First actually complete implementation

.gitignore

@@ -0,0 +1 @@
+dist-newstyle

app/Binding.hs

@@ -0,0 +1,49 @@
+module Binding 
+( newVar, unify, instantiate
+) where
+import Terms
+import Nondeterminism
+import Primitives
+import Control.Monad 
+import Control.Applicative
+
+newVar :: Name -> PrologStream Var
+newVar name = reversibly (newVarPrim name) unnewVarPrim
+
+getVar :: Var -> PrologStream (Maybe Term)
+getVar var = liftPrologIO (getVarPrim var)
+
+setVar :: Var -> Term -> PrologStream ()
+setVar var value = reversibly set unset
+  where 
+  set = do 
+    oldValue <- getVarPrim var
+    when (oldValue /= Nothing) (errorPrim "cannot set var that is currently set")
+    setVarPrim var (Just value)
+  unset = do
+    oldValue <- getVarPrim var
+    when (oldValue == Nothing) (errorPrim "cannot unset var that is already unset")
+    setVarPrim var Nothing
+
+unify :: Term -> Term -> PrologStream ()
+unify (Compound h0 args0) (Compound h1 args1) = do
+  when (h0 /= h1) empty
+  when (length args0 /= length args1) empty
+  forM_ (zip args0 args1) $ \(arg0, arg1) -> unify arg0 arg1
+unify (Var v0) (Var v1) | v0 == v1 = return ()
+unify (Var v0) t1 = do
+  binding <- getVar v0
+  case binding of
+    Just t0 -> unify t0 t1 
+    Nothing -> setVar v0 t1
+unify t0 t1@(Var _) = unify t1 t0
+
+instantiate :: Term -> PrologStream Term
+instantiate t@(Var v) = do
+  binding <- getVar v
+  case binding of 
+    Just t0 -> instantiate t0
+    Nothing -> return t
+instantiate (Compound h args) = do
+  instantiatedArgs <- mapM instantiate args
+  return (Compound h instantiatedArgs)

app/Classes.hs

@@ -0,0 +1,6 @@
+module Classes
+( LiftIO (..)
+) where
+
+class Monad m => LiftIO m where
+  liftIO :: IO a -> m a

app/Database.hs

@@ -0,0 +1,30 @@
+module Database
+( Clause(..)
+, Database(..)
+, seekAndInstantiate, seek
+) where
+import Binding
+import Nondeterminism
+import Terms
+import Control.Monad
+import Control.Applicative
+
+data Clause = Clause Term [Term]
+
+data Database = Database
+  { clauses :: [PrologStream Clause]
+  }
+
+seekAndInstantiate :: Term -> Database -> PrologStream Term
+seekAndInstantiate goal db = do
+  seek goal db
+  instantiate goal
+
+seek :: Term -> Database -> PrologStream ()
+seek goal db = do
+  foldr (<|>) mzero (map doClause (clauses db))
+  where 
+  doClause clause = do
+    Clause chead body <- clause
+    unify goal chead
+    forM_ body $ \newGoal -> seek newGoal db

app/Main.hs

@@ -1,237 +1,50 @@
 {-# LANGUAGE InstanceSigs #-}
 module Main where
 
-import Control.Monad 
+import Binding
-import qualified Data.Map.Lazy as M
+import Classes
-import Data.IORef
+import Database
-import Control.Monad.RWS (MonadTrans(lift))
+import Nondeterminism 
-import Control.Applicative
+import Terms
 
-data Atomic
+demoProgram :: Database
-  = AString String
+demoProgram = Database 
-  deriving (Eq, Show)
+  { clauses = 
+    [ do
+      who <- newVar "who"
+      action <- newVar "action"
+      return $ Clause (Compound "act" [Var who, Var action]) 
+        [ Compound "is_vampire" [Var who]
+        , Compound "vampire_behavior" [Var action]
+        ]
+    , do 
+      progenitor <- newVar "progenitor"
+      progeny <- newVar "progeny"
+      return $ Clause (Compound "is_vampire" [Var progeny])
+        [ Compound "has_progenitor" [Var progenitor, Var progeny]
+        , Compound "is_vampire" [Var progenitor]
+        ]
+    , return $ Clause (Compound "act" [Compound "Sam" [], Compound "goes to law school" []]) []
 
-data Name
+    , return $ Clause (Compound "vampire_behavior" [Compound "drinks blood" []]) []
-  = NString String
+    , return $ Clause (Compound "vampire_behavior" [Compound "spits venom" []]) []
-  deriving (Ord, Eq, Show)
+    , return $ Clause (Compound "is_vampire" [Compound "Nyeogmi" []]) []
-
+    , return $ Clause (Compound "has_progenitor" [Compound "Nyeogmi" [], Compound "Pyrex" []]) []
-data Term 
+    , return $ Clause (Compound "has_progenitor" [Compound "Sam" [], Compound "Alex" []]) []
-  = TCompound Atomic [Term] -- functor, args
+    -- , return $ Clause (Compound "is_vampire" [Compound "Sam" []]) []
-  | TVariable Name  -- name
+    ]
-  deriving (Eq, Show)
-
-type Vars = M.Map Name Term
-
-emptyVars :: Vars 
-emptyVars = M.empty
-
-
-data Globals = Globals {
-  psVars :: IORef Vars
-}
-
-newtype PrologIO a = PrologIO (Globals -> IO a)
-
-instance Functor PrologIO where
-  fmap :: (a -> b) -> PrologIO a -> PrologIO b
-  fmap f (PrologIO a) = PrologIO (\globals -> fmap f (a globals))
-
-instance Applicative PrologIO where
-  pure :: a -> PrologIO a
-  pure x = PrologIO (\_ -> pure x)
-
-  (PrologIO f) <*> (PrologIO x) = PrologIO (
-    \globals -> do
-      f' <- f globals
-      x' <- x globals
-      pure (f' x')
-    )
-
-instance Monad PrologIO where
-  return :: a -> PrologIO a
-  return = pure
-
-  (PrologIO a) >>= f = PrologIO (
-    \globals -> do
-      a' <- a globals
-      let PrologIO b' = f a'
-      b' globals
-    )
-
-runPrologIO :: Globals -> PrologIO a -> IO a
-runPrologIO globals (PrologIO a) = a globals
-
-data Suspend a = Yield a | Suspend
-
-instance Functor Suspend where
-  fmap f (Yield a) = Yield (f a)
-  fmap _ Suspend = Suspend
-
-data PrologStream a 
-  = PYes (Suspend a) (PrologIO (PrologStream a))
-  | PNo
-
-instance Functor PrologStream where
-  fmap :: (a -> b) -> PrologStream a -> PrologStream b
-  fmap f (PYes a next) = PYes (fmap f a) (fmap (fmap f) next)
-  fmap _ PNo = PNo
-
-instance Applicative PrologStream where
-  pure :: a -> PrologStream a
-  pure x = PYes (Yield x) (pure PNo)
-
-  (<*>) :: PrologStream (a -> b) -> PrologStream a -> PrologStream b
-  f <*> x = f >>= (\f' -> x >>= (\x' -> return (f' x')))
-
-instance Monad PrologStream where
-  return :: a -> PrologStream a
-  return = pure
-
-  (>>=) :: PrologStream a -> (a -> PrologStream b) -> PrologStream b
-  PNo >>= _ = PNo
-  PYes (Yield x) xs >>= f = f x <|> PYes Suspend (fmap (>>= f) xs)
-  PYes Suspend xs >>= f = PYes Suspend (fmap (>>= f) xs)
-
-instance Alternative PrologStream where 
-  empty :: PrologStream a
-  empty = PNo
-
-  (<|>) :: PrologStream a -> PrologStream a -> PrologStream a
-  (<|>) = append
-
-
-instance MonadPlus PrologStream where
-  mzero :: PrologStream a
-  mzero = empty
-
-  mplus :: PrologStream a -> PrologStream a -> PrologStream a
-  mplus = (<|>)
-
-
-append :: PrologStream a -> PrologStream a -> PrologStream a
-append (PYes x xs) next = PYes x (liftM2 append xs (return next))
-append PNo next = next
-
-
-data Thread a = Thread 
-  { threadGlobals :: Globals
-  , threadStream :: IORef (PrologStream a)
   }
 
-startThread :: PrologStream a -> IO (Thread a)
-startThread stream = do
-  vars <- newIORef emptyVars
-  streamRef <- newIORef stream 
-  let globals = Globals { psVars = vars }
-  return (Thread { threadGlobals = globals, threadStream = streamRef })
-
-advanceThread :: Thread a -> IO (Maybe a)
-advanceThread (Thread { threadGlobals = globals, threadStream = streamRef }) = do
-  stream <- readIORef streamRef
-  (value, newStream) <- runPrologIO globals (squeeze stream)
-  writeIORef streamRef newStream
-  return value
-
-completeThread :: Thread a -> IO [a]
-completeThread thread = do
-  value <- advanceThread thread
-  case value of 
-    Nothing -> return []
-    Just x -> fmap (x:) (completeThread thread)
-
-squeeze :: PrologStream a -> PrologIO (Maybe a, PrologStream a)
-squeeze PNo = return (Nothing, PNo)
-squeeze (PYes Suspend rest) = rest >>= squeeze
-squeeze (PYes (Yield x) rest) = fmap ((,) (Just x)) rest 
-
-exitUndo :: PrologIO a -> PrologIO () -> PrologStream a
-exitUndo exit undo = 
-  PYes Suspend $ exit >>= \v -> return $ 
-    PYes (Yield v) $ undo >>= \_ -> return $
-      PNo
-
-liftPrologIO :: PrologIO a -> PrologStream a
-liftPrologIO exit = exitUndo exit (return ())
-
-liftIO :: IO a -> PrologStream a
-liftIO x = liftPrologIO $ PrologIO (\_ -> x)
-
-setVar :: Name -> Term -> PrologStream ()
-setVar name value = exitUndo exit undo
-  where 
-  exit = PrologIO (
-    \globals -> do 
-      let varsRef = psVars globals
-      modifyIORef varsRef (M.insert name value)
-    )
-  undo = PrologIO (
-    \globals -> do
-      let varsRef = psVars globals
-      modifyIORef varsRef (M.delete name)
-    )
-
-getVar :: Name -> PrologStream (Maybe Term)
-getVar name = exitUndo exit undo
-  where
-  exit = PrologIO (
-    \globals -> do
-      let varsRef = psVars globals
-      vars <- readIORef varsRef
-      return (M.lookup name vars)
-    )
-  undo = PrologIO (\_ -> return ())
-
-  
-demoProgram :: PrologStream ()
-demoProgram = do
-  setVar (NString "bat") (TCompound (AString "Nyeogmi") []) <|> setVar (NString "bat") (TCompound (AString "Pyrex") []) 
-  setVar (NString "activity") (TCompound (AString "drinks blood") []) <|> setVar (NString "activity") (TCompound (AString "spits venom") []) 
-  who <- getVar (NString "bat")
-  what <- getVar (NString "activity")
-  liftIO (print ("Value: ", who, "What: ", what))
-
-
 main :: IO ()
 main = do
-  thread <- startThread demoProgram
+  thread <- newThread runDemoProgram
-  results <- completeThread thread
+  _ <- takeAllRemainingSolutions thread
-  putStrLn ("Results: " ++ show results)
+  return ()
-{-
-no :: Prolog a
-no = Prolog (\_ -> [])
-
-readVars :: Prolog Vars
-readVars = Prolog (\vars -> [(vars, vars)])
-
-writeVars :: Vars -> Prolog ()
-writeVars vars = Prolog (\_ -> [((), vars)])
-
-unify :: Term -> Term -> Prolog ()
-unify = unify_ []
   where 
-  unify_ :: [Name] -> Term -> Term -> Prolog ()
+    runDemoProgram = do
-  unify_ names (TCompound h0 args0) (TCompound h1 args1) = do
+      who <- newVar "who"
-    when (h0 /= h1) no
+      what <- newVar "what"
-    when (length args0 /= length args1) no
+      let goal = Compound "act" [Var who, Var what]
-    forM_ (zip args0 args1) $ \(arg0, arg1) -> unify_ names arg0 arg1
+      seek goal demoProgram
-  unify_ _ (TVariable name0) (TVariable name1) | name0 == name1 = return ()
+      instantiatedGoal <- instantiate goal
-  unify_ names (TVariable name) y = do
+      liftIO $ putStrLn ("Got " ++ show instantiatedGoal)
-    -- occurs check
-    when (name `elem` names) no
-    -- unify as usual
-    (Vars vs0) <- readVars
-    case M.lookup name vs0 of
-      Just existing -> unify_ (name:names) existing y
-      Nothing -> do
-        let vs1 = M.insert name y vs0
-        writeVars (Vars vs1)
-  unify_ names x@(TCompound _ _) y@(TVariable _) = unify_ names y x
-
-
-
-
-
-main :: IO ()
-main = putStrLn "Hello, Haskell!"
-
--}

app/Main.hs.old1

@@ -1,99 +0,0 @@
-{-# LANGUAGE InstanceSigs #-}
-module Main where
-
-import Control.Monad 
-import Control.Monad.State.Lazy
-import qualified Data.Map.Lazy as M
-import GHC.Conc (TVar(TVar))
-
-data Atomic
-  = AString String
-  deriving (Eq, Show)
-
-data Name
-  = NString String
-  deriving (Ord, Eq, Show)
-
-data Term 
-  = TCompound Atomic [Term] -- functor, args
-  | TVariable Name  -- name
-  deriving (Eq, Show)
-
-data Vars
-  = Vars { varsBindings :: M.Map Name Term }
-
-emptyVars :: Vars 
-emptyVars = Vars { varsBindings = M.empty }
-
-newtype Prolog a = Prolog (Vars -> [(a, Vars)])
-
-instance Functor Prolog where
-  fmap :: (a -> b) -> Prolog a -> Prolog b
-  fmap f (Prolog x) = Prolog (\v0 -> [(f a, v1) | (a, v1) <- x v0])
-
-instance Applicative Prolog where
-  pure :: a -> Prolog a
-  pure x = Prolog (\_ -> [(x, emptyVars)])
-
-  (<*>) :: Prolog (a -> b) -> Prolog a -> Prolog b
-  (Prolog f) <*> (Prolog x) = Prolog body
-    where 
-    body vars0 
-      = [
-        (function argument, vars2) |
-        (function, vars1) <- f vars0,
-        (argument, vars2) <- x vars1
-      ]
-
-instance Monad Prolog where
-  return :: a -> Prolog a
-  return = pure
-
-  (>>=) :: Prolog a -> (a -> Prolog b) -> Prolog b
-  (Prolog a0) >>= f = Prolog body
-    where
-    body vars0 
-      = [
-        (b1, vars2) |
-        (a1, vars1) <- a0 vars0,
-        let Prolog b = f a1,
-        (b1, vars2) <- (b vars1)
-      ]
-
-
-no :: Prolog a
-no = Prolog (\_ -> [])
-
-readVars :: Prolog Vars
-readVars = Prolog (\vars -> [(vars, vars)])
-
-writeVars :: Vars -> Prolog ()
-writeVars vars = Prolog (\_ -> [((), vars)])
-
-unify :: Term -> Term -> Prolog ()
-unify = unify_ []
-  where
-  unify_ :: [Name] -> Term -> Term -> Prolog ()
-  unify_ names (TCompound h0 args0) (TCompound h1 args1) = do
-    when (h0 /= h1) no
-    when (length args0 /= length args1) no
-    forM_ (zip args0 args1) $ \(arg0, arg1) -> unify_ names arg0 arg1
-  unify_ _ (TVariable name0) (TVariable name1) | name0 == name1 = return ()
-  unify_ names (TVariable name) y = do
-    -- occurs check
-    when (name `elem` names) no
-    -- unify as usual
-    (Vars vs0) <- readVars
-    case M.lookup name vs0 of
-      Just existing -> unify_ (name:names) existing y
-      Nothing -> do
-        let vs1 = M.insert name y vs0
-        writeVars (Vars vs1)
-  unify_ names x@(TCompound _ _) y@(TVariable _) = unify_ names y x
-
-
-
-
-
-main :: IO ()
-main = putStrLn "Hello, Haskell!"

app/Main.hs.old2

@@ -1,167 +0,0 @@
-{-# LANGUAGE InstanceSigs #-}
-module Main where
-
-import Control.Monad 
-import qualified Data.Map.Lazy as M
-import Data.IORef
-
-data Atomic
-  = AString String
-  deriving (Eq, Show)
-
-data Name
-  = NString String
-  deriving (Ord, Eq, Show)
-
-data Term 
-  = TCompound Atomic [Term] -- functor, args
-  | TVariable Name  -- name
-  deriving (Eq, Show)
-
-data Vars
-  = Vars { varsBindings :: M.Map Name Term }
-
-emptyVars :: Vars 
-emptyVars = Vars { varsBindings = M.empty }
-
-
-data Globals = Globals {
-  psVars :: IORef Vars
-}
-
-newtype PrologIO a = PrologIO (Globals -> IO a)
-
-instance Functor PrologIO where
-  fmap :: (a -> b) -> PrologIO a -> PrologIO b
-  fmap f (PrologIO a) = PrologIO (\globals -> fmap f (a globals))
-
-instance Applicative PrologIO where
-  pure :: a -> PrologIO a
-  pure x = PrologIO (\_ -> pure x)
-
-  (PrologIO f) <*> (PrologIO x) = PrologIO (
-    \globals -> do
-      f' <- f globals
-      x' <- x globals
-      pure (f' x')
-    )
-
-instance Monad PrologIO where
-  return :: a -> PrologIO a
-  return = pure
-
-  (PrologIO a) >>= f = PrologIO (
-    \globals -> do
-      a' <- a globals
-      let PrologIO b' = f a'
-      b' globals
-    )
-
-runPrologIO :: Globals -> PrologIO a -> IO a
-runPrologIO globals (PrologIO a) = a globals
-
-data PrologStream a 
-  = PYes (Maybe a) (PrologIO (PrologStream a))
-  | PNo
-
-instance Functor PrologStream where
-  fmap :: (a -> b) -> PrologStream a -> PrologStream b
-  fmap f (PYes a next) = PYes (fmap f a) (fmap (fmap f) next)
-  fmap _ PNo = PNo
-
-instance Monad PrologStream where
-  return :: a -> PrologStream a
-  return = pure
-
-  (>>=) :: PrologStream a -> (a -> PrologStream b) -> PrologStream b
-  x >>= f = join_ (fmap f x)
-
-instance Applicative PrologStream where
-  pure :: a -> PrologStream a
-  pure x = PYes (Just x) (pure PNo)
-
-  (<*>) :: PrologStream (a -> b) -> PrologStream a -> PrologStream b
-  f <*> x = f >>= (\f' -> x >>= (\x' -> return (f' x')))
-
-
-join_ :: PrologStream (PrologStream a) -> PrologStream a
-join_ (PNo) = PNo
-join_ (PYes Nothing rest) = PYes Nothing (fmap join_ rest)
-join_ (PYes (Just PNo) rest) = PYes Nothing (fmap join_ rest)
-join_ (PYes (Just (PYes a xs)) rest) = 
-  let joinedRest = liftM2 append xs (fmap join_ rest) in
-  PYes a joinedRest
-
-append :: PrologStream a -> PrologStream a -> PrologStream a
-append (PYes x xs) next = PYes x (liftM2 append xs (return next))
-append PNo next = next
-
-
-data Thread a = Thread 
-  { threadGlobals :: Globals
-  , threadStream :: IORef (PrologStream a)
-  }
-
-startThread :: PrologStream a -> IO (Thread a)
-startThread stream = do
-  vars <- newIORef emptyVars
-  streamRef <- newIORef stream 
-  let globals = Globals { psVars = vars }
-  return (Thread { threadGlobals = globals, threadStream = streamRef })
-
-advanceThread :: Thread a -> IO (Maybe a)
-advanceThread (Thread { threadGlobals = globals, threadStream = streamRef }) = do
-  stream <- readIORef streamRef
-  (value, newStream) <- runPrologIO globals (squeeze stream)
-  writeIORef streamRef newStream
-  return value
-
-
-squeeze :: PrologStream a -> PrologIO (Maybe a, PrologStream a)
-squeeze PNo = return (Nothing, PNo)
-squeeze (PYes (Nothing) rest) = rest >>= squeeze
-squeeze (PYes (Just x) rest) = fmap ((,) (Just x)) rest 
-
-
-main :: IO ()
-main = do
-  putStrLn "Hello, world!"
-{-
-no :: Prolog a
-no = Prolog (\_ -> [])
-
-readVars :: Prolog Vars
-readVars = Prolog (\vars -> [(vars, vars)])
-
-writeVars :: Vars -> Prolog ()
-writeVars vars = Prolog (\_ -> [((), vars)])
-
-unify :: Term -> Term -> Prolog ()
-unify = unify_ []
-  where
-  unify_ :: [Name] -> Term -> Term -> Prolog ()
-  unify_ names (TCompound h0 args0) (TCompound h1 args1) = do
-    when (h0 /= h1) no
-    when (length args0 /= length args1) no
-    forM_ (zip args0 args1) $ \(arg0, arg1) -> unify_ names arg0 arg1
-  unify_ _ (TVariable name0) (TVariable name1) | name0 == name1 = return ()
-  unify_ names (TVariable name) y = do
-    -- occurs check
-    when (name `elem` names) no
-    -- unify as usual
-    (Vars vs0) <- readVars
-    case M.lookup name vs0 of
-      Just existing -> unify_ (name:names) existing y
-      Nothing -> do
-        let vs1 = M.insert name y vs0
-        writeVars (Vars vs1)
-  unify_ names x@(TCompound _ _) y@(TVariable _) = unify_ names y x
-
-
-
-
-
-main :: IO ()
-main = putStrLn "Hello, Haskell!"
-
--}

app/Nondeterminism.hs

@@ -0,0 +1,104 @@
+{-# LANGUAGE InstanceSigs #-}
+module Nondeterminism
+( PrologStream
+, newThread
+, takeNextSolution, takeAllRemainingSolutions
+, reversibly
+, liftPrologIO
+) where
+import Primitives (Globals, PrologIO(..), newGlobals)
+import Control.Applicative
+import Control.Monad
+import Classes (LiftIO (..))
+import GHC.IORef
+
+data Suspend a = Yield a | Suspend
+
+instance Functor Suspend where
+  fmap f (Yield a) = Yield (f a)
+  fmap _ Suspend = Suspend
+
+data PrologStream a 
+  = PYes (Suspend a) (PrologIO (PrologStream a))
+  | PNo
+
+instance Functor PrologStream where
+  fmap :: (a -> b) -> PrologStream a -> PrologStream b
+  fmap f (PYes a next) = PYes (fmap f a) (fmap (fmap f) next)
+  fmap _ PNo = PNo
+
+instance Applicative PrologStream where
+  pure :: a -> PrologStream a
+  pure x = PYes (Yield x) (pure PNo)
+
+  (<*>) :: PrologStream (a -> b) -> PrologStream a -> PrologStream b
+  f <*> x = f >>= (\f' -> x >>= (\x' -> return (f' x')))
+
+instance Monad PrologStream where
+  return :: a -> PrologStream a
+  return = pure
+
+  (>>=) :: PrologStream a -> (a -> PrologStream b) -> PrologStream b
+  PNo >>= _ = PNo
+  PYes (Yield x) xs >>= f = f x <|> PYes Suspend (fmap (>>= f) xs)
+  PYes Suspend xs >>= f = PYes Suspend (fmap (>>= f) xs)
+
+instance Alternative PrologStream where 
+  empty :: PrologStream a
+  empty = PNo
+
+  (<|>) :: PrologStream a -> PrologStream a -> PrologStream a
+  (PYes x xs) <|> next = PYes x (liftM2 (<|>) xs (return next))
+  PNo <|> next = next
+
+instance MonadPlus PrologStream where
+  mzero :: PrologStream a
+  mzero = empty
+
+  mplus :: PrologStream a -> PrologStream a -> PrologStream a
+  mplus = (<|>)
+
+data Thread a = Thread
+  { threadGlobals :: Globals
+  , threadStream :: IORef (PrologStream a)
+  }
+
+newThread :: PrologStream a -> IO (Thread a)
+newThread stream = do
+  globals <- newGlobals
+  streamRef <- newIORef stream
+  return $ Thread 
+    { threadGlobals = globals
+    , threadStream = streamRef
+    }
+
+takeNextSolution :: Thread a -> IO (Maybe a)
+takeNextSolution (Thread { threadGlobals = globals, threadStream = streamRef }) = do
+  stream <- readIORef streamRef
+  (value, newStream) <- runPrologIO (squeeze stream) globals
+  writeIORef streamRef newStream
+  return value
+
+takeAllRemainingSolutions :: Thread a -> IO [a]
+takeAllRemainingSolutions thread = do
+  value <- takeNextSolution thread
+  case value of
+    Nothing -> return []
+    Just x -> fmap (x:) (takeAllRemainingSolutions thread)
+
+squeeze :: PrologStream a -> PrologIO (Maybe a, PrologStream a)
+squeeze PNo = return (Nothing, PNo)
+squeeze (PYes Suspend rest) = rest >>= squeeze
+squeeze (PYes (Yield x) rest) = fmap ((,) (Just x)) rest
+
+reversibly :: PrologIO a -> PrologIO () -> PrologStream a
+reversibly forward backward =
+  PYes Suspend $ forward >>= \v -> return $
+    PYes (Yield v) $ backward >>= \_ -> return $
+      PNo
+
+liftPrologIO :: PrologIO a -> PrologStream a
+liftPrologIO forward = reversibly forward (return ())
+
+instance LiftIO PrologStream where
+  liftIO x = liftPrologIO (liftIO x)

app/Primitives.hs

@@ -0,0 +1,85 @@
+{-# LANGUAGE InstanceSigs #-}
+module Primitives
+  ( PrologIO(..)
+  , Globals
+  , newGlobals
+  , newVarPrim, unnewVarPrim
+  , getVarPrim, setVarPrim
+  , errorPrim
+  )
+where
+
+import Data.IORef
+import qualified Data.IntMap.Strict as M
+import Terms
+import Classes (LiftIO(..))
+
+newtype PrologIO a = PrologIO { runPrologIO :: Globals -> IO a }
+
+instance Functor PrologIO where
+  fmap :: (a -> b) -> PrologIO a -> PrologIO b
+  fmap f (PrologIO a) = PrologIO (\globals -> fmap f (a globals))
+
+instance Applicative PrologIO where
+  pure :: a -> PrologIO a
+  pure x = PrologIO (\_ -> pure x)
+
+  (PrologIO f) <*> (PrologIO x) = PrologIO (
+    \globals -> do
+      f' <- f globals
+      x' <- x globals
+      pure (f' x')
+    )
+
+instance Monad PrologIO where
+  return :: a -> PrologIO a
+  return = pure
+
+  (PrologIO a) >>= f = PrologIO (
+    \globals -> do
+      a' <- a globals
+      let PrologIO b' = f a'
+      b' globals
+    )
+
+data Globals = Globals
+  { values :: IORef (M.IntMap Term)
+  , names :: IORef (M.IntMap Name)
+  , nextVar :: IORef Int
+  }
+
+newGlobals :: IO (Globals)
+newGlobals = do
+  vs <- newIORef (M.empty)
+  ns <- newIORef (M.empty)
+  next <- newIORef 0
+  return $ Globals { values = vs, names = ns, nextVar = next }
+
+newVarPrim :: Name -> PrologIO Var
+newVarPrim name = PrologIO $ \globals -> do
+  var <- readIORef (nextVar globals) 
+  modifyIORef (names globals) (M.insert var name)
+  modifyIORef (nextVar globals) succ
+  return (VInt var)
+
+unnewVarPrim :: PrologIO ()
+unnewVarPrim = PrologIO $ \globals -> do
+  modifyIORef (nextVar globals) pred
+  var <- readIORef (nextVar globals) 
+  modifyIORef (names globals) (M.delete var)
+  return ()
+
+getVarPrim :: Var -> PrologIO (Maybe Term) 
+getVarPrim (VInt var) = PrologIO $ \globals ->
+  fmap (M.lookup var) (readIORef (values globals))
+
+setVarPrim :: Var -> Maybe Term -> PrologIO ()
+setVarPrim (VInt var) term = PrologIO $ \globals -> do
+  modifyIORef (values globals) (M.alter (\_ -> term) var)
+  return ()
+
+errorPrim :: String -> PrologIO a
+errorPrim message = error message
+
+instance LiftIO PrologIO where 
+  liftIO x = PrologIO (\_ -> x)

app/Terms.hs

@@ -0,0 +1,17 @@
+module Terms(
+    Atom,
+    Name,
+    Term(..),
+    Var(..),
+) where
+
+type Atom = String
+type Name = String
+
+data Term 
+  = Compound Atom [Term] -- functor, args
+  | Var Var  
+  deriving (Eq, Show)
+
+newtype Var = VInt Int
+  deriving (Ord, Eq, Show)

prolog-in-haskell.cabal

@@ -61,13 +61,13 @@ executable prolog-in-haskell
     main-is:          Main.hs
 
     -- Modules included in this executable, other than Main.
-    -- other-modules:
+    other-modules:    Binding, Classes, Database, Nondeterminism, Primitives, Terms
 
     -- LANGUAGE extensions used by modules in this package.
     -- other-extensions:
 
     -- Other library packages from which modules are imported.
-    build-depends:    base ^>=4.20.0.0, containers^>=0.8, mtl^>=2.3
+    build-depends:    base ^>=4.20.0.0, containers^>=0.8
 
     -- Directories containing source files.
     hs-source-dirs:   app