables
+#-}
+
+{-|
+Description: Deterministic finite state transducers
+-}
+module Data.String.DFST
+  ( DFST(..)
+  , runDFST
+  ) where
+
+import Data.Map.Strict (Map, (!?))
+import qualified Data.Map.Strict as Map
+
+import Data.Set (Set)
+import qualified Data.Set as Set
+
+data DFST state = DFST
+  { stInitial :: state
+  , stTransition :: Map (state, Char) (state, Maybe Char)
+    -- ^ 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{..} str = ($ []) <$> go stInitial str id
+  where
+    go :: state -> String -> (String -> String) -> Maybe (String -> String)
+    go st [] acc
+      | st `Set.member` stAccept = Just acc
+      | otherwise = Nothing
+    go st (c:cs) acc
+      | Just (st', mc') <- stTransition !? (st, c)
+      = go st' cs $ acc . maybe id (:) mc'
+      | otherwise
+      = go st cs acc