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futhark-0.15.2: src/Language/Futhark/Interpreter.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE OverloadedStrings #-}
module Language.Futhark.Interpreter
  ( Ctx(..)
  , Env
  , InterpreterError
  , initialCtx
  , interpretExp
  , interpretDec
  , interpretImport
  , interpretFunction
  , ExtOp(..)
  , StackFrame(..)
  , typeCheckerEnv
  , Value (ValuePrim, ValueArray, ValueRecord)
  , fromTuple
  , isEmptyArray
  , prettyEmptyArray
  ) where

import Control.Monad.Trans.Maybe
import Control.Monad.Free.Church
import Control.Monad.Except
import Control.Monad.State
import Control.Monad.Reader
import Data.Array
import Data.Bifunctor (first)
import Data.List hiding (break)
import Data.Maybe
import qualified Data.Map as M
import qualified Data.List.NonEmpty as NE
import Data.Monoid hiding (Sum)
import Data.Loc

import Language.Futhark hiding (Value, matchDims)
import qualified Language.Futhark as F
import Futhark.Representation.Primitive (intValue, floatValue)
import qualified Futhark.Representation.Primitive as P
import qualified Language.Futhark.Semantic as T

import Futhark.Util.Pretty hiding (apply, bool)
import Futhark.Util (chunk, splitFromEnd, maybeHead)

import Prelude hiding (mod, break)

data StackFrame = StackFrame { stackFrameLoc :: Loc
                             , stackFrameCtx :: Ctx
                             }

instance Located StackFrame where
  locOf = stackFrameLoc

data ExtOp a = ExtOpTrace Loc String a
             | ExtOpBreak (NE.NonEmpty StackFrame) a
             | ExtOpError InterpreterError

instance Functor ExtOp where
  fmap f (ExtOpTrace w s x) = ExtOpTrace w s $ f x
  fmap f (ExtOpBreak backtrace x) = ExtOpBreak backtrace $ f x
  fmap _ (ExtOpError err) = ExtOpError err

type Stack = [StackFrame]

type Sizes = M.Map VName Int32

-- | The monad in which evaluation takes place.
newtype EvalM a = EvalM (ReaderT (Stack, M.Map FilePath Env)
                         (StateT Sizes (F ExtOp)) a)
  deriving (Monad, Applicative, Functor,
            MonadFree ExtOp,
            MonadReader (Stack, M.Map FilePath Env),
            MonadState Sizes)

runEvalM :: M.Map FilePath Env -> EvalM a -> F ExtOp a
runEvalM imports (EvalM m) = evalStateT (runReaderT m (mempty, imports)) mempty

stacking :: SrcLoc -> Env -> EvalM a -> EvalM a
stacking loc env = local $ \(ss, imports) ->
  if isNoLoc loc
  then (ss, imports)
  else let s = StackFrame (locOf loc) (Ctx env imports)
       in (s:ss, imports)
  where isNoLoc :: SrcLoc -> Bool
        isNoLoc = (==NoLoc) . locOf

stacktrace :: EvalM [Loc]
stacktrace = asks $ map stackFrameLoc . fst

lookupImport :: FilePath -> EvalM (Maybe Env)
lookupImport f = asks $ M.lookup f . snd

putExtSize :: VName -> Int32 -> EvalM ()
putExtSize v x = modify $ M.insert v x

getSizes :: EvalM Sizes
getSizes = get

extSizeEnv :: EvalM Env
extSizeEnv = i32Env <$> getSizes

prettyRecord :: Pretty a => M.Map Name a -> Doc
prettyRecord m
  | Just vs <- areTupleFields m =
      parens $ commasep $ map ppr vs
  | otherwise =
      braces $ commasep $ map field $ M.toList m
      where field (k, v) = ppr k <+> equals <+> ppr v

valueStructType :: ValueType -> StructType
valueStructType = first (ConstDim . fromIntegral)

-- | A shape is a tree to accomodate the case of records.  It is
-- parameterised over the representation of dimensions.
data Shape d = ShapeDim d (Shape d)
             | ShapeLeaf
             | ShapeRecord (M.Map Name (Shape d))
             | ShapeSum (M.Map Name [Shape d])
             deriving (Eq, Show, Functor, Foldable, Traversable)

type ValueShape = Shape Int32

instance Pretty d => Pretty (Shape d) where
  ppr ShapeLeaf = mempty
  ppr (ShapeDim d s) = brackets (ppr d) <> ppr s
  ppr (ShapeRecord m) = prettyRecord m
  ppr (ShapeSum cs) =
    mconcat (punctuate (text " | ") cs')
    where ppConstr (name, fs) = sep $ (text "#" <> ppr name) : map ppr fs
          cs' = map ppConstr $ M.toList cs

emptyShape :: ValueShape -> Bool
emptyShape (ShapeDim d s) = d == 0 || emptyShape s
emptyShape _ = False

typeShape :: M.Map VName (Shape d) -> TypeBase d () -> Shape d
typeShape shapes = go
  where go (Array _ _ et shape) =
          foldr ShapeDim (go (Scalar et)) $ shapeDims shape
        go (Scalar (Record fs)) =
          ShapeRecord $ M.map go fs
        go (Scalar (Sum cs)) =
          ShapeSum $ M.map (map go) cs
        go (Scalar (TypeVar _ _ (TypeName [] v) []))
          | Just shape <- M.lookup v shapes =
              shape
        go _ =
          ShapeLeaf

structTypeShape :: M.Map VName ValueShape -> StructType -> Shape (Maybe Int32)
structTypeShape shapes = fmap dim . typeShape shapes'
  where dim (ConstDim d) = Just $ fromIntegral d
        dim _ = Nothing
        shapes' = M.map (fmap $ ConstDim . fromIntegral) shapes

resolveTypeParams :: [VName] -> StructType -> StructType -> Env
resolveTypeParams names = match
  where match (Scalar (TypeVar _ _ tn _)) t
          | typeLeaf tn `elem` names =
              typeEnv $ M.singleton (typeLeaf tn) t
        match (Scalar (Record poly_fields)) (Scalar (Record fields)) =
          mconcat $ M.elems $
          M.intersectionWith match poly_fields fields
        match (Scalar (Sum poly_fields)) (Scalar (Sum fields)) =
          mconcat $ map mconcat $ M.elems $
          M.intersectionWith (zipWith match) poly_fields fields
        match (Scalar (Arrow _ _  poly_t1 poly_t2)) (Scalar (Arrow _ _ t1 t2)) =
          match poly_t1 t1 <> match poly_t2 t2
        match poly_t t
          | d1 : _ <- shapeDims (arrayShape poly_t),
            d2 : _ <- shapeDims (arrayShape t) =
              matchDims d1 d2 <> match (stripArray 1 poly_t) (stripArray 1 t)
        match _ _ = mempty

        matchDims (NamedDim (QualName _ d1)) (ConstDim d2)
          | d1 `elem` names =
              i32Env $ M.singleton d1 $ fromIntegral d2
        matchDims _ _ = mempty

resolveExistentials :: [VName] -> StructType -> ValueShape -> M.Map VName Int32
resolveExistentials names = match
  where match (Scalar (Record poly_fields)) (ShapeRecord fields) =
          mconcat $ M.elems $
          M.intersectionWith match poly_fields fields
        match (Scalar (Sum poly_fields)) (ShapeSum fields) =
          mconcat $ map mconcat $ M.elems $
          M.intersectionWith (zipWith match) poly_fields fields
        match poly_t (ShapeDim d2 rowshape)
          | d1 : _ <- shapeDims (arrayShape poly_t) =
              matchDims d1 d2 <> match (stripArray 1 poly_t) rowshape
        match _ _ = mempty

        matchDims (NamedDim (QualName _ d1)) d2
          | d1 `elem` names = M.singleton d1 d2
        matchDims _ _ = mempty

-- | A fully evaluated Futhark value.
data Value = ValuePrim !PrimValue
           | ValueArray ValueShape !(Array Int Value)
             -- Stores the full shape.
           | ValueRecord (M.Map Name Value)
           | ValueFun (Value -> EvalM Value)
           | ValueSum ValueShape Name [Value]
             -- Stores the full shape.

instance Eq Value where
  ValuePrim x == ValuePrim y = x == y
  ValueArray _ x == ValueArray _ y = x == y
  ValueRecord x == ValueRecord y = x == y
  ValueSum _ n1 vs1 == ValueSum _ n2 vs2 = n1 == n2 && vs1 == vs2
  _ == _ = False

instance Pretty Value where
  ppr (ValuePrim v)  = ppr v
  ppr (ValueArray _ a) =
    let elements  = elems a -- [Value]
        (x:_)     = elements
        separator = case x of
                      ValueArray _ _ -> comma <> line
                      _              -> comma <> space
     in brackets $ cat $ punctuate separator (map ppr elements)

  ppr (ValueRecord m) = prettyRecord m
  ppr ValueFun{} = text "#<fun>"
  ppr (ValueSum _ n vs) = text "#" <> sep (ppr n : map ppr vs)

valueShape :: Value -> ValueShape
valueShape (ValueArray shape _) = shape
valueShape (ValueRecord fs) = ShapeRecord $ M.map valueShape fs
valueShape (ValueSum shape _ _) = shape
valueShape _ = ShapeLeaf

checkShape :: Shape (Maybe Int32) -> ValueShape -> Maybe ValueShape
checkShape (ShapeDim Nothing shape1) (ShapeDim d2 shape2) =
  ShapeDim d2 <$> checkShape shape1 shape2
checkShape (ShapeDim (Just d1) shape1) (ShapeDim d2 shape2) = do
  guard $ d1 == d2
  ShapeDim d2 <$> checkShape shape1 shape2
checkShape (ShapeDim d1 shape1) ShapeLeaf =
  -- This case is for handling polymorphism, when a function doesn't
  -- know that the array it produced actually has more dimensions.
  ShapeDim (fromMaybe 0 d1) <$> checkShape shape1 ShapeLeaf
checkShape (ShapeRecord shapes1) (ShapeRecord shapes2) =
  ShapeRecord <$> sequence (M.intersectionWith checkShape shapes1 shapes2)
checkShape (ShapeRecord shapes1) ShapeLeaf =
  Just $ fromMaybe 0 <$> ShapeRecord shapes1
checkShape (ShapeSum shapes1) (ShapeSum shapes2) =
  ShapeSum <$> sequence (M.intersectionWith (zipWithM checkShape) shapes1 shapes2)
checkShape (ShapeSum shapes1) ShapeLeaf =
  Just $ fromMaybe 0 <$> ShapeSum shapes1
checkShape _ shape2 =
  Just shape2

-- | Does the value correspond to an empty array?
isEmptyArray :: Value -> Bool
isEmptyArray = emptyShape . valueShape

-- | String representation of an empty array with the provided element
-- type.  This is pretty ad-hoc - don't expect good results unless the
-- element type is a primitive.
prettyEmptyArray :: TypeBase () () -> Value -> String
prettyEmptyArray t v =
  "empty(" ++ dims (valueShape v) ++ pretty t' ++ ")"
  where t' = stripArray (arrayRank t) t
        dims (ShapeDim n rowshape) =
          "[" ++ pretty n ++ "]" ++ dims rowshape
        dims _ = ""

-- | Create an array value; failing if that would result in an
-- irregular array.
mkArray :: TypeBase Int32 () -> [Value] -> Maybe Value
mkArray t [] =
  return $ toArray (typeShape mempty t) []
mkArray _ (v:vs) = do
  let v_shape = valueShape v
  guard $ all ((==v_shape) . valueShape) vs
  return $ toArray' v_shape $ v:vs

arrayLength :: Integral int => Array Int Value -> int
arrayLength = fromIntegral . (+1) . snd . bounds

toTuple :: [Value] -> Value
toTuple = ValueRecord . M.fromList . zip tupleFieldNames

fromTuple :: Value -> Maybe [Value]
fromTuple (ValueRecord m) = areTupleFields m
fromTuple _ = Nothing

asInteger :: Value -> Integer
asInteger (ValuePrim (SignedValue v)) = P.valueIntegral v
asInteger (ValuePrim (UnsignedValue v)) =
  toInteger (P.valueIntegral (P.doZExt v Int64) :: Word64)
asInteger v = error $ "Unexpectedly not an integer: " ++ pretty v

asInt :: Value -> Int
asInt = fromIntegral . asInteger

asSigned :: Value -> IntValue
asSigned (ValuePrim (SignedValue v)) = v
asSigned v = error $ "Unexpected not a signed integer: " ++ pretty v

asInt32 :: Value -> Int32
asInt32 = fromIntegral . asInteger

asBool :: Value -> Bool
asBool (ValuePrim (BoolValue x)) = x
asBool v = error $ "Unexpectedly not a boolean: " ++ pretty v

lookupInEnv :: (Env -> M.Map VName x)
            -> QualName VName -> Env -> Maybe x
lookupInEnv onEnv qv env = f env $ qualQuals qv
  where f m (q:qs) =
          case M.lookup q $ envTerm m of
            Just (TermModule (Module mod)) -> f mod qs
            _ -> Nothing
        f m [] = M.lookup (qualLeaf qv) $ onEnv m

lookupVar :: QualName VName -> Env -> Maybe TermBinding
lookupVar = lookupInEnv envTerm

lookupType :: QualName VName -> Env -> Maybe T.TypeBinding
lookupType = lookupInEnv envType

-- | A TermValue with a 'Nothing' type annotation is an intrinsic.
data TermBinding = TermValue (Maybe T.BoundV) Value
                 | TermPoly (Maybe T.BoundV) (StructType -> EvalM Value)
                   -- ^ A polymorphic value that must be instantiated.
                 | TermModule Module

data Module = Module Env
            | ModuleFun (Module -> EvalM Module)

data Env = Env { envTerm :: M.Map VName TermBinding
               , envType :: M.Map VName T.TypeBinding
               , envShapes :: M.Map VName ValueShape
                 -- ^ A mapping from type parameters to the shapes of
                 -- the value to which they were initially bound.
               }

instance Monoid Env where
  mempty = Env mempty mempty mempty

instance Semigroup Env where
  Env vm1 tm1 sm1 <> Env vm2 tm2 sm2 =
    Env (vm1 <> vm2) (tm1 <> tm2) (sm1 <> sm2)

newtype InterpreterError = InterpreterError String

valEnv :: M.Map VName (Maybe T.BoundV, Value) -> Env
valEnv m = Env { envTerm = M.map (uncurry TermValue) m
               , envType = mempty
               , envShapes = mempty
               }

modEnv :: M.Map VName Module -> Env
modEnv m = Env { envTerm = M.map TermModule m
               , envType = mempty
               , envShapes = mempty
               }

typeEnv :: M.Map VName StructType -> Env
typeEnv m = Env { envTerm = mempty
                , envType = M.map tbind m
                , envShapes = mempty
                }
  where tbind = T.TypeAbbr Unlifted []

i32Env :: M.Map VName Int32 -> Env
i32Env = valEnv . M.map f
  where f x = (Just $ T.BoundV [] $ Scalar $ Prim $ Signed Int32,
               ValuePrim $ SignedValue $ Int32Value x)

instance Show InterpreterError where
  show (InterpreterError s) = s

bad :: SrcLoc -> Env -> String -> EvalM a
bad loc env s = stacking loc env $ do
  ss <- map (locStr . srclocOf) <$> stacktrace
  liftF $ ExtOpError $ InterpreterError $ "Error at\n" ++ prettyStacktrace 0 ss ++ s

trace :: Value -> EvalM ()
trace v = do
  -- We take the second-to-top element of the stack, because any
  -- actual call to 'implicits.trace' is going to be in the trace
  -- function in the prelude, which is not interesting.
  top <- fromMaybe noLoc . maybeHead . drop 1 <$> stacktrace
  liftF $ ExtOpTrace top (pretty v) ()

typeCheckerEnv :: Env -> T.Env
typeCheckerEnv env =
  -- FIXME: some shadowing issues are probably not right here.
  let valMap (TermValue (Just t) _) = Just t
      valMap _ = Nothing
      vtable = M.mapMaybe valMap $ envTerm env
      nameMap k | k `M.member` vtable = Just ((T.Term, baseName k), qualName k)
                | otherwise = Nothing
  in mempty { T.envNameMap = M.fromList $ mapMaybe nameMap $ M.keys $ envTerm env
            , T.envVtable = vtable }

break :: EvalM ()
break = do
  -- We don't want the env of the function that is calling
  -- intrinsics.break, since that is just going to be the boring
  -- wrapper function (intrinsics are never called directly).
  -- This is why we go a step up the stack.
  backtrace <- asks $ drop 1 . fst
  case NE.nonEmpty backtrace of
    Nothing -> return ()
    Just backtrace' -> liftF $ ExtOpBreak backtrace' ()

fromArray :: Value -> (ValueShape, [Value])
fromArray (ValueArray shape as) = (shape, elems as)
fromArray v = error $ "Expected array value, but found: " ++ pretty v

toArray :: ValueShape -> [Value] -> Value
toArray shape vs = ValueArray shape (listArray (0, length vs - 1) vs)

toArray' :: ValueShape -> [Value] -> Value
toArray' rowshape vs = ValueArray shape (listArray (0, length vs - 1) vs)
  where shape = ShapeDim (genericLength vs) rowshape

apply :: SrcLoc -> Env -> Value -> Value -> EvalM Value
apply loc env (ValueFun f) v = stacking loc env (f v)
apply _ _ f _ = error $ "Cannot apply non-function: " ++ pretty f

apply2 :: SrcLoc -> Env -> Value -> Value -> Value -> EvalM Value
apply2 loc env f x y = stacking loc env $ do f' <- apply noLoc mempty f x
                                             apply noLoc mempty f' y

matchPattern :: Env -> Pattern -> Value -> EvalM Env
matchPattern env p v = do
  m <- runMaybeT $ patternMatch env p v
  case m of
    Nothing   -> error $ "matchPattern: missing case for " ++ pretty p ++ " and " ++ pretty v
    Just env' -> return env'

patternMatch :: Env -> Pattern -> Value -> MaybeT EvalM Env
patternMatch env (Id v (Info t) _) val =
  lift $ pure $
  valEnv (M.singleton v (Just $ T.BoundV [] $ toStruct t, val)) <> env
patternMatch env Wildcard{} _ =
  lift $ pure env
patternMatch env (TuplePattern ps _) (ValueRecord vs)
  | length ps == length vs' =
      foldM (\env' (p,v) -> patternMatch env' p v) env $
      zip ps (map snd $ sortFields vs)
    where vs' = sortFields vs
patternMatch env (RecordPattern ps _) (ValueRecord vs)
  | length ps == length vs' =
      foldM (\env' (p,v) -> patternMatch env' p v) env $
      zip (map snd $ sortFields $ M.fromList ps) (map snd $ sortFields vs)
    where vs' = sortFields vs
patternMatch env (PatternParens p _) v = patternMatch env p v
patternMatch env (PatternAscription p _ _) v =
  patternMatch env p v
patternMatch env (PatternLit e _ _) v = do
  v' <- lift $ eval env e
  if v == v'
    then pure env
    else mzero
patternMatch env (PatternConstr n _ ps _) (ValueSum _ n' vs)
  | n == n' =
      foldM (\env' (p,v) -> patternMatch env' p v) env $ zip ps vs
patternMatch _ _ _ = mzero

data Indexing = IndexingFix Int32
              | IndexingSlice (Maybe Int32) (Maybe Int32) (Maybe Int32)

instance Pretty Indexing where
  ppr (IndexingFix i) = ppr i
  ppr (IndexingSlice i j (Just s)) =
    maybe mempty ppr i <> text ":" <>
    maybe mempty ppr j <> text ":" <>
    ppr s
  ppr (IndexingSlice i (Just j) s) =
    maybe mempty ppr i <> text ":" <>
    ppr j <>
    maybe mempty ((text ":" <>) . ppr) s
  ppr (IndexingSlice i Nothing Nothing) =
    maybe mempty ppr i <> text ":"

indexesFor :: Maybe Int32 -> Maybe Int32 -> Maybe Int32
           -> Int32 -> Maybe [Int]
indexesFor start end stride n
  | (start', end', stride') <- slice,
    end' == start' || signum' (end' - start') == signum' stride',
    stride' /= 0,
    is <- [start', start'+stride' .. end'-signum stride'],
    all inBounds is =
      Just $ map fromIntegral is
  | otherwise =
      Nothing
  where inBounds i = i >= 0 && i < n

        slice =
          case (start, end, stride) of
            (Just start', _, _) ->
              let end' = fromMaybe n end
              in (start', end', fromMaybe 1 stride)
            (Nothing, Just end', _) ->
              let start' = 0
              in (start', end', fromMaybe 1 stride)
            (Nothing, Nothing, Just stride') ->
              (if stride' > 0 then 0 else n-1,
               if stride' > 0 then n else -1,
               stride')
            (Nothing, Nothing, Nothing) ->
              (0, n, 1)

-- | 'signum', but with 0 as 1.
signum' :: (Eq p, Num p) => p -> p
signum' 0 = 1
signum' x = signum x

indexShape :: [Indexing] -> ValueShape -> ValueShape
indexShape (IndexingFix{}:is) (ShapeDim _ shape) =
  indexShape is shape
indexShape (IndexingSlice start end stride:is) (ShapeDim d shape) =
  ShapeDim n $ indexShape is shape
  where n = maybe 0 genericLength $ indexesFor start end stride d
indexShape _ shape =
  shape

indexArray :: [Indexing] -> Value -> Maybe Value
indexArray (IndexingFix i:is) (ValueArray _ arr)
  | i >= 0, i < n =
      indexArray is $ arr ! fromIntegral i
  | otherwise =
      Nothing
  where n = arrayLength arr
indexArray (IndexingSlice start end stride:is) (ValueArray (ShapeDim _ rowshape) arr) = do
  js <- indexesFor start end stride $ arrayLength arr
  toArray' (indexShape is rowshape) <$> mapM (indexArray is . (arr!)) js
indexArray _ v = Just v

updateArray :: [Indexing] -> Value -> Value -> Maybe Value
updateArray (IndexingFix i:is) (ValueArray shape arr) v
  | i >= 0, i < n = do
      v' <- updateArray is (arr ! i') v
      Just $ ValueArray shape $ arr // [(i', v')]
  | otherwise =
      Nothing
  where n = arrayLength arr
        i' = fromIntegral i
updateArray (IndexingSlice start end stride:is) (ValueArray shape arr) (ValueArray _ v) = do
  arr_is <- indexesFor start end stride $ arrayLength arr
  guard $ length arr_is == arrayLength v
  let update arr' (i, v') = do
        x <- updateArray is (arr!i) v'
        return $ arr' // [(i, x)]
  fmap (ValueArray shape) $ foldM update arr $ zip arr_is $ elems v
updateArray _ _ v = Just v

evalDimIndex :: Env -> DimIndex -> EvalM Indexing
evalDimIndex env (DimFix x) =
  IndexingFix . asInt32 <$> eval env x
evalDimIndex env (DimSlice start end stride) =
  IndexingSlice <$> traverse (fmap asInt32 . eval env) start
                <*> traverse (fmap asInt32 . eval env) end
                <*> traverse (fmap asInt32 . eval env) stride

evalIndex :: SrcLoc -> Env -> [Indexing] -> Value -> EvalM Value
evalIndex loc env is arr = do
  let oob = bad loc env $ "Index [" <> intercalate ", " (map pretty is) <>
            "] out of bounds for array of shape " <>
            pretty (valueShape arr) <> "."
  maybe oob return $ indexArray is arr

-- | Expand type based on information that was not available at
-- type-checking time (the structure of abstract types).
evalType :: Env -> StructType -> StructType
evalType _ (Scalar (Prim pt)) = Scalar $ Prim pt
evalType env (Scalar (Record fs)) = Scalar $ Record $ fmap (evalType env) fs
evalType env (Scalar (Arrow () p t1 t2)) =
  Scalar $ Arrow () p (evalType env t1) (evalType env t2)
evalType env t@(Array _ u _ shape) =
  let et = stripArray (shapeRank shape) t
      et' = evalType env et
      shape' = fmap evalDim shape
  in arrayOf et' shape' u
  where evalDim (NamedDim qn)
          | Just (TermValue _ (ValuePrim (SignedValue (Int32Value x)))) <-
              lookupVar qn env =
              ConstDim $ fromIntegral x
        evalDim d = d
evalType env t@(Scalar (TypeVar () _ tn args)) =
  case lookupType (qualNameFromTypeName tn) env of
    Just (T.TypeAbbr _ ps t') ->
      let (substs, types) = mconcat $ zipWith matchPtoA ps args
          onDim (NamedDim v) = fromMaybe (NamedDim v) $ M.lookup (qualLeaf v) substs
          onDim d = d
      in if null ps then first onDim t'
         else evalType (Env mempty types mempty <> env) $ first onDim t'
    Nothing -> t

  where matchPtoA (TypeParamDim p _) (TypeArgDim (NamedDim qv) _) =
          (M.singleton p $ NamedDim qv, mempty)
        matchPtoA (TypeParamDim p _) (TypeArgDim (ConstDim k) _) =
          (M.singleton p $ ConstDim k, mempty)
        matchPtoA (TypeParamType l p _) (TypeArgType t' _) =
          let t'' = evalType env t'
          in (mempty, M.singleton p $ T.TypeAbbr l [] t'')
        matchPtoA _ _ = mempty
evalType env (Scalar (Sum cs)) = Scalar $ Sum $ (fmap . fmap) (evalType env) cs

evalTermVar :: Env -> QualName VName -> StructType -> EvalM Value
evalTermVar env qv t =
  case lookupVar qv env of
    Just (TermPoly _ v) -> do size_env <- extSizeEnv
                              v $ evalType (size_env <> env) t
    Just (TermValue _ v) -> return v
    _ -> error $ "`" <> pretty qv <> "` is not bound to a value."

typeValueShape :: Env -> StructType -> EvalM ValueShape
typeValueShape env t = do
  size_env <- extSizeEnv
  let t' = evalType (size_env <> env) t
  case traverse dim $ typeShape mempty t' of
    Nothing -> error $ "typeValueShape: failed to fully evaluate type " ++ pretty t'
    Just shape -> return shape
  where dim (ConstDim x) = Just $ fromIntegral x
        dim _ = Nothing

evalFunction :: Env -> [VName] -> [Pattern] -> Exp -> StructType -> EvalM Value

-- We treat zero-parameter lambdas as simply an expression to
-- evaluate immediately.  Note that this is *not* the same as a lambda
-- that takes an empty tuple '()' as argument!  Zero-parameter lambdas
-- can never occur in a well-formed Futhark program, but they are
-- convenient in the interpreter.
evalFunction env _ [] body rettype =
  -- Eta-expand the rest to make any sizes visible.
  etaExpand [] env rettype
  where etaExpand vs env' (Scalar (Arrow _ _ pt rt)) =
          return $ ValueFun $ \v -> do
          env'' <- matchPattern env' (Wildcard (Info $ fromStruct pt) noLoc) v
          etaExpand (v:vs) env'' rt
        etaExpand vs env' _ = do
          f <- eval env' body
          foldM (apply noLoc mempty) f $ reverse vs

evalFunction env missing_sizes (p:ps) body rettype =
  return $ ValueFun $ \v -> do
    env' <- matchPattern env p v
    -- Fix up the last sizes, if any.
    let env'' | null missing_sizes = env'
              | otherwise = env' <>
                            i32Env (resolveExistentials missing_sizes
                                    (patternStructType p) (valueShape v))
    evalFunction env'' missing_sizes ps body rettype

evalFunctionBinding :: Env
                    -> [TypeParam] -> [Pattern] -> StructType -> [VName] -> Exp
                    -> EvalM TermBinding
evalFunctionBinding env tparams ps ret retext fbody = do
  let ret' = evalType env ret
      arrow (xp, xt) yt = Scalar $ Arrow () xp xt yt
      ftype = foldr (arrow . patternParam) ret' ps

  -- Distinguish polymorphic and non-polymorphic bindings here.
  if null tparams
  then TermValue (Just $ T.BoundV [] ftype) <$>
       (returned env ret retext =<< evalFunction env [] ps fbody ret')
  else return $ TermPoly (Just $ T.BoundV [] ftype) $ \ftype' -> do
         let tparam_names = map typeParamName tparams
             env' = resolveTypeParams tparam_names ftype ftype' <> env

             -- In some cases (abstract lifted types) there may be
             -- missing sizes that were not fixed by the type
             -- instantiation.  These will have to be set by looking
             -- at the actual function arguments.
             missing_sizes =
               filter (`M.notMember` envTerm env') $
               map typeParamName (filter isSizeParam tparams)
         returned env ret retext =<< evalFunction env' missing_sizes ps fbody ret'

evalArg :: Env -> Exp -> Maybe VName -> EvalM Value
evalArg env e ext = do
  v <- eval env e
  case ext of Just ext' -> putExtSize ext' $ asInt32 v
              Nothing -> return ()
  return v

returned :: Env -> TypeBase (DimDecl VName) als -> [VName] -> Value -> EvalM Value
returned _ _ [] v = return v
returned env ret retext v = do
  mapM_ (uncurry putExtSize) $ M.toList $
    resolveExistentials retext (evalType env $ toStruct ret) $ valueShape v
  return v

eval :: Env -> Exp -> EvalM Value

eval _ (Literal v _) = return $ ValuePrim v

eval env (Parens e _ ) = eval env e

eval env (QualParens _ e _ ) = eval env e

eval env (TupLit vs _) = toTuple <$> mapM (eval env) vs

eval env (RecordLit fields _) =
  ValueRecord . M.fromList <$> mapM evalField fields
  where evalField (RecordFieldExplicit k e _) = do
          v <- eval env e
          return (k, v)
        evalField (RecordFieldImplicit k t loc) = do
          v <- eval env $ Var (qualName k) t loc
          return (baseName k, v)

eval _ (StringLit vs _) =
  return $ toArray' ShapeLeaf $
  map (ValuePrim . UnsignedValue . Int8Value . fromIntegral) vs

eval env (ArrayLit [] (Info t) _) = do
  t' <- typeValueShape env $ toStruct t
  return $ toArray t' []

eval env (ArrayLit (v:vs) _ _) = do
  v' <- eval env v
  vs' <- mapM (eval env) vs
  return $ toArray' (valueShape v') (v':vs')

eval env (Range start maybe_second end (Info t, Info retext) loc) = do
  start' <- asInteger <$> eval env start
  maybe_second' <- traverse (fmap asInteger . eval env) maybe_second
  end' <- traverse (fmap asInteger . eval env) end

  let (end_adj, step, ok) =
        case (end', maybe_second') of
          (DownToExclusive end'', Nothing) ->
            (end'' + 1, -1, start' >= end'')
          (DownToExclusive end'', Just second') ->
            (end'' + 1, second' - start', start' >= end'' && second' < start')

          (ToInclusive end'', Nothing) ->
            (end'', 1, start' <= end'')
          (ToInclusive end'', Just second')
            | second' > start' ->
                (end'', second' - start', start' <= end'')
            | otherwise ->
                (end'', second' - start', start' >= end'' && second' /= start')

          (UpToExclusive x, Nothing) ->
            (x-1, 1, start' <= x)
          (UpToExclusive x, Just second') ->
            (x-1, second' - start', start' <= x && second' > start')

  if ok
    then returned env t retext $
         toArray' ShapeLeaf $ map toInt [start',start'+step..end_adj]
    else bad loc env $ badRange start' maybe_second' end'

  where toInt =
          case stripArray 1 t of
            Scalar (Prim (Signed t')) ->
              ValuePrim . SignedValue . intValue t'
            Scalar (Prim (Unsigned t')) ->
              ValuePrim . UnsignedValue . intValue t'
            _ -> error $ "Nonsensical range type: " ++ show t

        badRange start' maybe_second' end' =
          "Range " ++ pretty start' ++
          (case maybe_second' of
             Nothing -> ""
             Just second' -> ".." ++ pretty second') ++
          (case end' of
             DownToExclusive x -> "..>" ++ pretty x
             ToInclusive x -> "..." ++ pretty x
             UpToExclusive x -> "..<"++ pretty x) ++
          " is invalid."

eval env (Var qv (Info t) _) = evalTermVar env qv (toStruct t)

eval env (Ascript e _ _ ) = eval env e

eval env (Coerce e td (Info ret, Info retext) loc) = do
  v <- returned env ret retext =<< eval env e
  let t = evalType env $ unInfo $ expandedType td
  case checkShape (structTypeShape (envShapes env) t) (valueShape v) of
    Just _ -> return v
    Nothing ->
      bad loc env $ "Value `" <> pretty v <> "` of shape `" ++ pretty (valueShape v) ++
      "` cannot match shape of type `" <>
      pretty (declaredType td) <> "` (`" <> pretty t <> "`)"

eval env (LetPat p e body (Info ret, Info retext) _) = do
  v <- eval env e
  env' <- matchPattern env p v
  returned env ret retext =<< eval env' body

eval env (LetFun f (tparams, ps, _, Info ret, fbody) body _ _) = do
  binding <- evalFunctionBinding env tparams ps ret [] fbody
  eval (env { envTerm = M.insert f binding $ envTerm env }) body

eval _ (IntLit v (Info t) _) =
  case t of
    Scalar (Prim (Signed it)) ->
      return $ ValuePrim $ SignedValue $ intValue it v
    Scalar (Prim (Unsigned it)) ->
      return $ ValuePrim $ UnsignedValue $ intValue it v
    Scalar (Prim (FloatType ft)) ->
      return $ ValuePrim $ FloatValue $ floatValue ft v
    _ -> error $ "eval: nonsensical type for integer literal: " ++ pretty t

eval _ (FloatLit v (Info t) _) =
  case t of
    Scalar (Prim (FloatType ft)) ->
      return $ ValuePrim $ FloatValue $ floatValue ft v
    _ -> error $ "eval: nonsensical type for float literal: " ++ pretty t

eval env (BinOp (op, _) op_t
          (x, Info (_, xext)) (y, Info (_, yext))
          (Info t) (Info retext) loc)
  | baseString (qualLeaf op) == "&&" = do
      x' <- asBool <$> eval env x
      if x'
        then eval env y
        else return $ ValuePrim $ BoolValue False
  | baseString (qualLeaf op) == "||" = do
      x' <- asBool <$> eval env x
      if x'
        then return $ ValuePrim $ BoolValue True
        else eval env y
  | otherwise = do
      op' <- eval env $ Var op op_t loc
      x' <- evalArg env x xext
      y' <- evalArg env y yext
      returned env t retext =<< apply2 loc env op' x' y'

eval env (If cond e1 e2 (Info ret, Info retext) _) = do
  cond' <- asBool <$> eval env cond
  returned env ret retext =<<
    if cond' then eval env e1 else eval env e2

eval env (Apply f x (Info (_, ext)) (Info t, Info retext) loc) = do
  -- It is important that 'x' is evaluated first in order to bring any
  -- sizes into scope that may be used in the type of 'f'.
  x' <- evalArg env x ext
  f' <- eval env f
  returned env t retext =<< apply loc env f' x'

eval env (Negate e _) = do
  ev <- eval env e
  ValuePrim <$> case ev of
    ValuePrim (SignedValue (Int8Value v)) -> return $ SignedValue $ Int8Value (-v)
    ValuePrim (SignedValue (Int16Value v)) -> return $ SignedValue $ Int16Value (-v)
    ValuePrim (SignedValue (Int32Value v)) -> return $ SignedValue $ Int32Value (-v)
    ValuePrim (SignedValue (Int64Value v)) -> return $ SignedValue $ Int64Value (-v)
    ValuePrim (UnsignedValue (Int8Value v)) -> return $ UnsignedValue $ Int8Value (-v)
    ValuePrim (UnsignedValue (Int16Value v)) -> return $ UnsignedValue $ Int16Value (-v)
    ValuePrim (UnsignedValue (Int32Value v)) -> return $ UnsignedValue $ Int32Value (-v)
    ValuePrim (UnsignedValue (Int64Value v)) -> return $ UnsignedValue $ Int64Value (-v)
    ValuePrim (FloatValue (Float32Value v)) -> return $ FloatValue $ Float32Value (-v)
    ValuePrim (FloatValue (Float64Value v)) -> return $ FloatValue $ Float64Value (-v)
    _ -> error $ "Cannot negate " ++ pretty ev

eval env (Index e is (Info t, Info retext) loc) = do
  is' <- mapM (evalDimIndex env) is
  arr <- eval env e
  returned env t retext =<< evalIndex loc env is' arr

eval env (Update src is v loc) =
  maybe oob return =<<
  updateArray <$> mapM (evalDimIndex env) is <*> eval env src <*> eval env v
  where oob = bad loc env "Bad update"

eval env (RecordUpdate src all_fs v _ _) =
  update <$> eval env src <*> pure all_fs <*> eval env v
  where update _ [] v' = v'
        update (ValueRecord src') (f:fs) v'
          | Just f_v <- M.lookup f src' =
              ValueRecord $ M.insert f (update f_v fs v') src'
        update _ _ _ = error "eval RecordUpdate: invalid value."

eval env (LetWith dest src is v body _ loc) = do
  let Ident src_vn (Info src_t) _ = src
  dest' <- maybe oob return =<<
    updateArray <$> mapM (evalDimIndex env) is <*>
    evalTermVar env (qualName src_vn) (toStruct src_t) <*> eval env v
  let t = T.BoundV [] $ toStruct $ unInfo $ identType dest
  eval (valEnv (M.singleton (identName dest) (Just t, dest')) <> env) body
  where oob = bad loc env "Bad update"

-- We treat zero-parameter lambdas as simply an expression to
-- evaluate immediately.  Note that this is *not* the same as a lambda
-- that takes an empty tuple '()' as argument!  Zero-parameter lambdas
-- can never occur in a well-formed Futhark program, but they are
-- convenient in the interpreter.
eval env (Lambda ps body _ (Info (_, rt)) _) =
  evalFunction env [] ps body rt

eval env (OpSection qv (Info t) _) = evalTermVar env qv $ toStruct t

eval env (OpSectionLeft qv _ e (Info (_, argext), _) (Info t, Info retext) loc) = do
  v <- evalArg env e argext
  f <- evalTermVar env qv (toStruct t)
  returned env t retext =<< apply loc env f v

eval env (OpSectionRight qv _ e (Info _, Info (_, argext)) (Info t) loc) = do
  y <- evalArg env e argext
  return $ ValueFun $ \x -> do
    f <- evalTermVar env qv $ toStruct t
    apply2 loc env f x y

eval env (IndexSection is _ loc) = do
  is' <- mapM (evalDimIndex env) is
  return $ ValueFun $ evalIndex loc env is'

eval _ (ProjectSection ks _ _) =
  return $ ValueFun $ flip (foldM walk) ks
  where walk (ValueRecord fs) f
          | Just v' <- M.lookup f fs = return v'
        walk _ _ = error "Value does not have expected field."

eval env (DoLoop sparams pat init_e form body (Info (ret, retext)) _) = do
  init_v <- eval env init_e
  returned env ret retext =<<
    case form of For iv bound -> do
                   bound' <- asSigned <$> eval env bound
                   forLoop (identName iv) bound' (zero bound') init_v
                 ForIn in_pat in_e -> do
                   (_, in_vs) <- fromArray <$> eval env in_e
                   foldM (forInLoop in_pat) init_v in_vs
                 While cond ->
                   whileLoop cond init_v
  where withLoopParams v =
          let sparams' =
                resolveExistentials sparams
                (patternStructType pat) (valueShape v)
          in matchPattern (i32Env sparams' <> env) pat v

        inc = (`P.doAdd` Int64Value 1)
        zero = (`P.doMul` Int64Value 0)

        forLoop iv bound i v
          | i >= bound = return v
          | otherwise = do
              env' <- withLoopParams v
              forLoop iv bound (inc i) =<<
                eval (valEnv (M.singleton iv (Just $ T.BoundV [] $ Scalar $ Prim $ Signed Int32,
                                              ValuePrim (SignedValue i))) <> env') body

        whileLoop cond v = do
          env' <- withLoopParams v
          continue <- asBool <$> eval env' cond
          if continue
            then whileLoop cond =<< eval env' body
            else return v

        forInLoop in_pat v in_v = do
          env' <- withLoopParams v
          env'' <- matchPattern env' in_pat in_v
          eval env'' body

eval env (Project f e _ _) = do
  v <- eval env e
  case v of
    ValueRecord fs | Just v' <- M.lookup f fs -> return v'
    _ -> error "Value does not have expected field."

eval env (Unsafe e _) = eval env e

eval env (Assert what e (Info s) loc) = do
  cond <- asBool <$> eval env what
  unless cond $ bad loc env s
  eval env e

eval env (Constr c es (Info t) _) = do
  vs <- mapM (eval env) es
  shape <- typeValueShape env $ toStruct t
  return $ ValueSum shape c vs

eval env (Match e cs (Info ret, Info retext) _) = do
  v <- eval env e
  returned env ret retext =<< match v (NE.toList cs)
  where match _ [] =
          error "Pattern match failure."
        match v (c:cs') = do
          c' <- evalCase v env c
          case c' of
            Just v' -> return v'
            Nothing -> match v cs'

evalCase :: Value -> Env -> CaseBase Info VName
         -> EvalM (Maybe Value)
evalCase v env (CasePat p cExp _) = runMaybeT $ do
  env' <- patternMatch env p v
  lift $ eval env' cExp

substituteInModule :: M.Map VName VName -> Module -> Module
substituteInModule substs = onModule
  where
    rev_substs = reverseSubstitutions substs
    replace v = fromMaybe v $ M.lookup v rev_substs
    replaceQ v = maybe v qualName $ M.lookup (qualLeaf v) rev_substs
    replaceM f m = M.fromList $ do
      (k, v) <- M.toList m
      return (replace k, f v)
    onModule (Module (Env terms types _)) =
      Module $ Env (replaceM onTerm terms) (replaceM onType types) mempty
    onModule (ModuleFun f) =
      ModuleFun $ \m -> onModule <$> f (substituteInModule rev_substs m)
    onTerm (TermValue t v) = TermValue t v
    onTerm (TermPoly t v) = TermPoly t v
    onTerm (TermModule m) = TermModule $ onModule m
    onType (T.TypeAbbr l ps t) = T.TypeAbbr l ps $ first onDim t
    onDim (NamedDim v) = NamedDim $ replaceQ v
    onDim (ConstDim x) = ConstDim x
    onDim AnyDim = AnyDim

reverseSubstitutions :: M.Map VName VName -> M.Map VName VName
reverseSubstitutions = M.fromList . map (uncurry $ flip (,)) . M.toList

evalModuleVar :: Env -> QualName VName -> EvalM Module
evalModuleVar env qv =
  case lookupVar qv env of
    Just (TermModule m) -> return m
    _ -> error $ quote (pretty qv) <> " is not bound to a module."

evalModExp :: Env -> ModExp -> EvalM Module

evalModExp _ (ModImport _ (Info f) _) = do
  f' <- lookupImport f
  case f' of Nothing -> error $ "Unknown import " ++ show f
             Just m -> return $ Module m

evalModExp env (ModDecs ds _) = do
  Env terms types _ <- foldM evalDec env ds
  -- Remove everything that was present in the original Env.
  return $ Module $ Env (terms `M.difference` envTerm env)
                        (types `M.difference` envType env)
                        mempty

evalModExp env (ModVar qv _) =
  evalModuleVar env qv

evalModExp env (ModAscript me _ (Info substs) _) =
  substituteInModule substs <$> evalModExp env me

evalModExp env (ModParens me _) = evalModExp env me

evalModExp env (ModLambda p ret e loc) =
  return $ ModuleFun $ \am -> do
  let env' = env { envTerm = M.insert (modParamName p) (TermModule am) $ envTerm env }
  evalModExp env' $ case ret of
    Nothing -> e
    Just (se, rsubsts) -> ModAscript e se rsubsts loc

evalModExp env (ModApply f e (Info psubst) (Info rsubst) _) = do
  f' <- evalModExp env f
  case f' of
    ModuleFun f'' -> do
      e' <- evalModExp env e
      substituteInModule rsubst <$> f'' (substituteInModule psubst e')
    _ -> error "Expected ModuleFun."

evalDec :: Env -> Dec -> EvalM Env

evalDec env (ValDec (ValBind _ v _ (Info (ret, retext)) tparams ps fbody _ _)) = do
  binding <- evalFunctionBinding env tparams ps ret retext fbody
  return $ env { envTerm = M.insert v binding $ envTerm env }

evalDec env (OpenDec me _) = do
  me' <- evalModExp env me
  case me' of
    Module me'' -> return $ me'' <> env
    _ -> error "Expected Module"

evalDec env (ImportDec name name' loc) =
  evalDec env $ LocalDec (OpenDec (ModImport name name' loc) loc) loc

evalDec env (LocalDec d _) = evalDec env d
evalDec env SigDec{} = return env
evalDec env (TypeDec (TypeBind v l ps t _ _)) = do
  let abbr = T.TypeAbbr l ps $
             evalType env $ unInfo $ expandedType t
  return env { envType = M.insert v abbr $ envType env }
evalDec env (ModDec (ModBind v ps ret body _ loc)) = do
  mod <- evalModExp env $ wrapInLambda ps
  return $ modEnv (M.singleton v mod) <> env
  where wrapInLambda [] = case ret of
                            Just (se, substs) -> ModAscript body se substs loc
                            Nothing           -> body
        wrapInLambda [p] = ModLambda p ret body loc
        wrapInLambda (p:ps') = ModLambda p Nothing (wrapInLambda ps') loc

data Ctx = Ctx { ctxEnv :: Env
               , ctxImports :: M.Map FilePath Env
               }

-- | The initial environment contains definitions of the various intrinsic functions.
initialCtx :: Ctx
initialCtx =
  Ctx (Env (M.insert (VName (nameFromString "intrinsics") 0)
            (TermModule (Module $ Env terms types mempty)) terms)
        types mempty)
      mempty
  where
    terms = M.mapMaybeWithKey (const . def . baseString) intrinsics
    types = M.mapMaybeWithKey (const . tdef . baseString) intrinsics

    sintOp f = [ (getS, putS, P.doBinOp (f Int8))
               , (getS, putS, P.doBinOp (f Int16))
               , (getS, putS, P.doBinOp (f Int32))
               , (getS, putS, P.doBinOp (f Int64))]
    uintOp f = [ (getU, putU, P.doBinOp (f Int8))
               , (getU, putU, P.doBinOp (f Int16))
               , (getU, putU, P.doBinOp (f Int32))
               , (getU, putU, P.doBinOp (f Int64))]
    intOp f = sintOp f ++ uintOp f
    floatOp f = [ (getF, putF, P.doBinOp (f Float32))
                , (getF, putF, P.doBinOp (f Float64))]
    arithOp f g = Just $ bopDef $ intOp f ++ floatOp g

    flipCmps = map (\(f, g, h) -> (f, g, flip h))
    sintCmp f = [ (getS, Just . BoolValue, P.doCmpOp (f Int8))
                , (getS, Just . BoolValue, P.doCmpOp (f Int16))
                , (getS, Just . BoolValue, P.doCmpOp (f Int32))
                , (getS, Just . BoolValue, P.doCmpOp (f Int64))]
    uintCmp f = [ (getU, Just . BoolValue, P.doCmpOp (f Int8))
                , (getU, Just . BoolValue, P.doCmpOp (f Int16))
                , (getU, Just . BoolValue, P.doCmpOp (f Int32))
                , (getU, Just . BoolValue, P.doCmpOp (f Int64))]
    floatCmp f = [ (getF, Just . BoolValue, P.doCmpOp (f Float32))
                 , (getF, Just . BoolValue, P.doCmpOp (f Float64))]
    boolCmp f = [ (getB, Just . BoolValue, P.doCmpOp f) ]

    getV (SignedValue x) = Just $ P.IntValue x
    getV (UnsignedValue x) = Just $ P.IntValue x
    getV (FloatValue x) = Just $ P.FloatValue x
    getV (BoolValue x) = Just $ P.BoolValue x
    putV (P.IntValue x) = SignedValue x
    putV (P.FloatValue x) = FloatValue x
    putV (P.BoolValue x) = BoolValue x
    putV P.Checked = BoolValue True

    getS (SignedValue x) = Just $ P.IntValue x
    getS _               = Nothing
    putS (P.IntValue x) = Just $ SignedValue x
    putS _              = Nothing

    getU (UnsignedValue x) = Just $ P.IntValue x
    getU _                 = Nothing
    putU (P.IntValue x) = Just $ UnsignedValue x
    putU _              = Nothing

    getF (FloatValue x) = Just $ P.FloatValue x
    getF _              = Nothing
    putF (P.FloatValue x) = Just $ FloatValue x
    putF _                = Nothing

    getB (BoolValue x) = Just $ P.BoolValue x
    getB _             = Nothing
    putB (P.BoolValue x) = Just $ BoolValue x
    putB _               = Nothing

    fun1 f =
      TermValue Nothing $ ValueFun $ \x -> f x
    fun2 f =
      TermValue Nothing $ ValueFun $ \x ->
      return $ ValueFun $ \y -> f x y
    fun2t f =
      TermValue Nothing $ ValueFun $ \v ->
      case fromTuple v of Just [x,y] -> f x y
                          _ -> error $ "Expected pair; got: " ++ pretty v
    fun3t f =
      TermValue Nothing $ ValueFun $ \v ->
      case fromTuple v of Just [x,y,z] -> f x y z
                          _ -> error $ "Expected triple; got: " ++ pretty v

    fun6t f =
      TermValue Nothing $ ValueFun $ \v ->
      case fromTuple v of Just [x,y,z,a,b,c] -> f x y z a b c
                          _ -> error $ "Expected sextuple; got: " ++ pretty v

    bopDef fs = fun2 $ \x y ->
      case (x, y) of
        (ValuePrim x', ValuePrim y')
          | Just z <- msum $ map (`bopDef'` (x', y')) fs ->
              return $ ValuePrim z
        _ ->
          bad noLoc mempty $ "Cannot apply operator to arguments " <>
          quote (pretty x) <> " and " <> quote (pretty y) <> "."
      where bopDef' (valf, retf, op) (x, y) = do
              x' <- valf x
              y' <- valf y
              retf =<< op x' y'

    unopDef fs = fun1 $ \x ->
      case x of
        (ValuePrim x')
          | Just r <- msum $ map (`unopDef'` x') fs ->
              return $ ValuePrim r
        _ ->
          bad noLoc mempty $ "Cannot apply function to argument " <>
          quote (pretty x) <> "."
      where unopDef' (valf, retf, op) x = do
              x' <- valf x
              retf =<< op x'

    tbopDef f = fun1 $ \v ->
      case fromTuple v of
        Just [ValuePrim x, ValuePrim y]
          | Just x' <- getV x,
            Just y' <- getV y,
            Just z <- f x' y' ->
              return $ ValuePrim $ putV z
        _ ->
          bad noLoc mempty $ "Cannot apply operator to argument " <>
          quote (pretty v) <> "."

    def "!" = Just $ unopDef [ (getS, putS, P.doUnOp $ P.Complement Int8)
                             , (getS, putS, P.doUnOp $ P.Complement Int16)
                             , (getS, putS, P.doUnOp $ P.Complement Int32)
                             , (getS, putS, P.doUnOp $ P.Complement Int64)
                             , (getU, putU, P.doUnOp $ P.Complement Int8)
                             , (getU, putU, P.doUnOp $ P.Complement Int16)
                             , (getU, putU, P.doUnOp $ P.Complement Int32)
                             , (getU, putU, P.doUnOp $ P.Complement Int64)
                             , (getB, putB, P.doUnOp P.Not) ]

    def "+" = arithOp P.Add P.FAdd
    def "-" = arithOp P.Sub P.FSub
    def "*" = arithOp P.Mul P.FMul
    def "**" = arithOp P.Pow P.FPow
    def "/" = Just $ bopDef $ sintOp P.SDiv ++ uintOp P.UDiv ++ floatOp P.FDiv
    def "%" = Just $ bopDef $ sintOp P.SMod ++ uintOp P.UMod ++ floatOp P.FMod
    def "//" = Just $ bopDef $ sintOp P.SQuot ++ uintOp P.UDiv
    def "%%" = Just $ bopDef $ sintOp P.SRem ++ uintOp P.UMod
    def "^" = Just $ bopDef $ intOp P.Xor
    def "&" = Just $ bopDef $ intOp P.And
    def "|" = Just $ bopDef $ intOp P.Or
    def ">>" = Just $ bopDef $ sintOp P.AShr ++ uintOp P.LShr
    def "<<" = Just $ bopDef $ intOp P.Shl
    def ">>>" = Just $ bopDef $ sintOp P.LShr ++ uintOp P.LShr
    def "==" = Just $ fun2 $
               \xs ys -> return $ ValuePrim $ BoolValue $ xs == ys
    def "!=" = Just $ fun2 $
               \xs ys -> return $ ValuePrim $ BoolValue $ xs /= ys

    -- The short-circuiting is handled directly in 'eval'; these cases
    -- are only used when partially applying and such.
    def "&&" = Just $ fun2 $ \x y ->
      return $ ValuePrim $ BoolValue $ asBool x && asBool y
    def "||" = Just $ fun2 $ \x y ->
      return $ ValuePrim $ BoolValue $ asBool x || asBool y

    def "<" = Just $ bopDef $
              sintCmp P.CmpSlt ++ uintCmp P.CmpUlt ++
              floatCmp P.FCmpLt ++ boolCmp P.CmpLlt
    def ">" = Just $ bopDef $ flipCmps $
              sintCmp P.CmpSlt ++ uintCmp P.CmpUlt ++
              floatCmp P.FCmpLt ++ boolCmp P.CmpLlt
    def "<=" = Just $ bopDef $
               sintCmp P.CmpSle ++ uintCmp P.CmpUle ++
               floatCmp P.FCmpLe ++ boolCmp P.CmpLle
    def ">=" = Just $ bopDef $ flipCmps $
               sintCmp P.CmpSle ++ uintCmp P.CmpUle ++
               floatCmp P.FCmpLe ++ boolCmp P.CmpLle

    def s
      | Just bop <- find ((s==) . pretty) P.allBinOps =
          Just $ tbopDef $ P.doBinOp bop
      | Just unop <- find ((s==) . pretty) P.allCmpOps =
          Just $ tbopDef $ \x y -> P.BoolValue <$> P.doCmpOp unop x y
      | Just cop <- find ((s==) . pretty) P.allConvOps =
          Just $ unopDef [(getV, Just . putV, P.doConvOp cop)]
      | Just unop <- find ((s==) . pretty) P.allUnOps =
          Just $ unopDef [(getV, Just . putV, P.doUnOp unop)]

      | Just (pts, _, f) <- M.lookup s P.primFuns =
          case length pts of
            1 -> Just $ unopDef [(getV, Just . putV, f . pure)]
            _ -> Just $ fun1 $ \x -> do
              let getV' (ValuePrim v) = getV v
                  getV' _ = Nothing
              case f =<< mapM getV' =<< fromTuple x of
                Just res ->
                  return $ ValuePrim $ putV res
                _ ->
                  error $ "Cannot apply " ++ pretty s ++ " to " ++ pretty x

      | "sign_" `isPrefixOf` s =
          Just $ fun1 $ \x ->
          case x of (ValuePrim (UnsignedValue x')) ->
                      return $ ValuePrim $ SignedValue x'
                    _ -> error $ "Cannot sign: " ++ pretty x
      | "unsign_" `isPrefixOf` s =
          Just $ fun1 $ \x ->
          case x of (ValuePrim (SignedValue x')) ->
                      return $ ValuePrim $ UnsignedValue x'
                    _ -> error $ "Cannot unsign: " ++ pretty x

    def s | "map_stream" `isPrefixOf` s =
              Just $ fun2t stream

    def s | "reduce_stream" `isPrefixOf` s =
              Just $ fun3t $ \_ f arg -> stream f arg

    def "map" = Just $ TermPoly Nothing $ \t -> return $ ValueFun $ \v ->
      case (fromTuple v, unfoldFunType t) of
        (Just [f, xs], ([_], ret_t))
          | Just rowshape <- typeRowShape ret_t ->
              toArray' rowshape <$> mapM (apply noLoc mempty f) (snd $ fromArray xs)
          | otherwise ->
              error $ "Bad return type: " ++ pretty ret_t
        _ ->
          error $ "Invalid arguments to map intrinsic:\n" ++
          unlines [pretty t, pretty v]
      where typeRowShape = traverse id . structTypeShape mempty . stripArray 1

    def s | "reduce" `isPrefixOf` s = Just $ fun3t $ \f ne xs ->
      foldM (apply2 noLoc mempty f) ne $ snd $ fromArray xs

    def "scan" = Just $ fun3t $ \f ne xs -> do
      let next (out, acc) x = do
            x' <- apply2 noLoc mempty f acc x
            return (x':out, x')
      toArray' (valueShape ne) . reverse . fst <$>
        foldM next ([], ne) (snd $ fromArray xs)

    def "scatter" = Just $ fun3t $ \arr is vs ->
      case arr of
        ValueArray shape arr' ->
          return $ ValueArray shape $ foldl' update arr' $
          zip (map asInt $ snd $ fromArray is) (snd $ fromArray vs)
        _ ->
          error $ "scatter expects array, but got: " ++ pretty arr
      where update arr' (i, v) =
              if i >= 0 && i < arrayLength arr'
              then arr' // [(i, v)] else arr'

    def "hist" = Just $ fun6t $ \_ arr fun _ is vs ->
      case arr of
        ValueArray shape arr' ->
          ValueArray shape <$> foldM (update fun) arr'
          (zip (map asInt $ snd $ fromArray is) (snd $ fromArray vs))
        _ ->
          error $ "hist expects array, but got: " ++ pretty arr
      where update fun arr' (i, v) =
              if i >= 0 && i < arrayLength arr'
              then do
                v' <- apply2 noLoc mempty fun (arr' ! i) v
                return $ arr' // [(i, v')]
              else return arr'

    def "partition" = Just $ fun3t $ \k f xs -> do
      let (ShapeDim _ rowshape, xs') = fromArray xs

          next outs x = do
            i <- asInt <$> apply noLoc mempty f x
            return $ insertAt i x outs
          pack parts =
            toTuple [toArray' rowshape $ concat parts,
                     toArray' rowshape $
                     map (ValuePrim . SignedValue . Int32Value . genericLength) parts]

      pack . map reverse <$>
        foldM next (replicate (asInt k) []) xs'
      where insertAt 0 x (l:ls) = (x:l):ls
            insertAt i x (l:ls) = l:insertAt (i-1) x ls
            insertAt _ _ ls = ls

    def "cmp_threshold" = Just $ fun2t $ \_ _ ->
      return $ ValuePrim $ BoolValue True

    def "unzip" = Just $ fun1 $ \x -> do
      let ShapeDim _ (ShapeRecord fs) = valueShape x
          Just [xs_shape, ys_shape] = areTupleFields fs
          listPair (xs, ys) =
            [toArray' xs_shape xs, toArray' ys_shape ys]

      return $ toTuple $ listPair $ unzip $ map (fromPair . fromTuple) $ snd $ fromArray x
      where fromPair (Just [x,y]) = (x,y)
            fromPair l = error $ "Not a pair: " ++ pretty l

    def "zip" = Just $ fun2t $ \xs ys -> do
      let ShapeDim _ xs_rowshape = valueShape xs
          ShapeDim _ ys_rowshape = valueShape ys
      return $ toArray' (ShapeRecord (tupleFields [xs_rowshape, ys_rowshape])) $
        map toTuple $ transpose [snd $ fromArray xs, snd $ fromArray ys]

    def "concat" = Just $ fun2t $ \xs ys -> do
      let (ShapeDim _ rowshape, xs') = fromArray xs
          (_, ys') = fromArray ys
      return $ toArray' rowshape $ xs' ++ ys'

    def "transpose" = Just $ fun1 $ \xs -> do
      let (ShapeDim n (ShapeDim m shape), xs') = fromArray xs
      return $ toArray (ShapeDim m (ShapeDim n shape)) $
        map (toArray (ShapeDim n shape)) $ transpose $ map (snd . fromArray) xs'

    def "rotate" = Just $ fun2t $ \i xs -> do
      let (shape, xs') = fromArray xs
      return $
        if asInt i > 0
        then let (bef, aft) = splitAt (asInt i) xs'
             in toArray shape $ aft ++ bef
        else let (bef, aft) = splitFromEnd (-asInt i) xs'
             in toArray shape $ aft ++ bef

    def "flatten" = Just $ fun1 $ \xs -> do
      let (ShapeDim n (ShapeDim m shape), xs') = fromArray xs
      return $ toArray (ShapeDim (n*m) shape) $ concatMap (snd . fromArray) xs'

    def "unflatten" = Just $ fun3t $ \n m xs -> do
      let (ShapeDim _ innershape, xs') = fromArray xs
          rowshape = ShapeDim (asInt32 m) innershape
          shape = ShapeDim (asInt32 n) rowshape
      return $ toArray shape $ map (toArray rowshape) $ chunk (asInt m) xs'

    def "opaque" = Just $ fun1 return

    def "trace" = Just $ fun1 $ \v -> trace v >> return v

    def "break" = Just $ fun1 $ \v -> do
      break
      return v

    def s | nameFromString s `M.member` namesToPrimTypes = Nothing

    def s = error $ "Missing intrinsic: " ++ s

    tdef s = do
      t <- nameFromString s `M.lookup` namesToPrimTypes
      return $ T.TypeAbbr Unlifted [] $ Scalar $ Prim t

    stream f arg@(ValueArray _ xs) =
      let n = ValuePrim $ SignedValue $ Int32Value $ arrayLength xs
      in apply2 noLoc mempty f n arg
    stream _ arg = error $ "Cannot stream: " ++ pretty arg


interpretExp :: Ctx -> Exp -> F ExtOp Value
interpretExp ctx e = runEvalM (ctxImports ctx) $ eval (ctxEnv ctx) e

interpretDec :: Ctx -> Dec -> F ExtOp Ctx
interpretDec ctx d = do
  env <- runEvalM (ctxImports ctx) $ evalDec (ctxEnv ctx) d
  return ctx { ctxEnv = env }

interpretImport :: Ctx -> (FilePath, Prog) -> F ExtOp Ctx
interpretImport ctx (fp, prog) = do
  env <- runEvalM (ctxImports ctx) $ foldM evalDec (ctxEnv ctx) $ progDecs prog
  return ctx { ctxImports = M.insert fp env $ ctxImports ctx }

checkEntryArgs :: VName -> [F.Value] -> StructType -> Either String ()
checkEntryArgs entry args entry_t
  | args_ts == param_ts =
      return ()
  | otherwise =
      Left $ pretty $ expected </>
      "Got input of types" </>
      indent 2 (stack (map ppr args_ts))
  where (param_ts, _) = unfoldFunType entry_t
        args_ts = map (valueStructType . valueType) args
        expected
          | null param_ts =
              "Entry point " <> pquote (pprName entry) <> " is not a function."
          | otherwise =
              "Entry point " <> pquote (pprName entry) <> " expects input of type(s)" </>
              indent 2 (stack (map ppr param_ts))

-- | Execute the named function on the given arguments; may fail
-- horribly if these are ill-typed.
interpretFunction :: Ctx -> VName -> [F.Value] -> Either String (F ExtOp Value)
interpretFunction ctx fname vs = do
  ft <- case lookupVar (qualName fname) $ ctxEnv ctx of
          Just (TermValue (Just (T.BoundV _ t)) _) ->
            Right $ updateType (map valueType vs) t
          Just (TermPoly (Just (T.BoundV _ t)) _) ->
            Right $ updateType (map valueType vs) t
          _ ->
            Left $ "Unknown function `" <> prettyName fname <> "`."

  vs' <- case mapM convertValue vs of
           Just vs' -> Right vs'
           Nothing -> Left "Invalid input: irregular array."

  checkEntryArgs fname vs ft

  Right $ runEvalM (ctxImports ctx) $ do
    f <- evalTermVar (ctxEnv ctx) (qualName fname) ft
    foldM (apply noLoc mempty) f vs'

  where updateType (vt:vts) (Scalar (Arrow als u _ rt)) =
          Scalar $ Arrow als u (valueStructType vt) $ updateType vts rt
        updateType _ t = t

        convertValue (F.PrimValue p) = Just $ ValuePrim p
        convertValue (F.ArrayValue arr t) = mkArray t =<< mapM convertValue (elems arr)