feat: empty function parameters become unit param (#13)

Directly match out an empty list rather than using empty string "" for
label.

bindgen/caml.ml

@@ -32,13 +32,20 @@ let rec core_type_from_ir typ =
   | Ir.Prim Char -> Typ.constr (lid "char") []
   | Ir.Prim Void -> Typ.constr (lid "unit") []
   | Ir.Ptr t -> core_type_from_ir t
-  | Ir.Func { fn_ret; fn_params } ->
-      List.fold_left
-        (fun acc (name, typ) ->
-          let label = Asttypes.Labelled name in
-          let typ = core_type_from_ir typ in
-          Typ.arrow label typ acc)
-        (core_type_from_ir fn_ret) fn_params
+  | Ir.Func { fn_ret; fn_params } -> (
+      match fn_params with
+      | [] ->
+          (* If the C function declaration has no parameters we must introduce a `unit` param *)
+          Typ.arrow Asttypes.Nolabel
+            (core_type_from_ir (Ir.Prim Void))
+            (core_type_from_ir fn_ret)
+      | params ->
+          List.fold_left
+            (fun acc (name, typ) ->
+              let label = Asttypes.Labelled name in
+              let typ = core_type_from_ir typ in
+              Typ.arrow label typ acc)
+            (core_type_from_ir fn_ret) params)
 
 let type_from_ir typ =
   match typ with

examples/caml_doggo.c

@@ -31,4 +31,10 @@ void caml_eleven_out_of_ten_majestic_af(value caml_pupper) {
   CAMLreturn0;
 }
 
+void caml_no_input_no_output() {
+  CAMLparam0();
+  no_input_no_output();
+  CAMLreturn0;
+}
+
 

examples/doggo.c

@@ -13,3 +13,6 @@ void eleven_out_of_ten_majestic_af(Doggo* pupper) {
   printf("doggo is a %s\n", BreedToString[pupper->breed]);
 }
 
+void no_input_no_output(void) {
+  printf("We are doing nothing (of importance)\n");
+}

examples/doggo.h

@@ -12,3 +12,5 @@ typedef struct Doggo {
 } Doggo;
 
 void eleven_out_of_ten_majestic_af(Doggo* pupper);
+
+void no_input_no_output(void);

examples/doggo.ml

@@ -10,3 +10,4 @@ type nonrec doggo = {
   wow: char }
 external eleven_out_of_ten_majestic_af :
   pupper:doggo -> unit = "caml_eleven_out_of_ten_majestic_af"
+external no_input_no_output : unit -> unit = "caml_no_input_no_output"