Evidences that Content is actually covariant in its type argument.

We do not declare Content itself as covariant in the type definition because this would leak into the Thereafter.Aux pattern and thereby break the compiler's type inference.

Runs either empty or single after the computation f has run.

• If f does not produce a value of type A, empty is run. If empty completes normally, f is returned. If empty throws, the thrown exception is incorporated into f as follows:
  • If f did not throw an exception, the exception overrides the result of f.
  • If f did throw an exception, the exception from empty is added as a suppressed exception.
• Otherwise, if f produces a single value a of type A, then the behavior is equivalent to f.map(single).

This method is more general than thereafter because single can transform the value. This is necessary for instances such as cats.data.Nested where the nested computation must be transformed, which is not possible with thereafter for the outer computation. Note that a plain functor on the outer computation does not suffice because the transformation of the inner computation could throw and it is not guaranteed that the functor properly chains the exceptions. Nested computations are useful for keeping the several computations distinguishable, such as the synchronous and asynchronous parts of message processing.

The signature of single separates the Shape from the value to ensure that single only transforms the value, but does not change the shape. This is crucial for cats.data.Nested as we would otherwise have to be able to replace the inner computation with an empty content of the outer, which doesn't make sense.

Plugs the value x into the hole of the shape interpreted as a one-hole context.

Runs body after the computation f has completed.

runs body after the computation f has completed