Raise an error or warning in qml.compile if basis_set contains operations instead of strings.

[dwierichs]

Feature details

As the title suggests: qml.compile takes an argument basis_set, which is expected to be a sequence of operation names, i.e. strings.
If a user passes a sequence of PennyLane operator types, no error is raised but the basis set becomes equivalent to [].
It would be nice to raise a warning, or even an error, if users pass non-empty sequences that contain operators instead of operator names.

Implementation

Straightforward.

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

No response

[albi3ro]

Or we could update the stopping condition to:

class_types = tuple(o for o in basis_set if isinstance(o, type)
class_names = set(o for o in basis_set if isinstance(o, str))
def stopping_condition(obj):
    return obj.name in class_names or isinstance(obj, class_types)

Why raise an error when we can just support the operation types?

I think we could even turn this into a good first issue.

[dwierichs]

Haha, yes, this works, too, of course 🚀

[AnuravModak]

Hi @albi3ro and @dwierichs , if this is available for external contributors and want contribution on this, do let me know! Will be happy to take this up!

[albi3ro]

@AnuravModak Yes this issue is open for external contributions.

It will involve updating this function here:

pennylane/pennylane/transforms/compile.py

Line 185 in 2f9631b

def stop_at(obj):

to accept types specified in basis_set in addition to string based names. The docstring and type hints will also need to be updated.

We may still want to consider an error if basis_set contains anything that is not a string or an Operator subclass.