handler

Reusable handlers allow defining a set of effect handlers that can be included elsewhere:

handler ignore-io with
  on io.write(Text) => continue with nothing;
  on io.read(Text) => continue with "Placeholder";
end

With a handle block we can use this as:

handle
  let Name = io read;
  io write: "Hello, [Name]";
with
  use ignore-io;
end

These handlers can also be parameterised and include initialisation code:

type io(input is cell<list<text>>, output is cell<list<text>>);

handler capture-io io: (IO is io) do
  let Input = IO.input;
  let Output = IO.output;
with
  on io.write(Text) do
    Output <- Output value append: Text;
    continue with nothing;
  end

  on io.read() do
    let Value = Input value first;
    Input <- Input value rest;
    continue with Value;
  end
end

A handler can be automatically installed in the program by marking it as default. This causes all code to use this handler, unless a more specific handle block redefines its handled effects:

default handler ignore-io;