feat!: Implement error handling for modified variables outside modifiers by nicolaassolini-qntm · Pull Request #1712 · Quantinuum/guppylang

nicolaassolini-qntm · 2026-05-01T15:34:12Z

This PR add checks to raise an error when we use variables outside a modifier block when their value was assigned inside that modifier block. Moreover it improve the error rendering for undefined variables when the variables are only defined inside the modifier block.

Track variables assigned inside modifier blocks during CFG analysis.
Report Variable assigned in modifier block when a previously defined variable is assigned in a modifier and then used outside it.
Add help notes for variables that are defined only inside a modifier block and later used outside it.
Preserve valid reassignment behavior when a variable is reassigned after the modifier block before being used.
Add regression coverage for nested modifiers, sequential modifiers, branches, captured classical variables, and variables defined only inside modifiers.

BREAKING CHANGE: Using a variable outside a modifier block after it was assigned inside that modifier block now raises a compilation error.

BEGIN_COMMIT_OVERRIDE
feat!: Error handling for outside use of variables scoped to modifier blocks (#1712)
END_COMMIT_OVERRIDE

…difier blocks

github-actions · 2026-05-01T15:37:18Z

Bencher Report

Branch	na/1700-raise-an-error-when-variables-are-used-outside-a-modifier-block
Testbed	Linux

Click to view all benchmark results

Benchmark	Latency	Benchmark Result microseconds (µs) (Result Δ%)	Upper Boundary microseconds (µs) (Limit %)
tests/benchmarks/test_big_array.py::test_big_array_check	📈 view plot 🚷 view threshold	870,780.05 µs (-14.09%) Baseline: 1,013,581.58 µs	1,064,260.65 µs (81.82%)
tests/benchmarks/test_big_array.py::test_big_array_compile	📈 view plot 🚷 view threshold	2,121,526.13 µs (-0.18%) Baseline: 2,125,295.27 µs	2,231,560.03 µs (95.07%)
tests/benchmarks/test_big_array.py::test_big_array_executable	📈 view plot 🚷 view threshold	8,777,533.78 µs (+1.93%) Baseline: 8,611,724.33 µs	9,042,310.55 µs (97.07%)
tests/benchmarks/test_ctrl_flow.py::test_many_ctrl_flow_check	📈 view plot 🚷 view threshold	109,183.00 µs (-4.36%) Baseline: 114,159.33 µs	119,867.30 µs (91.09%)
tests/benchmarks/test_ctrl_flow.py::test_many_ctrl_flow_compile	📈 view plot 🚷 view threshold	232,023.20 µs (-2.35%) Baseline: 237,598.15 µs	249,478.06 µs (93.00%)
tests/benchmarks/test_ctrl_flow.py::test_many_ctrl_flow_executable	📈 view plot 🚷 view threshold	952,387.91 µs (-27.05%) Baseline: 1,305,526.15 µs	1,370,802.46 µs (69.48%)
tests/benchmarks/test_prelude.py::test_import_guppy	📈 view plot 🚷 view threshold	51.21 µs (-5.92%) Baseline: 54.44 µs	57.16 µs (89.60%)
tests/benchmarks/test_queue_push_pop.py::test_queue_push_benchmark	📈 view plot 🚷 view threshold	463,308.71 µs (-2.51%) Baseline: 475,216.79 µs	498,977.63 µs (92.85%)
tests/benchmarks/test_queue_push_pop.py::test_queue_push_pop_benchmark	📈 view plot 🚷 view threshold	619,643.00 µs (-1.36%) Baseline: 628,161.81 µs	659,569.90 µs (93.95%)

🐰 View full continuous benchmarking report in Bencher

github-actions · 2026-05-01T15:37:19Z

Bencher Report

Branch	na/1700-raise-an-error-when-variables-are-used-outside-a-modifier-block
Testbed	Linux

Click to view all benchmark results

Benchmark	hugr_bytes	Benchmark Result bytes x 1e3 (Result Δ%)	Upper Boundary bytes x 1e3 (Limit %)	hugr_nodes	Benchmark Result nodes (Result Δ%)	Upper Boundary nodes (Limit %)
tests/benchmarks/test_big_array.py::test_big_array_compile	📈 view plot 🚷 view threshold	158.77 x 1e3 (0.00%) Baseline: 158.77 x 1e3	160.36 x 1e3 (99.01%)	📈 view plot 🚷 view threshold	6,641.00 (0.00%) Baseline: 6,641.00	6,707.41 (99.01%)
tests/benchmarks/test_ctrl_flow.py::test_many_ctrl_flow_compile	📈 view plot 🚷 view threshold	27.54 x 1e3 (0.00%) Baseline: 27.54 x 1e3	27.81 x 1e3 (99.01%)	📈 view plot 🚷 view threshold	1,074.00 (0.00%) Baseline: 1,074.00	1,084.74 (99.01%)
tests/benchmarks/test_queue_push_pop.py::test_queue_push_benchmark_compile	📈 view plot 🚷 view threshold	10.91 x 1e3 (0.00%) Baseline: 10.91 x 1e3	11.02 x 1e3 (99.01%)	📈 view plot 🚷 view threshold	308.00 (0.00%) Baseline: 308.00	311.08 (99.01%)
tests/benchmarks/test_queue_push_pop.py::test_queue_push_pop_benchmark_compile	📈 view plot 🚷 view threshold	14.84 x 1e3 (0.00%) Baseline: 14.84 x 1e3	14.99 x 1e3 (99.01%)	📈 view plot 🚷 view threshold	435.00 (0.00%) Baseline: 435.00	439.35 (99.01%)

🐰 View full continuous benchmarking report in Bencher

codecov-commenter · 2026-05-01T15:37:39Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (561882f) to head (88095f1).
⚠️ Report is 50 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1712      +/-   ##
==========================================
- Coverage   93.47%   92.93%   -0.54%     
==========================================
  Files         133      136       +3     
  Lines       12697    13263     +566     
==========================================
+ Hits        11868    12326     +458     
- Misses        829      937     +108

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Copilot

Pull request overview

This PR addresses #1700 by detecting when classical/copyable variables captured by a modifier block are assigned within that block, and raising an error if the stale outer binding is used afterward.

Changes:

Track captured variables that are assigned inside modifier blocks (CheckedModifiedBlock.modified_captured), including nested modifier bodies.
Extend linearity checking to mark such variables as unusable after the modifier and emit a dedicated diagnostic with a note pointing to the in-modifier assignment.
Add new error fixtures covering basic, sequential, nested, multiple-variable, and branching scenarios.

Reviewed changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/error/modifier_errors/captured_classical_modified.py	New error fixture: modified captured variable used after a modifier block.
tests/error/modifier_errors/captured_classical_modified.err	Expected diagnostic output for the above scenario.
tests/error/modifier_errors/captured_classical_modified_sequential.py	New error fixture: sequential modifier blocks reusing a modified captured variable.
tests/error/modifier_errors/captured_classical_modified_sequential.err	Expected diagnostic output for sequential modifier case.
tests/error/modifier_errors/captured_classical_modified_nested.py	New error fixture: modification occurs inside nested modifiers.
tests/error/modifier_errors/captured_classical_modified_nested.err	Expected diagnostic output for nested modifier case.
tests/error/modifier_errors/captured_classical_modified_multiple.py	New error fixture: multiple captured variables modified; verifies first reported.
tests/error/modifier_errors/captured_classical_modified_multiple.err	Expected diagnostic output for multiple modified captures.
tests/error/modifier_errors/captured_classical_modified_branch.py	New error fixture: usage in a branch after a modifier.
tests/error/modifier_errors/captured_classical_modified_branch.err	Expected diagnostic output for branch usage case.
tests/error/errors_on_usage/branch_in_modifier.py	New error fixture ensuring “not defined” behavior remains for vars only defined inside modifier branches.
tests/error/errors_on_usage/branch_in_modifier.err	Expected output for “Variable not defined” regression case.
guppylang-internals/src/guppylang_internals/nodes.py	Add `modified_captured` field to `CheckedModifiedBlock` for later diagnostics/linearity.
guppylang-internals/src/guppylang_internals/checker/stmt_checker.py	Minor rename cleanup: return the checked modified block variable consistently.
guppylang-internals/src/guppylang_internals/checker/modifier_checker.py	Compute `modified_captured` by scanning CFG BB assignments, recursing into nested modifier blocks.
guppylang-internals/src/guppylang_internals/checker/linearity_checker.py	Introduce `UseKind.DEFINED_IN_MODIFIER` and raise `ModifiedVariableUsedError` on later uses; mark modified copyable captures as stale.
guppylang-internals/src/guppylang_internals/checker/errors/linearity.py	Add new `ModifiedVariableUsedError` diagnostic with note/help.
guppylang-internals/src/guppylang_internals/checker/cfg_checker.py	Adds commented-out debug print (should be removed).
guppylang-internals/src/guppylang_internals/cfg/cfg.py	Adds `cfg_as_string()` helper (currently unused except for commented debug line).
.gitignore	Ignore `AGENTS.md`.

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…unction

codspeed-hq · 2026-05-01T16:09:52Z

Merging this PR will not alter performance

✅ 9 untouched benchmarks

_{Comparing na/1700-raise-an-error-when-variables-are-used-outside-a-modifier-block (88095f1) with main (06a551d)}

…ocks

mark-koch

I had a initial look through, but I'm not convinced that a new UseKind is the best way to implement this. Currently, you're marking variables as used that are actually not used in the block, which doesn't feel good. In particular, what we actually care about is assignments, so presumably the assignment should track whether it happened in modifier block, not the use?

Furthermore, I think it would be better to check this before linearity checking and maybe even before type checking. This would be more consistent with how we currently catch undefined variable errors (see check_bb in cfg_checker.py). Any way we can include this logic at that point?

…ier_block for clarity

mark-koch · 2026-06-02T16:30:49Z

+        for bb in node.cfg.bbs:
+            bb_stat = bb.compute_variable_stats()
+            # We update the `assigned_in_modifier_block` field of the stats for
+            # each BB.
+            for var, ast_node in (
+                # We union the two dicts in this order to prioritize variables assigned
+                # in the outer modifier block
+                bb_stat.assigned_in_modifier_block | bb_stat.assigned
+            ).items():
+                # if already present we keep the first assignment
+                if var not in self.stats.assigned_in_modifier_block:
+                    self.stats.assigned_in_modifier_block[var] = ast_node
+            stats[bb] = bb_stat


Not sure I see what you're trying to do here? Can't we populate the fields during the visiting without having to do post processing?

The assigned_in_modifier_block variables are the one assigned in the block bb_stat.assigned plus that are assigned_in_modifier_block in bb_stats, i.e. variables that are assigned in a modifier block inside bb.
We are not going to visit node.cfg.bbs directly, what I can do is trigger the compute_variable_stats and the get the info that I need.

…ty and update related error messages for variable scope in modifier blocks

Copilot

Pull request overview

Copilot reviewed 32 out of 32 changed files in this pull request and generated 2 comments.

mark-koch

Nice! This looks a lot cleaner now 👍

Just a couple of questions and maybe opportunities to simplify further

mark-koch · 2026-06-08T12:24:18Z

+            # We also check that the variables assigned in a modifier block are not
+            # in successor blocks
+            if x in bb.vars.assigned_in_modifier_block:
+                raise GuppyError(
+                    _assigned_in_modifier_error(
+                        x, use_bb.vars.used[x], bb.vars.assigned_in_modifier_block[x]
+                    )
+                )


I think it would make more sense to move this into the if x in cfg.assigned_somewhere branch since it only applies to locals.

In particular, cfg.assigned_somewhere should probably include variables assigned in modifier blocks?

We should change the Assigment analysis to have the variables assigned in the block registered in cfg.assigned_somewhere

mark-koch · 2026-06-08T12:37:42Z

-            x: b for x, b in live_after.items() if x not in stats.assigned
+            x: b
+            for x, b in live_after.items()
+            if x not in stats.assigned and x not in stats.assigned_in_modifier_block


Isn't assigned_in_modifier_block a subset of assigned?

no it is not

mark-koch · 2026-06-08T12:38:46Z

+            if (
+                x not in self.stats.assigned
+                and x not in self.stats.used
+                and x not in self.stats.assigned_in_modifier_block


Same here, assigned_in_modifier_block is a subset of assigned, so no need to check it?

mark-koch · 2026-06-08T12:39:32Z

+                # We union the two dicts in this order to prioritize variables assigned
+                # in the outer modifier block
+                bb_stat.assigned_in_modifier_block | bb_stat.assigned


Same here, bb_stat.assigned should already contain everything?

Before this pr, assigned didn't include variables assigned in the block; thus, I defined a new set of variables. In principle, I think we can use bb_stat.assigned for everything? Not sure if this can break how we detect that a variable defined in a modifier block inside a bb is badly used in another bb

Oh yes I see, sorry! In that case it's fine as is 👍

…improve related error messages

nicolaassolini-qntm added 2 commits May 1, 2026 16:29

feat: implement error handling for modified variables used outside mo…

06413e7

…difier blocks

typo

3a13ff6

nicolaassolini-qntm requested a review from mark-koch May 1, 2026 15:34

nicolaassolini-qntm requested a review from a team as a code owner May 1, 2026 15:34

nicolaassolini-qntm linked an issue May 1, 2026 that may be closed by this pull request

[Feature]: Raise an error when variables are used outside a modifier block #1700

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nicolaassolini-qntm changed the title ~~Implement error handling for modified variables outside modifiers~~ feat: Implement error handling for modified variables outside modifiers May 1, 2026

nicolaassolini-qntm requested a review from Copilot May 1, 2026 15:35

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Comment thread guppylang-internals/src/guppylang_internals/checker/cfg_checker.py Outdated

Comment thread guppylang-internals/src/guppylang_internals/cfg/cfg.py Outdated

refactor: remove unused cfg_as_string method and clean up check_cfg f…

8f4de34

…unction

nicolaassolini-qntm changed the title ~~feat: Implement error handling for modified variables outside modifiers~~ feat(guppylang_internals)!: Implement error handling for modified variables outside modifiers May 1, 2026

nicolaassolini-qntm marked this pull request as draft May 1, 2026 16:05

comment about an issue

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nicolaassolini-qntm mentioned this pull request May 8, 2026

[Feature]: Allow assigment in all modifier blocks #1639

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nicolaassolini-qntm added 5 commits May 8, 2026 16:21

feat: enhance error handling for modified variables in modifier blocks

713ed21

fix: correct error messages for variable modifications in modifier bl…

e98f002

…ocks

feat: add error handling for variable usage outside modifier blocks

e3ccd0d

chore: remove AGENTS.md from .gitignore

3bee804

refactor: remove unused cfg_as_string method from CFG class

675e968

nicolaassolini-qntm marked this pull request as ready for review May 11, 2026 07:05

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suggestions

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mark-koch reviewed May 21, 2026

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Comment thread guppylang-internals/src/guppylang_internals/nodes.py Outdated

Comment thread guppylang-internals/src/guppylang_internals/checker/linearity_checker.py Outdated

Comment thread guppylang-internals/src/guppylang_internals/checker/errors/linearity.py Outdated

TODO

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fix: typo in error message for variable scope in modifier blocks

8f85e4a

nicolaassolini-qntm force-pushed the na/1700-raise-an-error-when-variables-are-used-outside-a-modifier-block branch from f6fda46 to 8f85e4a Compare May 26, 2026 14:44

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refactor: rename last_assigned_in_modifier_block to assigned_in_modif…

deaa5bd

…ier_block for clarity

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refactor: rename last_assigned_in_mod to assigned_in_mod for consistency

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mark-koch reviewed Jun 2, 2026

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nicolaassolini-qntm added 4 commits June 3, 2026 16:11

refactor: rename assigned_in_mod to assigned_under_modifier for clari…

e33f3a6

…ty and update related error messages for variable scope in modifier blocks

test: add error handling for variable usage outside modifier blocks

a64d44d

improved implementation

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removed unused coments and code

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Comment thread guppylang-internals/src/guppylang_internals/checker/cfg_checker.py

Comment thread guppylang-internals/src/guppylang_internals/checker/cfg_checker.py Outdated

mark-koch reviewed Jun 8, 2026

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update error handling for variable usage outside modifier blocks and …

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…improve related error messages

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Merged via the queue into main with commit cab8077 Jun 8, 2026
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nicolaassolini-qntm deleted the na/1700-raise-an-error-when-variables-are-used-outside-a-modifier-block branch June 8, 2026 16:16

hugrbot mentioned this pull request Jun 8, 2026

chore: release guppylang-internals 1.0.0-a3 #1826

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