PEGO
E020: Value-Free Rule Twins
A CST rule can be called where its result is discarded and elsewhere where its tree is needed. A separate value-free twin avoids constructing the discarded tree without changing valued calls.
Mechanism and Invariants
twinOf creates an alternative compiled rule for CST rules without actions or
text types. Calls in discarded, atomic or eligible body contexts select it.
The twin has its own rule and memo identity; its nil result cannot contaminate
valued memo entries. Rules in left-recursion cycles are excluded because mixed
valued/value-free seed growth would change semantics. A callee's action is not
skipped by this CST-only transformation.
Scope and State
Default eligible twin lowering in closure, both VMs and generated Go Node, typed Go and TypeScript. The original valued rule remains available.
Reference Control
No maintained universal same-source twin-disable control exists. Recognition builds another program and is not a twin-only reference.
Evidence and Implementation
Record 001 measures twins together with body elision and transient-rule memo policy. Its JSON timing/allocation reductions cannot be assigned entirely to twins. No current isolated twin effect is reported.
Evidence records: 001.
Source owners: Compiler.twinOf and Rule.TwinOK, twin rule bytecode, all rule/twin generation, TypeScript twin rule generation.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.