halide-haskell-0.0.1.0: src/Language/Halide/Expr.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeApplications #-}
{-# OPTIONS_GHC -Wno-orphans #-}

-- |
-- Module      : Language.Halide.Expr
-- Description : Scalar expressions
-- Copyright   : (c) Tom Westerhout, 2023
module Language.Halide.Expr
  ( Expr (..)
  , Var
  , RVar
  , VarOrRVar
  , Int32
  , mkExpr
  , mkVar
  , mkRVar
  , cast
  , eq
  , neq
  , lt
  , lte
  , gt
  , gte
  , bool
  , undef
    -- | For debugging, it's often useful to observe the value of an expression when it's evaluated. If you
    -- have a complex expression that does not depend on any buffers or indices, you can 'evaluate' it.
  , evaluate
    -- | However, often an expression is only used within a definition of a pipeline, and it's impossible to
    -- call 'evaluate' on it. In such cases, it can be wrapped with 'printed' to indicate to Halide that the
    -- value of the expression should be dumped to screen when it's computed.
  , printed
  , toIntImm

    -- * Internal
  , exprToForeignPtr
  , cxxConstructExpr
  -- , wrapCxxExpr
  , wrapCxxRVar
  , wrapCxxVarOrRVar
  , wrapCxxParameter
  , asExpr
  , asVar
  , asRVar
  , asVarOrRVar
  , asScalarParam
  , asVectorOf
  , mkScalarParameter
  , withMany
  , binaryOp
  , unaryOp
  , checkType
  )
where

import Control.Exception (bracket)
import Control.Monad (unless)
import Data.IORef
import Data.Int (Int32)
import Data.Proxy
import Data.Ratio (denominator, numerator)
import Data.Text (Text, unpack)
import Data.Text.Encoding qualified as T
import Data.Vector.Storable.Mutable qualified as SM
import Foreign.ForeignPtr
import Foreign.Marshal (alloca, allocaArray, peekArray, toBool, with)
import Foreign.Ptr (Ptr, castPtr, nullPtr)
import Foreign.Storable (peek)
import GHC.Stack (HasCallStack)
import Language.C.Inline qualified as C
import Language.C.Inline.Cpp.Exception qualified as C
import Language.C.Inline.Unsafe qualified as CU
import Language.Halide.Buffer
import Language.Halide.Context
import Language.Halide.Type
import Language.Halide.Utils
import System.IO.Unsafe (unsafePerformIO)
import Prelude hiding (min)

importHalide

instanceCxxConstructible "Halide::Expr"
instanceCxxConstructible "Halide::Var"
instanceCxxConstructible "Halide::RVar"
instanceCxxConstructible "Halide::VarOrRVar"

defineIsHalideTypeInstances

instanceHasCxxVector "Halide::Expr"
instanceHasCxxVector "Halide::Var"
instanceHasCxxVector "Halide::RVar"
instanceHasCxxVector "Halide::VarOrRVar"

-- instanceCxxConstructible "Halide::Var"
-- instanceCxxConstructible "Halide::RVar"
-- instanceCxxConstructible "Halide::VarOrRVar"

instance IsHalideType Bool where
  halideTypeFor _ = HalideType HalideTypeUInt 1 1
  toCxxExpr (fromIntegral . fromEnum -> x) =
    cxxConstruct $ \ptr ->
      [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{cast(Halide::UInt(1), Halide::Expr{$(int x)})} } |]

type instance FromTuple (Expr a) = Arguments '[Expr a]

-- | A scalar expression in Halide.
--
-- To have a nice experience writing arithmetic expressions in terms of @Expr@s, we want to derive 'Num',
-- 'Floating' etc. instances for @Expr@. Unfortunately, that means that we encode v'Expr', v'Var', v'RVar',
-- and v'ScalarParam' by the same type, and passing an @Expr@ to a function that expects a @Var@ will produce
-- a runtime error.
data Expr a
  = -- | Scalar expression.
    Expr (ForeignPtr CxxExpr)
  | -- | Index variable.
    Var (ForeignPtr CxxVar)
  | -- | Reduction variable.
    RVar (ForeignPtr CxxRVar)
  | -- | Scalar parameter.
    --
    -- The 'IORef' is initialized with 'Nothing' and filled in on the first
    -- call to 'asExpr'.
    ScalarParam (IORef (Maybe (ForeignPtr CxxParameter)))

-- | A v'Var'.
type Var = Expr Int32

-- | An v'RVar'.
type RVar = Expr Int32

-- | Either v'Var' or v'RVar'.
type VarOrRVar = Expr Int32

-- | Create a scalar expression from a Haskell value.
mkExpr :: IsHalideType a => a -> Expr a
mkExpr x = unsafePerformIO $! Expr <$> toCxxExpr x

-- | Create a named index variable.
mkVar :: Text -> IO (Expr Int32)
mkVar (T.encodeUtf8 -> s) = fmap Var . cxxConstruct $ \ptr ->
  [CU.exp| void {
    new ($(Halide::Var* ptr)) Halide::Var{std::string{$bs-ptr:s, static_cast<size_t>($bs-len:s)}} } |]

-- | Create a named reduction variable.
--
-- For more information about reduction variables, see [@Halide::RDom@](https://halide-lang.org/docs/class_halide_1_1_r_dom.html).
mkRVar
  :: Text
  -- ^ name
  -> Expr Int32
  -- ^ min index
  -> Expr Int32
  -- ^ extent
  -> IO (Expr Int32)
mkRVar name min extent =
  asExpr min $ \min' ->
    asExpr extent $ \extent' ->
      wrapCxxRVar
        =<< [CU.exp| Halide::RVar* {
              new Halide::RVar{static_cast<Halide::RVar>(Halide::RDom{
                *$(const Halide::Expr* min'),
                *$(const Halide::Expr* extent'),
                std::string{$bs-ptr:s, static_cast<size_t>($bs-len:s)}
                })}
            } |]
  where
    s = T.encodeUtf8 name

-- | Return an undef value of the given type.
--
-- For more information, see [@Halide::undef@](https://halide-lang.org/docs/namespace_halide.html#a9389bcacbed602df70eae94826312e03).
undef :: forall a. IsHalideType a => Expr a
undef = unsafePerformIO $
  with (halideTypeFor (Proxy @a)) $ \tp ->
    cxxConstructExpr $ \ptr ->
      [CU.exp| void {
        new ($(Halide::Expr* ptr))
          Halide::Expr{Halide::undef(Halide::Type{*$(const halide_type_t* tp)})} } |]
{-# NOINLINE undef #-}

-- | Cast a scalar expression to a different type.
--
-- Use TypeApplications with this function, e.g. @cast \@Float x@.
cast :: forall to from. (IsHalideType to, IsHalideType from) => Expr from -> Expr to
cast expr = unsafePerformIO $
  asExpr expr $ \e ->
    with (halideTypeFor (Proxy @to)) $ \t ->
      cxxConstructExpr $ \ptr ->
        [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
          Halide::cast(Halide::Type{*$(halide_type_t* t)}, *$(Halide::Expr* e))} } |]

-- | Print the expression to stdout when it's evaluated.
--
-- This is useful for debugging Halide pipelines.
printed :: IsHalideType a => Expr a -> Expr a
printed = unaryOp $ \e ptr ->
  [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{print(*$(Halide::Expr* e))} } |]

infix 4 `eq`, `neq`, `lt`, `lte`, `gt`, `gte`

-- | '==' but lifted to return an 'Expr'.
eq :: IsHalideType a => Expr a -> Expr a -> Expr Bool
eq = binaryOp $ \a b ptr ->
  [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
    (*$(Halide::Expr* a)) == (*$(Halide::Expr* b))} } |]

-- | '/=' but lifted to return an 'Expr'.
neq :: IsHalideType a => Expr a -> Expr a -> Expr Bool
neq = binaryOp $ \a b ptr ->
  [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
    (*$(Halide::Expr* a)) != (*$(Halide::Expr* b))} } |]

-- | '<' but lifted to return an 'Expr'.
lt :: IsHalideType a => Expr a -> Expr a -> Expr Bool
lt = binaryOp $ \a b ptr ->
  [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
    (*$(Halide::Expr* a)) < (*$(Halide::Expr* b))} } |]

-- | '<=' but lifted to return an 'Expr'.
lte :: IsHalideType a => Expr a -> Expr a -> Expr Bool
lte = binaryOp $ \a b ptr ->
  [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
    (*$(Halide::Expr* a)) <= (*$(Halide::Expr* b))} } |]

-- | '>' but lifted to return an 'Expr'.
gt :: IsHalideType a => Expr a -> Expr a -> Expr Bool
gt = binaryOp $ \a b ptr ->
  [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
    (*$(Halide::Expr* a)) > (*$(Halide::Expr* b))} } |]

-- | '>=' but lifted to return an 'Expr'.
gte :: IsHalideType a => Expr a -> Expr a -> Expr Bool
gte = binaryOp $ \a b ptr ->
  [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
    (*$(Halide::Expr* a)) >= (*$(Halide::Expr* b))} } |]

-- | Similar to the standard 'Prelude.bool' function from Prelude except that it's
-- lifted to work with 'Expr' types.
bool :: IsHalideType a => Expr Bool -> Expr a -> Expr a -> Expr a
bool condExpr trueExpr falseExpr = unsafePerformIO $
  asExpr condExpr $ \p ->
    asExpr trueExpr $ \t ->
      asExpr falseExpr $ \f ->
        cxxConstructExpr $ \ptr ->
          [CU.exp| void {
            new ($(Halide::Expr* ptr)) Halide::Expr{
              Halide::select(*$(Halide::Expr* p),
                *$(Halide::Expr* t), *$(Halide::Expr* f))} } |]

-- | Evaluate a scalar expression.
--
-- It should contain no parameters. If it does contain parameters, an exception will be thrown.
evaluate :: forall a. IsHalideType a => Expr a -> IO a
evaluate expr =
  asExpr expr $ \e -> do
    out <- SM.new 1
    withHalideBuffer out $ \buffer -> do
      let b = castPtr (buffer :: Ptr (HalideBuffer 1 a))
      [C.throwBlock| void {
        handle_halide_exceptions([=]() {
          Halide::Func f;
          Halide::Var i;
          f(i) = *$(Halide::Expr* e);
          f.realize(Halide::Pipeline::RealizationArg{$(halide_buffer_t* b)});
        });
      } |]
    SM.read out 0

-- | Convert expression to integer immediate.
--
-- Tries to extract the value of an expression if it is a compile-time constant. If the expression
-- isn't known at compile-time of the Halide pipeline, returns 'Nothing'.
toIntImm :: IsHalideType a => Expr a -> Maybe Int
toIntImm expr = unsafePerformIO $
  asExpr expr $ \expr' -> do
    intPtr <-
      [CU.block| const int64_t* {
        auto expr = *$(const Halide::Expr* expr');
        Halide::Internal::IntImm const* node = expr.as<Halide::Internal::IntImm>();
        if (node == nullptr) return nullptr;
        return &node->value;
      } |]
    if intPtr == nullPtr
      then pure Nothing
      else Just . fromIntegral <$> peek intPtr

instance IsTuple (Arguments '[Expr a]) (Expr a) where
  toTuple (x ::: Nil) = x
  fromTuple x = x ::: Nil

instance IsHalideType a => Show (Expr a) where
  show (Expr expr) = unpack . unsafePerformIO $ do
    withForeignPtr expr $ \x ->
      peekAndDeleteCxxString
        =<< [CU.exp| std::string* { to_string_via_iostream(*$(const Halide::Expr* x)) } |]
  show (Var var) = unpack . unsafePerformIO $ do
    withForeignPtr var $ \x ->
      peekAndDeleteCxxString
        =<< [CU.exp| std::string* { to_string_via_iostream(*$(const Halide::Var* x)) } |]
  show (RVar rvar) = unpack . unsafePerformIO $ do
    withForeignPtr rvar $ \x ->
      peekAndDeleteCxxString
        =<< [CU.exp| std::string* { to_string_via_iostream(*$(const Halide::RVar* x)) } |]
  show (ScalarParam r) = unpack . unsafePerformIO $ do
    maybeParam <- readIORef r
    case maybeParam of
      Just fp ->
        withForeignPtr fp $ \x ->
          peekAndDeleteCxxString
            =<< [CU.exp| std::string* {
                  new std::string{$(const Halide::Internal::Parameter* x)->name()} } |]
      Nothing -> pure "ScalarParam"

instance (IsHalideType a, Num a) => Num (Expr a) where
  fromInteger :: Integer -> Expr a
  fromInteger x = mkExpr (fromInteger x :: a)
  (+) :: Expr a -> Expr a -> Expr a
  (+) = binaryOp $ \a b ptr ->
    [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{*$(Halide::Expr* a) + *$(Halide::Expr* b)} } |]
  (-) :: Expr a -> Expr a -> Expr a
  (-) = binaryOp $ \a b ptr ->
    [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{*$(Halide::Expr* a) - *$(Halide::Expr* b)} } |]
  (*) :: Expr a -> Expr a -> Expr a
  (*) = binaryOp $ \a b ptr ->
    [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{*$(Halide::Expr* a) * *$(Halide::Expr* b)} } |]

  abs :: Expr a -> Expr a
  abs = unaryOp $ \a ptr ->
    -- If the type is unsigned, then abs does nothing Also note that for signed
    -- integers, in Halide abs returns the unsigned version, so we manually
    -- cast it back.
    [CU.block| void {
      if ($(Halide::Expr* a)->type().is_uint()) {
        new ($(Halide::Expr* ptr)) Halide::Expr{*$(Halide::Expr* a)};
      }
      else {
        new ($(Halide::Expr* ptr)) Halide::Expr{
          Halide::cast($(Halide::Expr* a)->type(), Halide::abs(*$(Halide::Expr* a)))};
      }
    } |]
  negate :: Expr a -> Expr a
  negate = unaryOp $ \a ptr ->
    [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{ -(*$(Halide::Expr* a))} } |]
  signum :: Expr a -> Expr a
  signum = error "Num instance of (Expr a) does not implement signum"

instance (IsHalideType a, Fractional a) => Fractional (Expr a) where
  (/) :: Expr a -> Expr a -> Expr a
  (/) = binaryOp $ \a b ptr ->
    [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{*$(Halide::Expr* a) / *$(Halide::Expr* b)} } |]
  fromRational :: Rational -> Expr a
  fromRational r = fromInteger (numerator r) / fromInteger (denominator r)

instance (IsHalideType a, Floating a) => Floating (Expr a) where
  pi :: Expr a
  pi = cast @a @Double $! mkExpr (pi :: Double)
  exp :: Expr a -> Expr a
  exp = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::exp(*$(Halide::Expr* a))} } |]
  log :: Expr a -> Expr a
  log = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::log(*$(Halide::Expr* a))} } |]
  sqrt :: Expr a -> Expr a
  sqrt = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::sqrt(*$(Halide::Expr* a))} } |]
  (**) :: Expr a -> Expr a -> Expr a
  (**) = binaryOp $ \a b ptr ->
    [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::pow(*$(Halide::Expr* a), *$(Halide::Expr* b))} } |]
  sin :: Expr a -> Expr a
  sin = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::sin(*$(Halide::Expr* a))} } |]
  cos :: Expr a -> Expr a
  cos = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::cos(*$(Halide::Expr* a))} } |]
  tan :: Expr a -> Expr a
  tan = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::tan(*$(Halide::Expr* a))} } |]
  asin :: Expr a -> Expr a
  asin = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::asin(*$(Halide::Expr* a))} } |]
  acos :: Expr a -> Expr a
  acos = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::acos(*$(Halide::Expr* a))} } |]
  atan :: Expr a -> Expr a
  atan = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::atan(*$(Halide::Expr* a))} } |]
  sinh :: Expr a -> Expr a
  sinh = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::sinh(*$(Halide::Expr* a))} } |]
  cosh :: Expr a -> Expr a
  cosh = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::cosh(*$(Halide::Expr* a))} } |]
  tanh :: Expr a -> Expr a
  tanh = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::tanh(*$(Halide::Expr* a))} } |]
  asinh :: Expr a -> Expr a
  asinh = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::asinh(*$(Halide::Expr* a))} } |]
  acosh :: Expr a -> Expr a
  acosh = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::acosh(*$(Halide::Expr* a))} } |]
  atanh :: Expr a -> Expr a
  atanh = unaryOp $ \a ptr -> [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{Halide::atanh(*$(Halide::Expr* a))} } |]

-- | Wrap a raw @Halide::Expr@ pointer in a Haskell value.
--
-- __Note:__ This function checks the runtime type of the expression.
-- wrapCxxExpr :: forall a. (HasCallStack, IsHalideType a) => Ptr CxxExpr -> IO (Expr a)
-- wrapCxxExpr p = do
--   checkType @a p
--   Expr <$> newForeignPtr deleter p
--   where
--     deleter = [C.funPtr| void deleteExpr(Halide::Expr *p) { delete p; } |]
cxxConstructExpr :: forall a. (HasCallStack, IsHalideType a) => (Ptr CxxExpr -> IO ()) -> IO (Expr a)
cxxConstructExpr construct = do
  fp <- cxxConstruct construct
  withForeignPtr fp (checkType @a)
  pure (Expr fp)

-- | Wrap a raw @Halide::RVar@ pointer in a Haskell value.
--
-- __Note:__ v'RVar' objects correspond to expressions of type 'Int32'.
wrapCxxRVar :: Ptr CxxRVar -> IO (Expr Int32)
wrapCxxRVar = fmap RVar . newForeignPtr deleter
  where
    deleter = [C.funPtr| void deleteExpr(Halide::RVar *p) { delete p; } |]

wrapCxxVarOrRVar :: Ptr CxxVarOrRVar -> IO (Expr Int32)
wrapCxxVarOrRVar p = do
  isRVar <- toBool <$> [CU.exp| bool { $(const Halide::VarOrRVar* p)->is_rvar } |]
  expr <-
    if isRVar
      then wrapCxxRVar =<< [CU.exp| Halide::RVar* { new Halide::RVar{$(const Halide::VarOrRVar* p)->rvar} } |]
      else fmap Var . cxxConstruct $ \ptr ->
        [CU.exp| void { new ($(Halide::Var* ptr)) Halide::Var{$(const Halide::VarOrRVar* p)->var} } |]
  [CU.exp| void { delete $(const Halide::VarOrRVar* p) } |]
  pure expr

class HasHalideType a where
  getHalideType :: a -> IO HalideType

instance HasHalideType (Expr a) where
  getHalideType (Expr fp) =
    withForeignPtr fp $ \e -> alloca $ \t -> do
      [CU.block| void {
        *$(halide_type_t* t) = static_cast<halide_type_t>(
          $(Halide::Expr* e)->type()); } |]
      peek t
  getHalideType (Var fp) =
    withForeignPtr fp $ \e -> alloca $ \t -> do
      [CU.block| void {
        *$(halide_type_t* t) = static_cast<halide_type_t>(
          static_cast<Halide::Expr>(*$(Halide::Var* e)).type()); } |]
      peek t
  getHalideType (RVar fp) =
    withForeignPtr fp $ \e -> alloca $ \t -> do
      [CU.block| void {
        *$(halide_type_t* t) = static_cast<halide_type_t>(
          static_cast<Halide::Expr>(*$(Halide::RVar* e)).type()); } |]
      peek t
  getHalideType _ = error "not implemented"

instance HasHalideType (Ptr CxxExpr) where
  getHalideType e =
    alloca $ \t -> do
      [CU.block| void {
        *$(halide_type_t* t) = static_cast<halide_type_t>($(Halide::Expr* e)->type()); } |]
      peek t

instance HasHalideType (Ptr CxxVar) where
  getHalideType _ = pure $ halideTypeFor (Proxy @Int32)

instance HasHalideType (Ptr CxxRVar) where
  getHalideType _ = pure $ halideTypeFor (Proxy @Int32)

instance HasHalideType (Ptr CxxParameter) where
  getHalideType p =
    alloca $ \t -> do
      [CU.block| void {
        *$(halide_type_t* t) = static_cast<halide_type_t>($(Halide::Internal::Parameter* p)->type()); } |]
      peek t

-- | Wrap a raw @Halide::Internal::Parameter@ pointer in a Haskell value.
--
-- __Note:__ v'Var' objects correspond to expressions of type 'Int32'.
wrapCxxParameter :: Ptr CxxParameter -> IO (ForeignPtr CxxParameter)
wrapCxxParameter = newForeignPtr deleter
  where
    deleter = [C.funPtr| void deleteParameter(Halide::Internal::Parameter *p) { delete p; } |]

-- | Helper function to assert that the runtime type of the expression matches it's
-- compile-time type.
--
-- Essentially, given an @(x :: 'Expr' a)@, we check that @x.type()@ in C++ is equal to
-- @'halideTypeFor' (Proxy \@a)@ in Haskell.
checkType :: forall a t. (HasCallStack, IsHalideType a, HasHalideType t) => t -> IO ()
checkType x = do
  let hsType = halideTypeFor (Proxy @a)
  cxxType <- getHalideType x
  unless (cxxType == hsType) . error $
    "Type mismatch: C++ Expr has type "
      <> show cxxType
      <> ", but its Haskell counterpart has type "
      <> show hsType

mkScalarParameter :: forall a. IsHalideType a => Maybe Text -> IO (ForeignPtr CxxParameter)
mkScalarParameter maybeName = do
  with (halideTypeFor (Proxy @a)) $ \t -> do
    let createWithoutName =
          [CU.exp| Halide::Internal::Parameter* {
            new Halide::Internal::Parameter{Halide::Type{*$(halide_type_t* t)}, false, 0} } |]
        createWithName name =
          let s = T.encodeUtf8 name
           in [CU.exp| Halide::Internal::Parameter* {
                new Halide::Internal::Parameter{
                  Halide::Type{*$(halide_type_t* t)},
                  false,
                  0,
                  std::string{$bs-ptr:s, static_cast<size_t>($bs-len:s)}}
              } |]
    p <- maybe createWithoutName createWithName maybeName
    checkType @a p
    wrapCxxParameter p

getScalarParameter
  :: forall a
   . IsHalideType a
  => Maybe Text
  -> IORef (Maybe (ForeignPtr CxxParameter))
  -> IO (ForeignPtr CxxParameter)
getScalarParameter name r = do
  readIORef r >>= \case
    Just fp -> pure fp
    Nothing -> do
      fp <- mkScalarParameter @a name
      writeIORef r (Just fp)
      pure fp

-- | Make sure that the expression is fully constructed. That means that if we
-- are dealing with a 'ScalarParam' rather than an 'Expr', we force the
-- construction of the underlying @Halide::Internal::Parameter@ and convert it
-- to an 'Expr'.
forceExpr :: forall a. (HasCallStack, IsHalideType a) => Expr a -> IO (Expr a)
forceExpr x@(Expr _) = pure x
forceExpr (Var fp) =
  withForeignPtr fp $ \varPtr ->
    cxxConstructExpr $ \ptr ->
      [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
        static_cast<Halide::Expr>(*$(Halide::Var* varPtr))} } |]
forceExpr (RVar fp) =
  withForeignPtr fp $ \rvarPtr ->
    cxxConstructExpr $ \ptr ->
      [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
        static_cast<Halide::Expr>(*$(Halide::RVar* rvarPtr))} } |]
forceExpr (ScalarParam r) =
  getScalarParameter @a Nothing r >>= \fp -> withForeignPtr fp $ \paramPtr ->
    cxxConstructExpr $ \ptr ->
      [CU.exp| void { new ($(Halide::Expr* ptr)) Halide::Expr{
        Halide::Internal::Variable::make(
          $(Halide::Internal::Parameter* paramPtr)->type(),
          $(Halide::Internal::Parameter* paramPtr)->name(),
          *$(Halide::Internal::Parameter* paramPtr))} } |]

-- | Use the underlying @Halide::Expr@ in an 'IO' action.
asExpr :: IsHalideType a => Expr a -> (Ptr CxxExpr -> IO b) -> IO b
asExpr x = withForeignPtr (exprToForeignPtr x)

-- | Allows applying 'asExpr', 'asVar', 'asRVar', and 'asVarOrRVar' to multiple arguments.
--
-- Example usage:
--
-- > asVectorOf @((~) (Expr Int32)) asVarOrRVar (fromTuple args) $ \v -> do
-- >   withFunc func $ \f ->
-- >     [C.throwBlock| void { $(Halide::Func* f)->reorder(
-- >                             *$(std::vector<Halide::VarOrRVar>* v)); } |]
asVectorOf
  :: forall c k ts a
   . (All c ts, HasCxxVector k)
  => (forall t b. c t => t -> (Ptr k -> IO b) -> IO b)
  -> Arguments ts
  -> (Ptr (CxxVector k) -> IO a)
  -> IO a
asVectorOf asPtr args action =
  bracket (newCxxVector Nothing) deleteCxxVector (go args)
  where
    go
      :: All c ts'
      => Arguments ts'
      -> Ptr (CxxVector k)
      -> IO a
    go Nil v = action v
    go (x ::: xs) v = asPtr x $ \p -> cxxVectorPushBack v p >> go xs v

withMany
  :: forall k t a
   . (HasCxxVector k)
  => (t -> (Ptr k -> IO a) -> IO a)
  -> [t]
  -> (Ptr (CxxVector k) -> IO a)
  -> IO a
withMany asPtr args action =
  bracket (newCxxVector Nothing) deleteCxxVector (go args)
  where
    go [] v = action v
    go (x : xs) v = asPtr x $ \p -> cxxVectorPushBack v p >> go xs v

-- | Use the underlying @Halide::Var@ in an 'IO' action.
asVar :: HasCallStack => Expr Int32 -> (Ptr CxxVar -> IO b) -> IO b
asVar (Var fp) = withForeignPtr fp
asVar _ = error "the expression is not a Var"

-- | Use the underlying @Halide::RVar@ in an 'IO' action.
asRVar :: HasCallStack => Expr Int32 -> (Ptr CxxRVar -> IO b) -> IO b
asRVar (RVar fp) = withForeignPtr fp
asRVar _ = error "the expression is not an RVar"

-- | Use the underlying v'Var' or v'RVar' as @Halide::VarOrRVar@ in an 'IO' action.
asVarOrRVar :: HasCallStack => VarOrRVar -> (Ptr CxxVarOrRVar -> IO b) -> IO b
asVarOrRVar x action = case x of
  Var fp ->
    let allocate p = [CU.exp| Halide::VarOrRVar* { new Halide::VarOrRVar{*$(Halide::Var* p)} } |]
     in withForeignPtr fp (run . allocate)
  RVar fp ->
    let allocate p = [CU.exp| Halide::VarOrRVar* { new Halide::VarOrRVar{*$(Halide::RVar* p)} } |]
     in withForeignPtr fp (run . allocate)
  _ -> error "the expression is not a Var or an RVar"
  where
    destroy p = [CU.exp| void { delete $(Halide::VarOrRVar* p) } |]
    run allocate = bracket allocate destroy action

-- | Use the underlying @Halide::RVar@ in an 'IO' action.
asScalarParam :: forall a b. (HasCallStack, IsHalideType a) => Expr a -> (Ptr CxxParameter -> IO b) -> IO b
asScalarParam (ScalarParam r) action = do
  fp <- getScalarParameter @a Nothing r
  withForeignPtr fp action
asScalarParam _ _ = error "the expression is not a ScalarParam"

-- | Get the underlying 'ForeignPtr CxxExpr'.
exprToForeignPtr :: IsHalideType a => Expr a -> ForeignPtr CxxExpr
exprToForeignPtr x =
  unsafePerformIO $!
    forceExpr x >>= \case
      (Expr fp) -> pure fp
      _ -> error "this cannot happen"

-- | Lift a unary function working with @Halide::Expr@ to work with 'Expr'.
unaryOp :: IsHalideType a => (Ptr CxxExpr -> Ptr CxxExpr -> IO ()) -> Expr a -> Expr a
unaryOp f a = unsafePerformIO $
  asExpr a $ \aPtr ->
    cxxConstructExpr $ \destPtr ->
      f aPtr destPtr

-- | Lift a binary function working with @Halide::Expr@ to work with 'Expr'.
binaryOp
  :: (IsHalideType a, IsHalideType b, IsHalideType c)
  => (Ptr CxxExpr -> Ptr CxxExpr -> Ptr CxxExpr -> IO ())
  -> Expr a
  -> Expr b
  -> Expr c
binaryOp f a b = unsafePerformIO $
  asExpr a $ \aPtr -> asExpr b $ \bPtr ->
    cxxConstructExpr $ \destPtr ->
      f aPtr bPtr destPtr