Add print/println primitive

eval.ml

@@ -204,6 +204,24 @@ module External = struct
       |> (fun a -> String (Parser.assoc_to_string a))
     | _ -> failwith "get_op_assoc"
 
+  let print args =
+    let to_string = function
+      | Int n -> string_of_int n
+      | Float n -> string_of_float n
+      | Bool b -> string_of_bool b
+      | String s -> s
+      | Symbol s -> s
+      | _ -> failwith "print"
+    in
+    List.map to_string args
+    |> List.iter (Printf.printf "%s");
+    Nop
+
+  let println args =
+    ignore @@ print args;
+    Printf.printf "\n";
+    Nop
+
   let apply f args =
     let f = match f with
       | "sin" -> floatfun Float.sin
@@ -215,6 +233,8 @@ module External = struct
       | "get_op_pre" -> get_op_pre
       | "set_op_assoc" -> set_op_assoc
       | "get_op_assoc" -> get_op_assoc
+      | "print" -> print
+      | "println" -> println
       | _ -> raise @@ Invalid f
     in
     f args

main.ml

@@ -29,6 +29,7 @@ let stdlib = [
   "deg"; "rad";
   "get_op_pre"; "set_op_pre";
   "get_op_assoc"; "set_op_assoc";
+  "print"; "println";
 ]
   |> List.to_seq
   |> Seq.map (fun v -> v, External v)