PEGO
E050: Projected Repetition Values
A captured repetition often builds an element record only so an action can extract one field. Projection collects that field while matching, avoiding intermediate records and the later field traversal.
Mechanism and Invariants
RuleProjections accepts a capture used only as map($x, (e) => e.$f). No
predicate may read $x, positional body values must be unused, both capture
names must occur once, and every element must produce a non-nil f. These
checks prevent replacing a field-access error with a successful projection.
Operators Rule bodies are excluded. The VM lowers collection through NEXT mode
3; Go and TypeScript emit equivalent collection and action rewrites. Matching
and capture rollback still run normally.
Scope and State
Applied by default to eligible repetitions in closure, recursive VM, iterative
VM, generated Go Node, typed Go and TypeScript. It does not make arbitrary map
calls into fused operations.
Reference Control
Internal compilation option NoProjections constructs a same-source general
route for differential tests. Corpus comparisons use that engine reference to
check all optimized backends. It is not a public parser option or a standalone
disable selector for every generated output.
Evidence and Implementation
Historical record 052 reports generated CSV changing from 3.9 to 2.7 ms and 14.0 to 8.7 MB; 053 reports closure CSV from 6.1 to 4.7 ms and smaller improvements in both VMs. Corpus checks compare against the no-projection engine. Those historical timings do not establish the current isolated effect or TypeScript timing.
Source owners: RuleProjections,
Walker.ProjectRepeat,
closureBuilder.ProjectRepeat, projected repetition lowering,
Flow projectRepeat, which lowers the
projected repetitions of generated Go and TypeScript.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.