[Copilot]

Summary

A for-loop nested inside another for-loop failed to parse when the inner loop iterated over the same collection as the outer one. The parser raised KeyError: Symbol 'w' is not in scope:

r = arange(min_value=min_value, max_value=max_value)
outer_result = []
for v in r:
    inner_result = []
    for w in r:  # KeyError: Symbol 'w' is not in scope
        b = sub(w, x)
        inner_result.append(b)
    ...

Related Issue(s)

Resolves the nested-loop failure reported in #340.

Backward Compatibility

No breaking changes. Previously, referencing the iterated collection inside its own loop body raised a KeyError. It is now accepted. All existing unit tests pass.

Detailed Description

Cause. When a for-loop body was forked, the iterated symbol r was renamed to the loop variable v and r was dropped from the body's scope. An inner for w in r then could not find r, so w was never registered.

Changes in parsers/for_parser.py:

• parse_for_node keeps the iterated collection's port (r) in the body scope alongside the loop variable, via added_symbols. The body can then use r as a broadcast input while v is scattered from the same for-node port.
• _wire_inputs dedupes for-node inputs, since r can now appear as both a broadcast and a scattered port.

Resulting recipe. The outer for-node has inputs [x, r]. Its body has inputs [x, r, v], with v fed from r as the scattered edge and r fed from r as the broadcast edge.

Verification. The issue's workflow now parses and evaluates to 2.0, both called directly and via its recipe; I computed the expected value by hand. I added a regression test, TestNestedLoops, to tests/integration/parsers/test_parsing_for_nodes.py. It checks the ports of both the outer and inner for-nodes.

• Fixes Nested loops fail #340

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (6deaab4) to head (9174916).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #341   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files          53       53           
  Lines        4135     4135           
=======================================
  Hits         4133     4133           
  Misses          2        2           

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[samwaseda]

@liamhuber Maybe we should talk whether this is a valid way of using nested for loops, because we could also argue that arange should be written as a list of lists instead of a simple list.

[liamhuber]

Solution LGTM.

Discussion of why I agree we need a solution is over in #340