Implementation of classical expressions: RFC 10

[jakelishman]

What should we add?

This is the tracking epic for RFC 10: preliminary representation of classical expressions. Details of the proposal are in the linked document.

Required

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1. Add representation of types and expressions for initial classical expressions qiskit-terra#10223
   type: feature request +1
   
2. Add support for classical expressions to SwitchCaseOp, WhileLoopOp and IfElseOp qiskit-terra#10224
   type: feature request +1
   
3. Add support to QuantumCircuit and DAGCircuit methods involving Expr expressions qiskit-terra#10233
   7 of 7
   type: epic type: feature request +2
   
4. Add support for Expr and Type values in QPY serialisation qiskit-terra#10289
   mod: qpy type: feature request +2
   
5. Add support for exporting Expr values to OpenQASM 3 qiskit-terra#10225
   type: feature request +1
   
6. Add testing for Expr values through the transpiler pipelines qiskit-terra#10234
   type: qa +1
7. Add changelog entry for Expr and Type work qiskit-terra#10331
   +0
   

Strongly desired for 0.25

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1. Add support for using Expr values in the control-flow builder interface qiskit-terra#10228
   type: feature request +1
   

Long term required, not necessary for 0.25

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1. Add support for Expr values to SabreSwap and SabreLayout qiskit-terra#10232
   mod: transpiler type: feature request +2
   
2. Add support for visualising Expr values in control flow #10236
   mod: visualization priority: medium type: feature request +3
3. Add a more convenient builder interface for Expr values qiskit-terra#10238
   type: feature request +1

[jakelishman]

As of right now, there are PRs open for every "feature" component of the new Expr work that's required for 0.25. I still have some testing work to write, but assuming there's nothing catastrophic lurking low down in the PR chain, the rest should be some bug fixes and QA stuff.

[kdk]

Here's the review order for the PRs under this issue:

• Refactor output of OpenQASM 3 exporter to use fewer aliases #10249
  • Add Expr support to OpenQASM 3 exporter #10408
• Add Expr supprt to DAGCircuit simple constructors #10362
  • Add support for semantic equality checking of Exprs in DAGCircuit #10367
    • Add Expr support to QuantumCircuit.compose #10375
      • Add Expr support to DAGCircuit.compose #10377
        • Add Expr support to DAGCircuit.substitute_node_with_dag #10381
        • Add Expr support in DAGCircuit.substitute_node #10382
        • Add Expr support to DAGCircuit.replace_block_with_node #10383
    • Add Expr support to QPY for conditions and targets #10392
    • Add Expr support to the control-flow builders #10400

[jakelishman]

This epic is now complete for the purposes of Terra 0.25.0rc1. I'll leave it open for the time being because it's tracking a couple more things for the final 0.25.0, but in principle it's complete from a feature perspective.

[jakelishman]

And with the merge of #10503, that's this epic complete. We still need to add further visualisation support as tracked in #10236, but that's for Qiskit 0.45, and #10504 at least fixes the crashes.

[1ucian0]

With this epic closed, can https://github.com/Qiskit/RFCs/blob/master/0010-simple-classical-representations.md be considered Implemented?

[jakelishman]

Yes