Simplify use sites by passing by value

See https://sraka.xyz/posts/hs-bindgen-introduction.html.

For now, this is a shell-based workflow, rather than using Nix to build everything, i.e. `nix develop` works but `nix build` doesn't. And `cargo build` has to be called manually to create the C library, rather than `cabal` being clever enough to invoke it itself.

We ran `cargo cabal init` from the `rust` directory (`nix shell github:yvan-sraka/cargo-cabal`), which generated `hsbindgen.toml` (which we use), and `Setup.lhs` (which just added `extra-lib-dirs`, and with the wrong paths, so we dspecify those statically instead in `garnet.cabal`). We also follow its advice to use `staticlib`.

Also, after we ran the first `cargo build` (requiring a `mkdir rust/src` before it would run), we took the generated the Haskell file, and moved the main contents in to `Main.hs` manually.

cabal.project

@@ -1,5 +1,28 @@
 packages: haskell
 
+-- TODO a total hack
+package garnet
+  extra-lib-dirs:
+    /home/gthomas/code/garnet/rust/target/release
+    /home/gthomas/code/garnet/rust/target/debug
+  extra-include-dirs:
+    /home/gthomas/code/garnet/rust
+
+-- https://well-typed.com/blog/2026/02/hs-bindgen-alpha
+-- Haskell.nix doesn't seem to like `tag: release-0.1-alpha`, which the blog post suggests
+-- so we specify the equivalent commit SHAs manually instead
+source-repository-package
+  type: git
+  location: https://github.com/well-typed/hs-bindgen
+  tag: e2a9260678d9fa76dab602a5a07927acada3be4f
+  subdir: c-expr-dsl c-expr-runtime hs-bindgen hs-bindgen-runtime
+  --sha256: 0nrs3iq0l5ha5kxyhqnlmvgi7734pmzyp3zf7p8s1gb21ylh4sy0
+source-repository-package
+  type: git
+  location: https://github.com/well-typed/libclang
+  tag: b5ff712c91c039cde6720ffe2096a121d9f4d802
+  --sha256: 1lwjdxd2ahhkvyxrpli7z9z7ss4l94m2jaif8kg1i2yygbhksrb3
+
 allow-newer:
   *:base,
   *:containers,

flake.nix

@@ -33,17 +33,19 @@
               shell.tools.cabal = "latest";
               shell.withHoogle = false;
               shell.tools.haskell-language-server = {
-                  src = hls-2-13;
+                src = hls-2-13;
-                  sha256map = {
+                sha256map = {
-                    "https://github.com/snowleopard/alga"."d4e43fb42db05413459fb2df493361d5a666588a" = "0s1mlnl64wj7pkg3iipv5bb4syy3bhxwqzqv93zqlvkyfn64015i";
+                  "https://github.com/snowleopard/alga"."d4e43fb42db05413459fb2df493361d5a666588a" = "0s1mlnl64wj7pkg3iipv5bb4syy3bhxwqzqv93zqlvkyfn64015i";
-                  };
                 };
+              };
             };
         })
         (import rust-overlay)
       ];
       pkgs = import nixpkgs { inherit system overlays; inherit (haskellNix) config; };
       haskell = pkgs.hixProject.flake { };
+      # TODO we're not getting Rust libs from Nix
+      # i.e. running `cargo clean && cargo build` rebuilds dependencies
       rust = (crane.mkLib pkgs).overrideToolchain (p: p.rust-bin.selectLatestNightlyWith (
         toolchain: toolchain.default.override {
           extensions = [ "rust-src" ];
@@ -63,10 +65,6 @@
           rust-analyzer
         ];
       };
-      packages = {
-        haskell = haskell.packages."garnet:exe:garnet";
-        rust = rust.buildPackage { src = rust.cleanCargoSource ./rust; };
-      };
     }
     );
 }

generate-bindings

@@ -0,0 +1,128 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# TODO this is a complete vibe-coded hack, but the header patching at least is crucial
+
+# Generate Haskell FFI bindings from Rust source code.
+#
+# Pipeline: cargo build -> cbindgen -> patch header -> hs-bindgen
+#
+# Prerequisites: run inside the Nix dev shell (provides gcc, cabal, etc.)
+# cbindgen is fetched via `nix run nixpkgs#rust-cbindgen`.
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+RUST_DIR="$SCRIPT_DIR/rust"
+HASKELL_DIR="$SCRIPT_DIR/haskell"
+HEADER_NAME="garnet_rs.h"
+HEADER="$RUST_DIR/$HEADER_NAME"
+
+# --- Step 1: Build Rust static library ---
+echo "=== Building Rust library ==="
+cargo build --manifest-path "$RUST_DIR/Cargo.toml"
+
+# --- Step 2: Generate C header with cbindgen ---
+echo "=== Running cbindgen ==="
+nix run nixpkgs#rust-cbindgen -- \
+  --lang c \
+  --crate garnet-rs \
+  --output "$HEADER" \
+  "$RUST_DIR"
+
+echo "  Raw header written to $HEADER"
+
+# --- Step 3: Patch the header for hs-bindgen compatibility ---
+echo "=== Patching header ==="
+
+# Two patches are needed:
+#
+# Patch 1: Replace cbindgen's enum pattern with typedef + #defines.
+#   cbindgen emits:  enum Shape_Tag { Circle, Rectangle, }; typedef uint8_t Shape_Tag;
+#   hs-bindgen needs: typedef uint8_t Shape_Tag; #define Circle 0 ...
+#
+# Patch 2: Name anonymous unions inside structs.
+#   cbindgen emits:  union { ... };
+#   hs-bindgen needs: union { ... } body;
+
+awk '
+# State machine for enum->typedef+define transformation
+/^enum [A-Za-z_][A-Za-z0-9_]* \{$/ {
+    in_enum = 1
+    enum_name = $2
+    variant_count = 0
+    delete variants
+    next
+}
+in_enum && /^\};$/ {
+    # Next line should be: typedef <type> <enum_name>;
+    in_enum = 0
+    pending_enum = 1
+    next
+}
+in_enum {
+    # Collect variant names (strip trailing comma and whitespace)
+    v = $0
+    gsub(/^[[:space:]]+/, "", v)
+    gsub(/,[[:space:]]*$/, "", v)
+    if (v != "") {
+        variants[variant_count] = v
+        variant_count++
+    }
+    next
+}
+pending_enum && /^typedef [A-Za-z0-9_]+ / {
+    # Emit: typedef <type> <name>; then #define for each variant
+    print $0
+    for (i = 0; i < variant_count; i++) {
+        printf "#define %s  %d\n", variants[i], i
+    }
+    pending_enum = 0
+    next
+}
+
+# Name anonymous unions: }; at end of union block inside struct -> } body;
+/^  \};$/ && saw_union {
+    print "  } body;"
+    saw_union = 0
+    next
+}
+/^  union \{$/ {
+    saw_union = 1
+}
+
+{ print }
+' "$HEADER" > "${HEADER}.tmp" && mv "${HEADER}.tmp" "$HEADER"
+
+echo "  Patched header at $HEADER"
+
+# --- Step 4: Derive system include paths for hs-bindgen's libclang ---
+echo "=== Detecting system include paths ==="
+
+# hs-bindgen uses libclang directly, which doesn't know about NixOS's
+# non-standard include locations. We extract them from cpp -v and pass
+# all of them — extra paths are harmless.
+CLANG_OPTIONS=()
+while IFS= read -r dir; do
+  CLANG_OPTIONS+=("--clang-option" "-isystem$dir")
+done < <(echo | cpp -v 2>&1 | awk '/#include <\.\.\.> search starts here:/{f=1;next}/End of search list/{f=0}f{gsub(/^ +/,"");print}')
+
+if [ ${#CLANG_OPTIONS[@]} -eq 0 ]; then
+  echo "  WARNING: No system include paths detected. hs-bindgen may fail."
+else
+  echo "  Found ${#CLANG_OPTIONS[@]} clang options:"
+  for ((i=0; i<${#CLANG_OPTIONS[@]}; i+=2)); do
+    echo "    ${CLANG_OPTIONS[i+1]}"
+  done
+fi
+
+# --- Step 5: Run hs-bindgen ---
+echo "=== Running hs-bindgen ==="
+cabal run -- hs-bindgen-cli preprocess \
+  --overwrite-files --create-output-dirs \
+  --unique-id com.garnet --enable-record-dot \
+  --hs-output-dir "$HASKELL_DIR/generated" --module GarnetRs \
+  "${CLANG_OPTIONS[@]}" \
+  -I "$RUST_DIR" "$HEADER_NAME"
+
+echo "=== Done ==="
+echo "Generated Haskell bindings in $HASKELL_DIR/generated/"
+echo "Run 'cabal run garnet' to test."

haskell/Main.hs

@@ -1,5 +0,0 @@
-module Main (main) where
-
-main :: IO ()
-main = do
-    putStrLn "Hello from Haskell"

haskell/exe/Main.hs

@@ -0,0 +1,14 @@
+module Main (main) where
+
+import Data.ByteString
+import Foreign.C
+import GarnetRs
+import GarnetRs.Safe
+import HsBindgen.Runtime.PtrConst
+
+main :: IO ()
+main = do
+    useAsCString "Haskell" $ hello . unsafeFromPtr
+    hello_struct T{a = CBool 1, b = 42}
+    hello_shape $ Shape (Shape_Tag 0) $ set_shape_body_circle $ Circle_Body 3.14
+    hello_shape $ Shape (Shape_Tag 1) $ set_shape_body_rectangle $ Rectangle_Body 10.0 5.0

haskell/garnet.cabal

@@ -5,10 +5,23 @@ license-file: LICENSE
 author: Patrick Aldis
 maintainer: patricktaldis@gmail.com
 
+library garnet-generated
+    hs-source-dirs: generated
+    exposed-modules:
+        GarnetRs
+        GarnetRs.Safe
+        GarnetRs.Unsafe
+        GarnetRs.FunPtr
+    default-language: Haskell2010
+    extra-bundled-libraries: garnet_rs
+    build-depends:
+        base,
+        hs-bindgen-runtime,
+        primitive,
+
 executable garnet
     main-is: Main.hs
-    hs-source-dirs: .
+    hs-source-dirs: exe
-    other-modules:
     default-language: GHC2024
     default-extensions:
         BlockArguments
@@ -33,3 +46,6 @@ executable garnet
         -with-rtsopts=-N
     build-depends:
         base >= 4.14,
+        bytestring,
+        garnet-generated,
+        hs-bindgen-runtime,

haskell/generated/GarnetRs.hs

@@ -0,0 +1,476 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE ExplicitForAll #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE UnboxedTuples #-}
+{-# LANGUAGE UndecidableInstances #-}
+
+module GarnetRs where
+
+import qualified Data.Array.Byte
+import qualified Data.Bits as Bits
+import qualified Data.Ix as Ix
+import qualified Data.Primitive.Types
+import qualified Data.Proxy
+import qualified Foreign as F
+import qualified Foreign.C as FC
+import qualified GHC.Generics
+import qualified GHC.Ptr as Ptr
+import qualified GHC.Records
+import qualified HsBindgen.Runtime.HasCField
+import qualified HsBindgen.Runtime.Internal.Bitfield
+import qualified HsBindgen.Runtime.Internal.ByteArray
+import qualified HsBindgen.Runtime.Internal.HasFFIType
+import qualified HsBindgen.Runtime.Internal.SizedByteArray
+import qualified HsBindgen.Runtime.LibC
+import qualified HsBindgen.Runtime.Marshal
+import Data.Bits (FiniteBits)
+import HsBindgen.Runtime.Internal.TypeEquality (TyEq)
+import Prelude ((<*>), (>>), Bounded, Enum, Eq, Int, Integral, Num, Ord, Read, Real, Show, pure)
+
+{-| __C declaration:__ @struct T@
+
+    __defined at:__ @garnet_rs.h 6:16@
+
+    __exported by:__ @garnet_rs.h@
+-}
+data T = T
+  { a :: FC.CBool
+    {- ^ __C declaration:__ @a@
+
+         __defined at:__ @garnet_rs.h 7:8@
+
+         __exported by:__ @garnet_rs.h@
+    -}
+  , b :: HsBindgen.Runtime.LibC.Word8
+    {- ^ __C declaration:__ @b@
+
+         __defined at:__ @garnet_rs.h 8:11@
+
+         __exported by:__ @garnet_rs.h@
+    -}
+  }
+  deriving stock (GHC.Generics.Generic)
+  deriving stock (Eq, Show)
+
+instance HsBindgen.Runtime.Marshal.StaticSize T where
+
+  staticSizeOf = \_ -> (2 :: Int)
+
+  staticAlignment = \_ -> (1 :: Int)
+
+instance HsBindgen.Runtime.Marshal.ReadRaw T where
+
+  readRaw =
+    \ptr0 ->
+          pure T
+      <*> HsBindgen.Runtime.HasCField.readRaw (Data.Proxy.Proxy @"a") ptr0
+      <*> HsBindgen.Runtime.HasCField.readRaw (Data.Proxy.Proxy @"b") ptr0
+
+instance HsBindgen.Runtime.Marshal.WriteRaw T where
+
+  writeRaw =
+    \ptr0 ->
+      \s1 ->
+        case s1 of
+          T a2 b3 ->
+               HsBindgen.Runtime.HasCField.writeRaw (Data.Proxy.Proxy @"a") ptr0 a2
+            >> HsBindgen.Runtime.HasCField.writeRaw (Data.Proxy.Proxy @"b") ptr0 b3
+
+deriving via HsBindgen.Runtime.Marshal.EquivStorable T instance F.Storable T
+
+instance HsBindgen.Runtime.HasCField.HasCField T "a" where
+
+  type CFieldType T "a" = FC.CBool
+
+  offset# = \_ -> \_ -> 0
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType T) "a")
+         ) => GHC.Records.HasField "a" (Ptr.Ptr T) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"a")
+
+instance HsBindgen.Runtime.HasCField.HasCField T "b" where
+
+  type CFieldType T "b" = HsBindgen.Runtime.LibC.Word8
+
+  offset# = \_ -> \_ -> 1
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType T) "b")
+         ) => GHC.Records.HasField "b" (Ptr.Ptr T) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"b")
+
+{-| __C declaration:__ @Shape_Tag@
+
+    __defined at:__ @garnet_rs.h 11:17@
+
+    __exported by:__ @garnet_rs.h@
+-}
+newtype Shape_Tag = Shape_Tag
+  { unwrap :: HsBindgen.Runtime.LibC.Word8
+  }
+  deriving stock (GHC.Generics.Generic)
+  deriving stock (Eq, Ord, Read, Show)
+  deriving newtype
+    ( HsBindgen.Runtime.Marshal.StaticSize
+    , HsBindgen.Runtime.Marshal.ReadRaw
+    , HsBindgen.Runtime.Marshal.WriteRaw
+    , F.Storable
+    , HsBindgen.Runtime.Internal.HasFFIType.HasFFIType
+    , Data.Primitive.Types.Prim
+    , HsBindgen.Runtime.Internal.Bitfield.Bitfield
+    , Bits.Bits
+    , Bounded
+    , Enum
+    , FiniteBits
+    , Integral
+    , Ix.Ix
+    , Num
+    , Real
+    )
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType Shape_Tag) "unwrap")
+         ) => GHC.Records.HasField "unwrap" (Ptr.Ptr Shape_Tag) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"unwrap")
+
+instance HsBindgen.Runtime.HasCField.HasCField Shape_Tag "unwrap" where
+
+  type CFieldType Shape_Tag "unwrap" =
+    HsBindgen.Runtime.LibC.Word8
+
+  offset# = \_ -> \_ -> 0
+
+{-| __C declaration:__ @Circle@
+
+    __defined at:__ @garnet_rs.h 12:9@
+
+    __exported by:__ @garnet_rs.h@
+-}
+circle :: FC.CInt
+circle = (0 :: FC.CInt)
+
+{-| __C declaration:__ @Rectangle@
+
+    __defined at:__ @garnet_rs.h 13:9@
+
+    __exported by:__ @garnet_rs.h@
+-}
+rectangle :: FC.CInt
+rectangle = (1 :: FC.CInt)
+
+{-| __C declaration:__ @struct Circle_Body@
+
+    __defined at:__ @garnet_rs.h 15:16@
+
+    __exported by:__ @garnet_rs.h@
+-}
+data Circle_Body = Circle_Body
+  { radius :: FC.CDouble
+    {- ^ __C declaration:__ @radius@
+
+         __defined at:__ @garnet_rs.h 16:10@
+
+         __exported by:__ @garnet_rs.h@
+    -}
+  }
+  deriving stock (GHC.Generics.Generic)
+  deriving stock (Eq, Show)
+
+instance HsBindgen.Runtime.Marshal.StaticSize Circle_Body where
+
+  staticSizeOf = \_ -> (8 :: Int)
+
+  staticAlignment = \_ -> (8 :: Int)
+
+instance HsBindgen.Runtime.Marshal.ReadRaw Circle_Body where
+
+  readRaw =
+    \ptr0 ->
+          pure Circle_Body
+      <*> HsBindgen.Runtime.HasCField.readRaw (Data.Proxy.Proxy @"radius") ptr0
+
+instance HsBindgen.Runtime.Marshal.WriteRaw Circle_Body where
+
+  writeRaw =
+    \ptr0 ->
+      \s1 ->
+        case s1 of
+          Circle_Body radius2 ->
+            HsBindgen.Runtime.HasCField.writeRaw (Data.Proxy.Proxy @"radius") ptr0 radius2
+
+deriving via HsBindgen.Runtime.Marshal.EquivStorable Circle_Body instance F.Storable Circle_Body
+
+instance HsBindgen.Runtime.HasCField.HasCField Circle_Body "radius" where
+
+  type CFieldType Circle_Body "radius" = FC.CDouble
+
+  offset# = \_ -> \_ -> 0
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType Circle_Body) "radius")
+         ) => GHC.Records.HasField "radius" (Ptr.Ptr Circle_Body) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"radius")
+
+{-| __C declaration:__ @struct Rectangle_Body@
+
+    __defined at:__ @garnet_rs.h 19:16@
+
+    __exported by:__ @garnet_rs.h@
+-}
+data Rectangle_Body = Rectangle_Body
+  { width :: FC.CDouble
+    {- ^ __C declaration:__ @width@
+
+         __defined at:__ @garnet_rs.h 20:10@
+
+         __exported by:__ @garnet_rs.h@
+    -}
+  , height :: FC.CDouble
+    {- ^ __C declaration:__ @height@
+
+         __defined at:__ @garnet_rs.h 21:10@
+
+         __exported by:__ @garnet_rs.h@
+    -}
+  }
+  deriving stock (GHC.Generics.Generic)
+  deriving stock (Eq, Show)
+
+instance HsBindgen.Runtime.Marshal.StaticSize Rectangle_Body where
+
+  staticSizeOf = \_ -> (16 :: Int)
+
+  staticAlignment = \_ -> (8 :: Int)
+
+instance HsBindgen.Runtime.Marshal.ReadRaw Rectangle_Body where
+
+  readRaw =
+    \ptr0 ->
+          pure Rectangle_Body
+      <*> HsBindgen.Runtime.HasCField.readRaw (Data.Proxy.Proxy @"width") ptr0
+      <*> HsBindgen.Runtime.HasCField.readRaw (Data.Proxy.Proxy @"height") ptr0
+
+instance HsBindgen.Runtime.Marshal.WriteRaw Rectangle_Body where
+
+  writeRaw =
+    \ptr0 ->
+      \s1 ->
+        case s1 of
+          Rectangle_Body width2 height3 ->
+               HsBindgen.Runtime.HasCField.writeRaw (Data.Proxy.Proxy @"width") ptr0 width2
+            >> HsBindgen.Runtime.HasCField.writeRaw (Data.Proxy.Proxy @"height") ptr0 height3
+
+deriving via HsBindgen.Runtime.Marshal.EquivStorable Rectangle_Body instance F.Storable Rectangle_Body
+
+instance HsBindgen.Runtime.HasCField.HasCField Rectangle_Body "width" where
+
+  type CFieldType Rectangle_Body "width" = FC.CDouble
+
+  offset# = \_ -> \_ -> 0
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType Rectangle_Body) "width")
+         ) => GHC.Records.HasField "width" (Ptr.Ptr Rectangle_Body) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"width")
+
+instance HsBindgen.Runtime.HasCField.HasCField Rectangle_Body "height" where
+
+  type CFieldType Rectangle_Body "height" = FC.CDouble
+
+  offset# = \_ -> \_ -> 8
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType Rectangle_Body) "height")
+         ) => GHC.Records.HasField "height" (Ptr.Ptr Rectangle_Body) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"height")
+
+{-| __C declaration:__ @union \@Shape_body@
+
+    __defined at:__ @garnet_rs.h 26:3@
+
+    __exported by:__ @garnet_rs.h@
+-}
+newtype Shape_body = Shape_body
+  { unwrap :: Data.Array.Byte.ByteArray
+  }
+  deriving stock (GHC.Generics.Generic)
+
+deriving via (HsBindgen.Runtime.Internal.SizedByteArray.SizedByteArray 16) 8 instance HsBindgen.Runtime.Marshal.StaticSize Shape_body
+
+deriving via (HsBindgen.Runtime.Internal.SizedByteArray.SizedByteArray 16) 8 instance HsBindgen.Runtime.Marshal.ReadRaw Shape_body
+
+deriving via (HsBindgen.Runtime.Internal.SizedByteArray.SizedByteArray 16) 8 instance HsBindgen.Runtime.Marshal.WriteRaw Shape_body
+
+deriving via HsBindgen.Runtime.Marshal.EquivStorable Shape_body instance F.Storable Shape_body
+
+{-|
+
+  __See:__ 'set_shape_body_circle'
+
+__C declaration:__ @circle@
+
+__defined at:__ @garnet_rs.h 27:17@
+
+__exported by:__ @garnet_rs.h@
+-}
+get_shape_body_circle ::
+     Shape_body
+  -> Circle_Body
+get_shape_body_circle =
+  HsBindgen.Runtime.Internal.ByteArray.getUnionPayload
+
+{-|
+
+  __See:__ 'get_shape_body_circle'
+
+-}
+set_shape_body_circle ::
+     Circle_Body
+  -> Shape_body
+set_shape_body_circle =
+  HsBindgen.Runtime.Internal.ByteArray.setUnionPayload
+
+{-|
+
+  __See:__ 'set_shape_body_rectangle'
+
+__C declaration:__ @rectangle@
+
+__defined at:__ @garnet_rs.h 28:20@
+
+__exported by:__ @garnet_rs.h@
+-}
+get_shape_body_rectangle ::
+     Shape_body
+  -> Rectangle_Body
+get_shape_body_rectangle =
+  HsBindgen.Runtime.Internal.ByteArray.getUnionPayload
+
+{-|
+
+  __See:__ 'get_shape_body_rectangle'
+
+-}
+set_shape_body_rectangle ::
+     Rectangle_Body
+  -> Shape_body
+set_shape_body_rectangle =
+  HsBindgen.Runtime.Internal.ByteArray.setUnionPayload
+
+instance HsBindgen.Runtime.HasCField.HasCField Shape_body "circle" where
+
+  type CFieldType Shape_body "circle" = Circle_Body
+
+  offset# = \_ -> \_ -> 0
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType Shape_body) "circle")
+         ) => GHC.Records.HasField "circle" (Ptr.Ptr Shape_body) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"circle")
+
+instance HsBindgen.Runtime.HasCField.HasCField Shape_body "rectangle" where
+
+  type CFieldType Shape_body "rectangle" =
+    Rectangle_Body
+
+  offset# = \_ -> \_ -> 0
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType Shape_body) "rectangle")
+         ) => GHC.Records.HasField "rectangle" (Ptr.Ptr Shape_body) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"rectangle")
+
+{-| __C declaration:__ @struct Shape@
+
+    __defined at:__ @garnet_rs.h 24:16@
+
+    __exported by:__ @garnet_rs.h@
+-}
+data Shape = Shape
+  { tag :: Shape_Tag
+    {- ^ __C declaration:__ @tag@
+
+         __defined at:__ @garnet_rs.h 25:13@
+
+         __exported by:__ @garnet_rs.h@
+    -}
+  , body :: Shape_body
+    {- ^ __C declaration:__ @body@
+
+         __defined at:__ @garnet_rs.h 29:5@
+
+         __exported by:__ @garnet_rs.h@
+    -}
+  }
+  deriving stock (GHC.Generics.Generic)
+
+instance HsBindgen.Runtime.Marshal.StaticSize Shape where
+
+  staticSizeOf = \_ -> (24 :: Int)
+
+  staticAlignment = \_ -> (8 :: Int)
+
+instance HsBindgen.Runtime.Marshal.ReadRaw Shape where
+
+  readRaw =
+    \ptr0 ->
+          pure Shape
+      <*> HsBindgen.Runtime.HasCField.readRaw (Data.Proxy.Proxy @"tag") ptr0
+      <*> HsBindgen.Runtime.HasCField.readRaw (Data.Proxy.Proxy @"body") ptr0
+
+instance HsBindgen.Runtime.Marshal.WriteRaw Shape where
+
+  writeRaw =
+    \ptr0 ->
+      \s1 ->
+        case s1 of
+          Shape tag2 body3 ->
+               HsBindgen.Runtime.HasCField.writeRaw (Data.Proxy.Proxy @"tag") ptr0 tag2
+            >> HsBindgen.Runtime.HasCField.writeRaw (Data.Proxy.Proxy @"body") ptr0 body3
+
+deriving via HsBindgen.Runtime.Marshal.EquivStorable Shape instance F.Storable Shape
+
+instance HsBindgen.Runtime.HasCField.HasCField Shape "tag" where
+
+  type CFieldType Shape "tag" = Shape_Tag
+
+  offset# = \_ -> \_ -> 0
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType Shape) "tag")
+         ) => GHC.Records.HasField "tag" (Ptr.Ptr Shape) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"tag")
+
+instance HsBindgen.Runtime.HasCField.HasCField Shape "body" where
+
+  type CFieldType Shape "body" = Shape_body
+
+  offset# = \_ -> \_ -> 8
+
+instance ( TyEq ty ((HsBindgen.Runtime.HasCField.CFieldType Shape) "body")
+         ) => GHC.Records.HasField "body" (Ptr.Ptr Shape) (Ptr.Ptr ty) where
+
+  getField =
+    HsBindgen.Runtime.HasCField.fromPtr (Data.Proxy.Proxy @"body")

haskell/generated/GarnetRs/FunPtr.hs

@@ -0,0 +1,104 @@
+{-# LANGUAGE CApiFFI #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# OPTIONS_HADDOCK prune #-}
+
+module GarnetRs.FunPtr where
+
+import qualified Foreign.C as FC
+import qualified GHC.IO.Unsafe
+import qualified GHC.Ptr as Ptr
+import qualified HsBindgen.Runtime.Internal.CAPI
+import qualified HsBindgen.Runtime.Internal.HasFFIType
+import qualified HsBindgen.Runtime.PtrConst
+import Data.Void (Void)
+import GarnetRs
+import Prelude (IO)
+
+$(HsBindgen.Runtime.Internal.CAPI.addCSource (HsBindgen.Runtime.Internal.CAPI.unlines
+  [ "#include <garnet_rs.h>"
+  , "/* com_garnet_GarnetRs_get_hello */"
+  , "__attribute__ ((const))"
+  , "void (*hs_bindgen_faf62265b53521d3 (void)) ("
+  , "  char const *arg1"
+  , ")"
+  , "{"
+  , "  return &hello;"
+  , "}"
+  , "/* com_garnet_GarnetRs_get_hello_struct */"
+  , "__attribute__ ((const))"
+  , "void (*hs_bindgen_0f8c37ef19b17a6d (void)) ("
+  , "  struct T arg1"
+  , ")"
+  , "{"
+  , "  return &hello_struct;"
+  , "}"
+  , "/* com_garnet_GarnetRs_get_hello_shape */"
+  , "__attribute__ ((const))"
+  , "void (*hs_bindgen_287ff3ac660f333b (void)) ("
+  , "  struct Shape arg1"
+  , ")"
+  , "{"
+  , "  return &hello_shape;"
+  , "}"
+  ]))
+
+-- __unique:__ @com_garnet_GarnetRs_get_hello@
+foreign import ccall unsafe "hs_bindgen_faf62265b53521d3" hs_bindgen_faf62265b53521d3_base ::
+     IO (Ptr.FunPtr Void)
+
+-- __unique:__ @com_garnet_GarnetRs_get_hello@
+hs_bindgen_faf62265b53521d3 :: IO (Ptr.FunPtr ((HsBindgen.Runtime.PtrConst.PtrConst FC.CChar) -> IO ()))
+hs_bindgen_faf62265b53521d3 =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_faf62265b53521d3_base
+
+{-# NOINLINE hello #-}
+{-| __C declaration:__ @hello@
+
+    __defined at:__ @garnet_rs.h 32:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello :: Ptr.FunPtr ((HsBindgen.Runtime.PtrConst.PtrConst FC.CChar) -> IO ())
+hello =
+  GHC.IO.Unsafe.unsafePerformIO hs_bindgen_faf62265b53521d3
+
+-- __unique:__ @com_garnet_GarnetRs_get_hello_struct@
+foreign import ccall unsafe "hs_bindgen_0f8c37ef19b17a6d" hs_bindgen_0f8c37ef19b17a6d_base ::
+     IO (Ptr.FunPtr Void)
+
+-- __unique:__ @com_garnet_GarnetRs_get_hello_struct@
+hs_bindgen_0f8c37ef19b17a6d :: IO (Ptr.FunPtr (T -> IO ()))
+hs_bindgen_0f8c37ef19b17a6d =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_0f8c37ef19b17a6d_base
+
+{-# NOINLINE hello_struct #-}
+{-| __C declaration:__ @hello_struct@
+
+    __defined at:__ @garnet_rs.h 34:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello_struct :: Ptr.FunPtr (T -> IO ())
+hello_struct =
+  GHC.IO.Unsafe.unsafePerformIO hs_bindgen_0f8c37ef19b17a6d
+
+-- __unique:__ @com_garnet_GarnetRs_get_hello_shape@
+foreign import ccall unsafe "hs_bindgen_287ff3ac660f333b" hs_bindgen_287ff3ac660f333b_base ::
+     IO (Ptr.FunPtr Void)
+
+-- __unique:__ @com_garnet_GarnetRs_get_hello_shape@
+hs_bindgen_287ff3ac660f333b :: IO (Ptr.FunPtr (Shape -> IO ()))
+hs_bindgen_287ff3ac660f333b =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_287ff3ac660f333b_base
+
+{-# NOINLINE hello_shape #-}
+{-| __C declaration:__ @hello_shape@
+
+    __defined at:__ @garnet_rs.h 36:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello_shape :: Ptr.FunPtr (Shape -> IO ())
+hello_shape =
+  GHC.IO.Unsafe.unsafePerformIO hs_bindgen_287ff3ac660f333b

haskell/generated/GarnetRs/Safe.hs

@@ -0,0 +1,114 @@
+{-# LANGUAGE CApiFFI #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# OPTIONS_HADDOCK prune #-}
+
+module GarnetRs.Safe where
+
+import qualified Foreign as F
+import qualified Foreign.C as FC
+import qualified GHC.Ptr as Ptr
+import qualified HsBindgen.Runtime.Internal.CAPI
+import qualified HsBindgen.Runtime.Internal.HasFFIType
+import qualified HsBindgen.Runtime.PtrConst
+import Data.Void (Void)
+import GarnetRs
+import Prelude (IO)
+
+$(HsBindgen.Runtime.Internal.CAPI.addCSource (HsBindgen.Runtime.Internal.CAPI.unlines
+  [ "#include <garnet_rs.h>"
+  , "void hs_bindgen_433ea2a26af4e593 ("
+  , "  char const *arg1"
+  , ")"
+  , "{"
+  , "  hello(arg1);"
+  , "}"
+  , "void hs_bindgen_51157946af5519c9 ("
+  , "  struct T *arg1"
+  , ")"
+  , "{"
+  , "  hello_struct(*arg1);"
+  , "}"
+  , "void hs_bindgen_7de06f1fd827ca60 ("
+  , "  struct Shape *arg1"
+  , ")"
+  , "{"
+  , "  hello_shape(*arg1);"
+  , "}"
+  ]))
+
+-- __unique:__ @com_garnet_GarnetRs_Safe_hello@
+foreign import ccall safe "hs_bindgen_433ea2a26af4e593" hs_bindgen_433ea2a26af4e593_base ::
+     Ptr.Ptr Void
+  -> IO ()
+
+-- __unique:__ @com_garnet_GarnetRs_Safe_hello@
+hs_bindgen_433ea2a26af4e593 ::
+     HsBindgen.Runtime.PtrConst.PtrConst FC.CChar
+  -> IO ()
+hs_bindgen_433ea2a26af4e593 =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_433ea2a26af4e593_base
+
+{-| __C declaration:__ @hello@
+
+    __defined at:__ @garnet_rs.h 32:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello ::
+     HsBindgen.Runtime.PtrConst.PtrConst FC.CChar
+     -- ^ __C declaration:__ @c@
+  -> IO ()
+hello = hs_bindgen_433ea2a26af4e593
+
+-- __unique:__ @com_garnet_GarnetRs_Safe_hello_struct@
+foreign import ccall safe "hs_bindgen_51157946af5519c9" hs_bindgen_51157946af5519c9_base ::
+     Ptr.Ptr Void
+  -> IO ()
+
+-- __unique:__ @com_garnet_GarnetRs_Safe_hello_struct@
+hs_bindgen_51157946af5519c9 ::
+     Ptr.Ptr T
+  -> IO ()
+hs_bindgen_51157946af5519c9 =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_51157946af5519c9_base
+
+{-| __C declaration:__ @hello_struct@
+
+    __defined at:__ @garnet_rs.h 34:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello_struct ::
+     T
+     -- ^ __C declaration:__ @t@
+  -> IO ()
+hello_struct =
+  \t0 ->
+    F.with t0 (\t1 -> hs_bindgen_51157946af5519c9 t1)
+
+-- __unique:__ @com_garnet_GarnetRs_Safe_hello_shape@
+foreign import ccall safe "hs_bindgen_7de06f1fd827ca60" hs_bindgen_7de06f1fd827ca60_base ::
+     Ptr.Ptr Void
+  -> IO ()
+
+-- __unique:__ @com_garnet_GarnetRs_Safe_hello_shape@
+hs_bindgen_7de06f1fd827ca60 ::
+     Ptr.Ptr Shape
+  -> IO ()
+hs_bindgen_7de06f1fd827ca60 =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_7de06f1fd827ca60_base
+
+{-| __C declaration:__ @hello_shape@
+
+    __defined at:__ @garnet_rs.h 36:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello_shape ::
+     Shape
+     -- ^ __C declaration:__ @s@
+  -> IO ()
+hello_shape =
+  \s0 ->
+    F.with s0 (\s1 -> hs_bindgen_7de06f1fd827ca60 s1)

haskell/generated/GarnetRs/Unsafe.hs

@@ -0,0 +1,114 @@
+{-# LANGUAGE CApiFFI #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# OPTIONS_HADDOCK prune #-}
+
+module GarnetRs.Unsafe where
+
+import qualified Foreign as F
+import qualified Foreign.C as FC
+import qualified GHC.Ptr as Ptr
+import qualified HsBindgen.Runtime.Internal.CAPI
+import qualified HsBindgen.Runtime.Internal.HasFFIType
+import qualified HsBindgen.Runtime.PtrConst
+import Data.Void (Void)
+import GarnetRs
+import Prelude (IO)
+
+$(HsBindgen.Runtime.Internal.CAPI.addCSource (HsBindgen.Runtime.Internal.CAPI.unlines
+  [ "#include <garnet_rs.h>"
+  , "void hs_bindgen_2dfe97662a4d6377 ("
+  , "  char const *arg1"
+  , ")"
+  , "{"
+  , "  hello(arg1);"
+  , "}"
+  , "void hs_bindgen_29d823ada2bc7302 ("
+  , "  struct T *arg1"
+  , ")"
+  , "{"
+  , "  hello_struct(*arg1);"
+  , "}"
+  , "void hs_bindgen_b3f40a03f07eaa85 ("
+  , "  struct Shape *arg1"
+  , ")"
+  , "{"
+  , "  hello_shape(*arg1);"
+  , "}"
+  ]))
+
+-- __unique:__ @com_garnet_GarnetRs_Unsafe_hello@
+foreign import ccall unsafe "hs_bindgen_2dfe97662a4d6377" hs_bindgen_2dfe97662a4d6377_base ::
+     Ptr.Ptr Void
+  -> IO ()
+
+-- __unique:__ @com_garnet_GarnetRs_Unsafe_hello@
+hs_bindgen_2dfe97662a4d6377 ::
+     HsBindgen.Runtime.PtrConst.PtrConst FC.CChar
+  -> IO ()
+hs_bindgen_2dfe97662a4d6377 =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_2dfe97662a4d6377_base
+
+{-| __C declaration:__ @hello@
+
+    __defined at:__ @garnet_rs.h 32:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello ::
+     HsBindgen.Runtime.PtrConst.PtrConst FC.CChar
+     -- ^ __C declaration:__ @c@
+  -> IO ()
+hello = hs_bindgen_2dfe97662a4d6377
+
+-- __unique:__ @com_garnet_GarnetRs_Unsafe_hello_struct@
+foreign import ccall unsafe "hs_bindgen_29d823ada2bc7302" hs_bindgen_29d823ada2bc7302_base ::
+     Ptr.Ptr Void
+  -> IO ()
+
+-- __unique:__ @com_garnet_GarnetRs_Unsafe_hello_struct@
+hs_bindgen_29d823ada2bc7302 ::
+     Ptr.Ptr T
+  -> IO ()
+hs_bindgen_29d823ada2bc7302 =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_29d823ada2bc7302_base
+
+{-| __C declaration:__ @hello_struct@
+
+    __defined at:__ @garnet_rs.h 34:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello_struct ::
+     T
+     -- ^ __C declaration:__ @t@
+  -> IO ()
+hello_struct =
+  \t0 ->
+    F.with t0 (\t1 -> hs_bindgen_29d823ada2bc7302 t1)
+
+-- __unique:__ @com_garnet_GarnetRs_Unsafe_hello_shape@
+foreign import ccall unsafe "hs_bindgen_b3f40a03f07eaa85" hs_bindgen_b3f40a03f07eaa85_base ::
+     Ptr.Ptr Void
+  -> IO ()
+
+-- __unique:__ @com_garnet_GarnetRs_Unsafe_hello_shape@
+hs_bindgen_b3f40a03f07eaa85 ::
+     Ptr.Ptr Shape
+  -> IO ()
+hs_bindgen_b3f40a03f07eaa85 =
+  HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_b3f40a03f07eaa85_base
+
+{-| __C declaration:__ @hello_shape@
+
+    __defined at:__ @garnet_rs.h 36:6@
+
+    __exported by:__ @garnet_rs.h@
+-}
+hello_shape ::
+     Shape
+     -- ^ __C declaration:__ @s@
+  -> IO ()
+hello_shape =
+  \s0 ->
+    F.with s0 (\s1 -> hs_bindgen_b3f40a03f07eaa85 s1)

rust/Cargo.lock

@@ -3,5 +3,5 @@
 version = 4
 
 [[package]]
-name = "garnet"
+name = "garnet-rs"
 version = "0.1.0"

rust/Cargo.toml

@@ -1,8 +1,8 @@
 [package]
-name = "garnet"
+name = "garnet-rs"
 version = "0.1.0"
 edition = "2024"
 
-[[bin]]
+[lib]
-path = "main.rs"
+path = "lib.rs"
-name = "garnet"
+crate-type = ["staticlib"]

rust/garnet_rs.h

@@ -0,0 +1,36 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef struct T {
+  bool a;
+  uint8_t b;
+} T;
+
+typedef uint8_t Shape_Tag;
+#define Circle  0
+#define Rectangle  1
+
+typedef struct Circle_Body {
+  double radius;
+} Circle_Body;
+
+typedef struct Rectangle_Body {
+  double width;
+  double height;
+} Rectangle_Body;
+
+typedef struct Shape {
+  Shape_Tag tag;
+  union {
+    Circle_Body circle;
+    Rectangle_Body rectangle;
+  } body;
+} Shape;
+
+void hello(const char *c);
+
+void hello_struct(struct T t);
+
+void hello_shape(struct Shape s);

rust/lib.rs

@@ -0,0 +1,34 @@
+use std::ffi::{CStr, c_char};
+
+fn say_hello(name: &str) {
+    println!("Hello from Rust, {name}!");
+}
+
+#[unsafe(no_mangle)]
+extern "C" fn hello(c: *const c_char) -> () {
+    say_hello(unsafe { CStr::from_ptr(c) }.to_str().unwrap())
+}
+
+#[repr(C)]
+#[derive(Debug)]
+struct T {
+    a: bool,
+    b: u8,
+}
+#[unsafe(no_mangle)]
+extern "C" fn hello_struct(t: T) -> () {
+    say_hello(&format!("{:?}", t))
+}
+
+#[repr(C, u8)]
+#[allow(dead_code)]
+#[derive(Debug)]
+enum Shape {
+    Circle { radius: f64 },
+    Rectangle { width: f64, height: f64 },
+}
+
+#[unsafe(no_mangle)]
+extern "C" fn hello_shape(s: Shape) -> () {
+    say_hello(&format!("{:?}", s))
+}

rust/main.rs

@@ -1,3 +0,0 @@
-fn main() {
-    println!("Hello from Rust");
-}