AdaCore Blog

Using GNAT-LLVM to target Ada to WebAssembly

by Vadim Godunko

The GNAT-LLVM project provides an opportunity to port Ada to new platforms, one of which is WebAssembly. We conducted an experiment to evaluate the porting of Ada and the development of bindings to use Web API provided by the browser directly from Ada applications.

Subtotals

As a result of the experiment, the standard language library and runtime library were partially ported. Together with a binding for the Web API, this allowed us to write a simple example showing the possibility of using Ada for developing applications compiled into WebAssembly and executed inside the browser. At the same time, there are some limitations both of WebAssembly and of the current GNAT-LLVM implementation:

  • the inability to use tasks and protected types
  • support for exceptions limited to local propagation and the last chance handler
  • the inability to use nested subprograms

Example

Here is small example of an Ada program that shows/hides the text when pressing the button by manipulating attributes of document nodes.

with Web.DOM.Event_Listeners;
with Web.DOM.Events;
with Web.HTML.Buttons;
with Web.HTML.Elements;
with Web.Strings;
with Web.Window;

package body Demo is

   function "+" (Item : Wide_Wide_String) return Web.Strings.Web_String
     renames Web.Strings.To_Web_String;

   type Listener is
     limited new Web.DOM.Event_Listeners.Event_Listener with null record;

   overriding procedure Handle_Event
    (Self  : in out Listener;
     Event : in out Web.DOM.Events.Event'Class);

   L : aliased Listener;

   ------------------
   -- Handle_Event --
   ------------------

   overriding procedure Handle_Event
    (Self  : in out Listener;
     Event : in out Web.DOM.Events.Event'Class)
   is
      X : Web.HTML.Elements.HTML_Element
        := Web.Window.Document.Get_Element_By_Id (+"toggle_label");

   begin
      X.Set_Hidden (not X.Get_Hidden);
   end Handle_Event;

   ---------------------
   -- Initialize_Demo --
   ---------------------

   procedure Initialize_Demo is
      B : Web.HTML.Buttons.HTML_Button
        := Web.Window.Document.Get_Element_By_Id
            (+"toggle_button").As_HTML_Button;

   begin
      B.Add_Event_Listener (+"click", L'Access);
      B.Set_Disabled (False);
   end Initialize_Demo;

begin
   Initialize_Demo;
end Demo;

As you can see, it uses elaboration, tagged and interface types, and callbacks.

Live demo


Setup & Build

To compile the examples you need to setup GNAT-LLVM & GNAT WASM RTL following instructions in README.md file. 

To compile specific example use gprbuild to build application and open index.html in the browser to run it.

Next steps

The source code is published in a repository on GitHub and we invite everyone to participate in the project.

Attachments

Posted in #GNAT     #LLVM    #WebAssembly    #Programming