Add maybe/optional example

exe/Main.hs

@@ -2,6 +2,7 @@ module Main (main) where
 
 import Data.Vector.Storable qualified as V
 import GarnetRs.Wrapped
+import System.IO
 
 main :: IO ()
 main = do
@@ -13,3 +14,5 @@ main = do
     putStrLn $ "3 + 4 = " <> show (add 3 4)
     putStrLn $ "Tree sum: " <> show (sumTree (Fork (Fork (Leaf 1) (Fork (Leaf 2) (Leaf 3))) (Leaf 4)))
     putStrLn $ "Slice sum: " <> show (sumSlice $ V.fromList [0 .. 5])
+    putStrLn "Nothing." >> printOptional Nothing
+    putStr "Something: " >> hFlush stdout >> printOptional (Just 67)

lib/GarnetRs/Wrapped.hs

@@ -10,6 +10,7 @@ module GarnetRs.Wrapped (
     add,
     sumTree,
     sumSlice,
+    printOptional,
 ) where
 
 import Control.Monad.Cont
@@ -72,3 +73,8 @@ sumTree = unsafePerformIO . flip withBTree (flip with $ Raw.sum_tree . unsafeFro
 
 sumSlice :: V.Vector Int64 -> Int64
 sumSlice v = unsafePerformIO $ V.unsafeWith v \p -> Raw.sum_slice (unsafeFromPtr p) (fromIntegral $ V.length v)
+
+printOptional :: Maybe Int8 -> IO ()
+printOptional = \case
+    Nothing -> Raw.print_optional (unsafeFromPtr nullPtr)
+    Just t -> with t (Raw.print_optional . unsafeFromPtr)

rust/lib.rs

@@ -87,3 +87,11 @@ extern "C" fn sum_tree(t: &BTreeC) -> i64 {
 extern "C" fn sum_slice(v: *const i64, s: usize) -> i64 {
     unsafe { slice::from_raw_parts(v, s) }.iter().sum()
 }
+
+#[unsafe(no_mangle)]
+extern "C" fn print_optional(x: Option<&i8>) -> () {
+    match x {
+        Some(x) => println!("{}", x / 2),
+        None => {}
+    }
+}