diff --git a/exe/GarnetRs/Wrapped.hs b/exe/GarnetRs/Wrapped.hs
index d045576..ee80ce0 100644
--- a/exe/GarnetRs/Wrapped.hs
+++ b/exe/GarnetRs/Wrapped.hs
@@ -41,5 +41,5 @@ helloStruct = Raw.hello_struct . convertT
 helloShape :: Shape -> IO ()
 helloShape = Raw.hello_shape . convertShape
 
-add :: Int64 -> Int64 -> IO Int64
+add :: Int64 -> Int64 -> Int64
 add = Raw.add
diff --git a/exe/Main.hs b/exe/Main.hs
index 80f3f47..9084e3b 100644
--- a/exe/Main.hs
+++ b/exe/Main.hs
@@ -9,4 +9,4 @@ main = do
     helloStruct T{a = False, b = maxBound}
     helloShape $ Circle 3.14
     helloShape $ Rectangle 10.0 5.0
-    putStrLn . ("3 + 4 = " <>) . show =<< add 3 4
+    putStrLn $ "3 + 4 = " <> show (add 3 4)
diff --git a/generated/GarnetRs/FunPtr.hs b/generated/GarnetRs/FunPtr.hs
index 77c1f94..253311f 100644
--- a/generated/GarnetRs/FunPtr.hs
+++ b/generated/GarnetRs/FunPtr.hs
@@ -125,7 +125,7 @@ hs_bindgen_bbabdbe61cd1eeb2 =
 {-# NOINLINE add #-}
 {-| __C declaration:__ @add@
 
-    __defined at:__ @garnet_rs.h 38:9@
+    __defined at:__ @garnet_rs.h 38:32@
 
     __exported by:__ @garnet_rs.h@
 -}
diff --git a/generated/GarnetRs/Safe.hs b/generated/GarnetRs/Safe.hs
index 7b0bec2..40b04fa 100644
--- a/generated/GarnetRs/Safe.hs
+++ b/generated/GarnetRs/Safe.hs
@@ -126,26 +126,30 @@ hello_shape =
 foreign import ccall safe "hs_bindgen_1c0c71fa74c428a9" hs_bindgen_1c0c71fa74c428a9_base ::
      GHC.Int.Int64
   -> GHC.Int.Int64
-  -> IO GHC.Int.Int64
+  -> GHC.Int.Int64
 
 -- __unique:__ @com_garnet_GarnetRs_Safe_add@
 hs_bindgen_1c0c71fa74c428a9 ::
      HsBindgen.Runtime.LibC.Int64
   -> HsBindgen.Runtime.LibC.Int64
-  -> IO HsBindgen.Runtime.LibC.Int64
+  -> HsBindgen.Runtime.LibC.Int64
 hs_bindgen_1c0c71fa74c428a9 =
   HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_1c0c71fa74c428a9_base
 
-{-| __C declaration:__ @add@
+{-|
 
-    __defined at:__ @garnet_rs.h 38:9@
+  Marked @__attribute((const))__@
 
-    __exported by:__ @garnet_rs.h@
+__C declaration:__ @add@
+
+__defined at:__ @garnet_rs.h 38:32@
+
+__exported by:__ @garnet_rs.h@
 -}
 add ::
      HsBindgen.Runtime.LibC.Int64
      -- ^ __C declaration:__ @a@
   -> HsBindgen.Runtime.LibC.Int64
      -- ^ __C declaration:__ @b@
-  -> IO HsBindgen.Runtime.LibC.Int64
+  -> HsBindgen.Runtime.LibC.Int64
 add = hs_bindgen_1c0c71fa74c428a9
diff --git a/generated/GarnetRs/Unsafe.hs b/generated/GarnetRs/Unsafe.hs
index 87f47f8..0fa12e0 100644
--- a/generated/GarnetRs/Unsafe.hs
+++ b/generated/GarnetRs/Unsafe.hs
@@ -126,26 +126,30 @@ hello_shape =
 foreign import ccall unsafe "hs_bindgen_20eb651f0a8faf48" hs_bindgen_20eb651f0a8faf48_base ::
      GHC.Int.Int64
   -> GHC.Int.Int64
-  -> IO GHC.Int.Int64
+  -> GHC.Int.Int64
 
 -- __unique:__ @com_garnet_GarnetRs_Unsafe_add@
 hs_bindgen_20eb651f0a8faf48 ::
      HsBindgen.Runtime.LibC.Int64
   -> HsBindgen.Runtime.LibC.Int64
-  -> IO HsBindgen.Runtime.LibC.Int64
+  -> HsBindgen.Runtime.LibC.Int64
 hs_bindgen_20eb651f0a8faf48 =
   HsBindgen.Runtime.Internal.HasFFIType.fromFFIType hs_bindgen_20eb651f0a8faf48_base
 
-{-| __C declaration:__ @add@
+{-|
 
-    __defined at:__ @garnet_rs.h 38:9@
+  Marked @__attribute((const))__@
 
-    __exported by:__ @garnet_rs.h@
+__C declaration:__ @add@
+
+__defined at:__ @garnet_rs.h 38:32@
+
+__exported by:__ @garnet_rs.h@
 -}
 add ::
      HsBindgen.Runtime.LibC.Int64
      -- ^ __C declaration:__ @a@
   -> HsBindgen.Runtime.LibC.Int64
      -- ^ __C declaration:__ @b@
-  -> IO HsBindgen.Runtime.LibC.Int64
+  -> HsBindgen.Runtime.LibC.Int64
 add = hs_bindgen_20eb651f0a8faf48
diff --git a/rust/garnet_rs.h b/rust/garnet_rs.h
index 15aaba1..3bef3e6 100644
--- a/rust/garnet_rs.h
+++ b/rust/garnet_rs.h
@@ -35,4 +35,4 @@ void hello_struct(struct T t);
 
 void hello_shape(struct Shape s);
 
-int64_t add(int64_t a, int64_t b);
+__attribute__((const)) int64_t add(int64_t a, int64_t b);
diff --git a/rust/lib.rs b/rust/lib.rs
index 5be6262..3e2f64f 100644
--- a/rust/lib.rs
+++ b/rust/lib.rs
@@ -34,6 +34,7 @@ extern "C" fn hello_shape(s: Shape) -> () {
     say_hello(&format!("{:?}", s))
 }
 
+/// cbindgen:prefix=__attribute__((const))
 #[unsafe(no_mangle)]
 extern "C" fn add(a: i64, b: i64) -> i64 {
     a + b