Better drawings for control flow

What should we add?

Currently, if_else, while_loop and for_loop are represented in drawings as black boxes. Additionally, the tested condition is not explicit in the drawing.

I propose adding a more explicit representation. In the figure below I propose two alternatives, so that we can choose one or discuss an even better option.

[enavarro51]

@gabrieleagl I thought of something similar to your top drawing, but I think it’s probably simpler and more easily readable if we just put the condition for the IfElseOp in text at the top. And for the loops that would work as well. More demos to follow.

[jakelishman]

I’m keen to stick close to what Edwin thinks is reasonably implementable here, because we 100% do not have the resources to make Qiskit’s visualisations “perfect”. We’ve generally maintained the position that we would support a more full-featured visualisation package being responsible for the best drawings, and that Qiskit’s internal support is just best-effort.

As a direct response: I think drawing the box around all the affected wires (both qubit and clbit), and not doing something different between qubits and clbits is the best way forwards. There are many edge cases, such as blocks that aren’t on visually contiguous qubits/clbits, that are already considered for opaque instructions (i.e. we draw a box from the top-most qubit to bottom-most clbit and label unused wires), and I think it will be simplest to stick close to that type of approach. It will also mean that if a control-flow block contains one of these opaque instructions, we won’t suddenly end up with the case where an opaque instruction is sticking out of the surrounding control-flow box.

I agree that writing out the condition / loop indices above the box(es) will probably be the neatest (and easier to implement).