PEGO
M010: First-Character Dispatch
An ordered choice may call several rules that cannot begin with the next input character. A conservative first-terminal guard rejects those alternatives before constructing their call and memo state.
Mechanism and Invariants
firstTerminal recognizes a required starting literal or class through safe
expression wrappers and calls. Unknown, left-recursive and Operators Rule starts
retain ordinary matching. A rejected alternative records the expectation its
first terminal would have produced. The next character is examined, so a
Document edit can invalidate the result. Depth-limit behavior also constrains
skipped calls. The VMs encode guards in GUARD instructions; generated parsers
emit equivalent tests. Alternatives that pass still execute in their original
order.
A case-insensitive literal ("x"i, or a literal under #ignorecase) is guarded by
both cases of its first ASCII letter in the VMs and in the generated Go and TypeScript
parsers, and in the typed Go path through the Core first set (a lower-case letter with a
fold flag). Record 119 has the measurement.
Scope and State
Applied by default to eligible choices in closure, both VMs, generated Go Node/recognition, typed Go and TypeScript. It is not a reordering of PEG alternatives or a general lookahead prediction.
Reference Control
No common same-source guard-disable control is maintained across all backends.
Old bytecode without GUARD is compatibility input, not a current-source
reference compiler.
Evidence and Implementation
Historical records 049–051 report JSON parsing improvements of 13% in generated Node, 7% in closure, and 13%/15% in the VMs on their recorded runs. Record 050 also measures incremental reparse benefits. The revisions and operations differ; the percentages cannot be combined or generalized to unknown starts or current TypeScript workloads.
Evidence records: 049, 050, 051.
Source owners: firstTerminal,
Walker.ExprForm and Compiler.first,
closureBuilder.Choice and guard, GUARD emission, generated choice guards, TypeScript choice guards.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.