refactor: apply for-comprehension pattern across collections and argparse

[bobzhang]

Summary

Follow-up to #3444 consolidating three stacked batches (originally #3445, #3446, #3447 — superseded by this PR) plus an additional aggressive sweep of simple unconditional-map loops. Converts imperative `let xs = []` / `Array::new(...)` + `for ... { xs.push(...) }` patterns to MoonBit array comprehensions.

Sites converted (18 files)

argparse:

• `help_render.mbt`: `positional_entries`, `subcommand_entries`, `group_members` (filter-map)
• `parser.mbt`: `group_usage_expr` (`is Some(idx)` binding)
• `parser_lookup.mbt`: `split_long` (simple map)
• `parser_positionals.mbt`: `positional_args` (pattern filter)

core collections:

• `deque/deque.mbt`: `ToJson` impl
• `hashset/hashset.mbt`: `to_array` (`is Some({ key, .. })` binding), `ToJson` impl
• `list/list.mbt`: `to_array`, `ToJson` impl
• `priority_queue/priority_queue.mbt`: `ToJson` impl
• `set/linked_hash_set.mbt`: `ToJson` impl
• `sorted_map/map.mbt`: `to_array`
• `sorted_map/deprecated.mbt`: `keys`, `values` (two-binding `for k, _ in self` / `for _, v in self`)

immut collections:

• `immut/hashmap/HAMT.mbt`: `to_array` (two-binding)
• `immut/priority_queue/types.mbt`: `ToJson` impl
• `immut/sorted_map/map.mbt`: `to_array` (two-binding)
• `immut/sorted_set/immutable_set.mbt`: `ToJson` impl
• `immut/vector/vector.mbt`: `ToJson` impl

misc:

• `bytes/internal/regex_engine/translate.mbt`: `Alternation` arm (`is (cr, pref2)` destructure)
• `env/env_native.mbt`: `get_cli_args_internal`

Patterns used

• Unconditional map: `[for x in iter => f(x)]`
• Filter-map: `[for x in iter if cond => f(x)]`
• Is-pattern binding: `[for x in iter if f(x) is Some(y) => g(y)]`
• Two-binding for Iter2: `[for k, v in map => (k, v)]`

Discovered limitation

MoonBit rejects calls to raising functions inside comprehensions:

`error: calling function with error is not allowed inside list comprehension`

This blocks conversion of `Array::filter`, `Array::filter_map`, `ArrayView::filter`, `ArrayView::filter_map`, and similar functions whose predicate/mapper closures are typed `(T) -> ... raise?`. Those are intentionally left as imperative loops.

Sites deliberately skipped

• `Array::new(capacity=...)` hot paths where the explicit capacity hint is load-bearing (`Deque::to_array`, `Vector::to_array`, `Array::extract_if`, `Array::split`).
• `parser_globals_merge` two-optional-loop concat — cleaner as-is.
• Closure-based `each_intervals` callbacks in regex symbol_map.

Test plan

• `moon check` clean
• `moon test` passes all 6333 tests
• `moon fmt --check` clean
• `codex review --uncommitted` confirms behavior-preserving (no correctness, API, or performance regressions)

Supersedes #3445, #3446, #3447.

🤖 Generated with Claude Code

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[coveralls]

Coverage Report for CI Build 3877

Coverage decreased (-0.008%) to 94.906%

Details

• Coverage decreased (-0.008%) from the base build.
• Patch coverage: 30 of 30 lines across 16 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	15507
Covered Lines:	14717
Line Coverage:	94.91%
Coverage Strength:	220574.09 hits per line

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💛 - Coveralls

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.