PEGO
M040: Character Scan Loops
A value-free repetition of one character need not build an element value or create a full backtracking mark for every iteration. A scan loop performs the repeated membership or any-character test directly.
Mechanism and Invariants
The compiler recognizes eligible single-character class or dot repetitions only
when their result is unused. It preserves minimum/maximum repetition bounds,
input positions, terminal failure expectations and examined input. Captures,
actions or compound expressions that require per-element state remain on the
general repetition path. The bytecode compiler emits SCAN; generated emitters
produce an equivalent loop. Stream scans must retain reader loading behavior.
Scope and State
Applied by default to eligible value-free repetitions in closure, both VMs, generated Go Node/recognition, typed Go and TypeScript. Value-building repetitions are outside this specialization.
Reference Control
No shared same-source scan-disable comparator is maintained. A value-building parse changes the operation and is not an isolated reference.
Evidence and Implementation
Record 008 combines scan lowering with unmemoized-call changes and reports recognition reductions such as CSV 24 to 18 ms. Record 011 carries scan loops into generated Go. Those historical combined measurements do not assign the whole improvement to scanning or establish current performance in all six targets.
Evidence records: 008.
Source owners: Walker.scanRepeat,
closureBuilder.ScanRepeat,
SCAN lowering,
Scans pass,
Flow scan lowering, which lowers the
scans of generated Go and TypeScript.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.