htcc-parser-regression-workflow

name	htcc-parser-regression-workflow
description	Fix htcc parser regressions with focused component tests, malformed-input QA, and nontermination safeguards.
risk	safe
source	project
date_added	2026-05-18

Use this skill when

• Editing src/Htcc/Parser/** or parser-facing code in app/Main.hs.
• Review feedback mentions parse acceptance, parse errors, ATEmpty, manyTill, EOF, Megaparsec, scope restoration, declarations, initializers, or implicit calls.
• A malformed C input can hang, parse successfully by mistake, or produce the wrong diagnostic.

Do not use this skill when

• The change is pure codegen/output behavior after parsing has already succeeded.

Investigation steps

1. Read the exact parser function and the nearest combinators it composes with.
2. Search for zero-width success in recursive or repeated contexts: M.many, M.manyTill, M.option, ATEmpty, M.eof, M.try.
3. Identify whether the parser must fail, consume input, or produce ATEmpty intentionally.
4. Check existing component tests in test/Tests/ComponentsTests/Parser/Combinators.hs before adding new helpers.

Regression test placement

• Put parser unit regressions in test/Tests/ComponentsTests/Parser/Combinators.hs when direct parseProgram, parseAssignExpr, or helper assertions can reproduce the bug.
• Put CLI parser regressions in subprocess tests only when stderr/exit-code behavior is the relevant contract.
• Prefer adding cases under the existing Parser.Program.* group matching the construct, such as function-call, scalar-initializer, or function-pointer-arithmetic.
• For call argument lists, trailing commas such as f(1,) are malformed. For initializer lists, check existing accepted C-like trailing-comma behavior before turning a trailing comma into a rejection.
• For initializer EOF regressions, consider both file-scope and local-scope shapes such as int g =, int g = {1, and int main(){ int x =.

Nontermination safeguards

• Any bug involving EOF, repeated combinators, or manyTill must have a timeout-guarded manual QA command, for example:

printf 'int main(){ return f(' | timeout 5s stack exec htcc -- /dev/stdin

• The expected outcome for malformed input is non-zero exit before the timeout, not success.
• Use timeout or gtimeout, whichever exists on the host, and report the command used.
• Do not fix nontermination by accepting malformed input as ATEmpty.

Verification

• stylish-haskell -i <all changed .hs files, including app/Main.hs when touched> test/Tests/ComponentsTests/Parser/Combinators.hs
• lsp_diagnostics on edited Haskell files when the diagnostics tool is available. If unavailable, state that and substitute stack build plus the relevant test command.
• stack test --test-arguments components.
• Full stack test for parser-wide behavior or when the change affects common expression/statement parsing.
• stack build.
• Manual CLI QA for malformed inputs when user-visible parser behavior or hangs are involved.

Review focus

• Does the fix remove the bad acceptance path without breaking intended empty statements or empty for sections?
• Do the tests cover both minimal malformed shape and comma/trailing-argument shape where applicable?
• Does the parser fail with a parse error rather than looping or producing a misleading successful AST?