Fast impertive pointers (static, unboxing, etc.) with Struct library - pointers

Question

I am interested in using more efficient pointers for a project implementing an imperative language in Haskell. There is already a library for that: Struct. There is a blog post on it and brief documentation.
The problem is there is only a quite sophisticated example of linkcut trees. For someone like me who doesn't use Haskell on a daily basis, it is quite exhausting to battle little documented code, template haskell, etc.
I would need a simpler example to get started, along the lines of expressing either of those two data types:
import Data.IORef
data DLL a = DLL a (Maybe (IORef (DLL a))) (Maybe (IORef (DLL a)))
data DLLINT = DLLINT Int (Maybe (IORef DLLINT)) (Maybe (IORef DLLINT))
This should be just a few simple lines for someone who is fluent in Haskell/GHC.
How do I express one of the data types above with the Struct library?

Answer 1

I managed to get your DLL type working with Structs as follows:
{-# LANGUAGE TemplateHaskell, RoleAnnotations #-}
module DubLiList where
import Control.Monad.Primitive
import Data.Struct.TH
import Data.Struct
import Data.Struct.Internal
makeStruct [d|
data DLL a s = DLL
{ prev :: !(DLL a s)
, value :: a
, next :: !(DLL a s)
}
|]
new :: (PrimMonad m) => a -> m (DLL a (PrimState m))
new x = st $ newDLL Nil x Nil
insert :: (PrimMonad m) => a -> DLL a (PrimState m) -> m (DLL a (PrimState m))
insert x this = st $ do
prev' <- get prev this
new <- newDLL prev' x this
set prev this new
set next prev' new
return new
delete :: (PrimMonad m) => DLL a (PrimState m) -> m ()
delete this = st $ do
prev' <- get prev this
next' <- get next this
set next prev' next'
set prev next' prev'
toList :: (PrimMonad m) => DLL a (PrimState m) -> m [a]
toList this = st $ do
if isNil this then return [] else do
x <- getField value this
that <- get next this
(x:) <$> toList that
Here's an example of using it:
main :: IO ()
main = do
dll <- new "foo" -- [foo]
dll' <- insert "bar" dll -- [bar, foo]
insert "baz" dll -- [bar, baz, foo]
xs <- toList dll'
print xs