handle expression

The handle expression introduces effect handlers to a piece of code. It has the following syntax:

handle
  Some;
  Expressions;
  Go;
  Here;
with
  on some-effect.operation() do
    continue with "ok";
  end

  on some-effect.other-operation() do
    return "nope";
  end
end

The do blocks may use the arrow form, which accepts a single complex expression:

handle
  Code;
with
  on some-effect.operation() => continue with "ok";
end

Additionally, it’s possible to reference reusable handlers:

handle
  Code;
with
  use some-reusable-handler;
  use other-reusable-handler parameter: Argument;
end

Example

Continuations

Given the effect:

effect increase with
  one(value is integer);
end

We can write the following handle block:

handle
  [
    perform increase.one(1),
    perform increase.one(2),
  ]
with
  on increase.one(Value) do
    continue with Value + 1;
  end
end

This will result in:

[2, 3]

Returns

Given the effect:

effect non-local with
  result(Value);
end

We can write the following handle block:

let Items = [1, 2, 3, 4];

handle
  for Item in self do
    transcript show: Item;
    perform non-local.result(Item);
  end
with
  on non-local.result(Value) do
    return Value;
  end
end

This will result in the transcript showing 1, and immediatelly making that the result of the entire handle block, without touching any of the other items.