diff --git a/src/Rextra/Fa.hs b/src/Rextra/Fa.hs
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+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE NamedFieldPuns #-}
+
+-- | This module contains ways to represent finite automata and their
+-- execution/evaluation.
+
+module Rextra.Fa
+  ( FaState(..)
+  , Fa
+  , fa
+  , stateMap
+  , entryState
+  , exitStates
+  , states
+  , getState
+  , Executable(..)
+  , transitionAll
+  , execute
+  ) where
+
+import           Data.List
+import qualified Data.Map as Map
+import qualified Data.Set as Set
+
+-- | The state of a finite automaton.
+class FaState state where
+  -- | All the states that can be reached (by any sort of transition)
+  -- from this state.
+  canReach :: state s t -> Set.Set s
+
+-- | A finite automaton.
+data Fa state s t = Fa
+  { stateMap   :: Map.Map s (state s t)
+  -- ^ A 'Map.Map' from state indentifiers to states. See 'states',
+  -- 'getState'.
+  , entryState :: s
+  -- ^ The state at which execution of the automaton begins.
+  , exitStates :: Set.Set s
+  -- ^ The automaton's accepting states, i. e. the states that
+  -- determine whether the automaton accepts a certain word.
+  }
+
+-- | @'states' fa@ are the identifiers of all states contained in @fa@.
+states :: Fa state s t -> Set.Set s
+states Fa{stateMap} = Map.keysSet stateMap
+
+-- | @'getState' fa s@ retrieves the @state s t@ corresponding to the
+-- identifer @s@.
+--
+-- __Warning__: This function is unsafe. It only works if @s@ is a
+-- member of @'states' fa@, and errors otherwise.
+getState :: (Ord s) => Fa state s t -> s -> state s t
+getState Fa{stateMap} s = stateMap Map.! s
+
+integrityCheck :: (FaState state, Ord s) => Fa state s t -> Bool
+integrityCheck fa'@Fa{stateMap, entryState, exitStates} =
+  let reachable = map canReach $ Map.elems stateMap
+      mentioned = Set.unions $ Set.singleton entryState : exitStates : reachable
+  in  mentioned `Set.isSubsetOf` states fa'
+
+-- | Construct a 'Fa'.
+--
+-- This constructor function also checks whether the finite
+-- automaton's invariants (see 'getState') hold true.
+fa :: (FaState state, Ord s) => Map.Map s (state s t) -> s -> Set.Set s -> Maybe (Fa state s t)
+fa stateMap entryState exitStates =
+  let potentialFa = Fa{stateMap, entryState, exitStates}
+  in  if integrityCheck potentialFa then Just potentialFa else Nothing
+
+-- | A special type class for automata that can be executed. These
+-- automata must not necessarily be finite.
+class Executable a execState where
+  -- | The state at which execution of the automaton begins.
+  startState :: a s t -> execState
+  -- | A function that determines the automaton's next state based on
+  -- a token.
+  transition :: a s t -> execState -> t -> execState
+  -- | Whether the automaton acceps
+  accepts    :: a s t -> execState -> Bool
+
+-- | Perform all transitions corresponding to a word (or list) of tokens, in order.
+transitionAll :: (Executable a execState) => a s t -> execState -> [t] -> execState
+transitionAll a = foldl' (transition a)
+
+-- | Like 'transitionAll', starting with the automaton's 'startState'.
+--
+-- To check, whether an automaton @a@ accepts a word @w@, use
+-- 'accepts' like this:
+--
+-- > a `accepts` execute a w
+execute :: (Executable a execState) => a s t -> [t] -> execState
+execute a = transitionAll a (startState a)