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< All Decisions

Struct Copy-Update Syntax — Design Rationale

Problem Statement

Blink structs are intentionally immutable — no field mutation. To "update" a struct, you must reconstruct all fields manually:

let updated = Account { id: acct.id, owner: acct.owner, balance: acct.balance + amount }

This scales poorly. The self-hosting compiler's Node struct has 51 fields — updating one field requires spelling out all 51. This blocks the parser parallel-array migration (0jbhdq) and makes struct-heavy code fragile: adding a field to a type breaks every construction site.

Cross-Language Survey

  • Rust: Struct { field: val, ..other } — spread suffix, same type required, last position
  • OCaml: { record with field = val } — with keyword (taken in Blink for resource blocks)
  • Elm: { record | field = val } — pipe separator inside braces
  • JavaScript: { ...obj, field: val } — spread operator, any position, no type checking
  • Kotlin: data class + .copy(field = val) — method-based, compiler-generated

Panel Deliberation

Five panelists (systems, web/scripting, PLT, DevOps/tooling, AI/ML) independently proposed options, then voted.

Q1: Struct copy-update syntax (4-1 for Option A)

Option A: ..source suffix in struct literal

let updated = Account { balance: acct.balance + amount, ..acct }

Option B: copy keyword expression

let updated = copy acct { balance: acct.balance + amount }

Option C: update...with keyword expression

let updated = update acct with { balance: acct.balance + amount }
Expert Vote Reasoning
Systems A Zero-cost desugaring to flat field list in C. No new keywords — keywords have grammar-wide blast radius (copy and update are common identifiers). .. dual of pattern rest is structural symmetry, not coincidence.
Web/Scripting A Recognizable from Rust/JS. Symmetric with existing pattern .. — same sigil, opposite direction. Zero new vocabulary. Teachable in one sentence.
PLT A Pattern/construction duality is principled: .. means "fields I didn't name" in both directions. Sound typing rule — source must be same nominal type. Desugars before type inference.
DevOps A Single sigil means one formatter/LSP code path. Zero keyword-colorization edge cases. Error attribution is precise. Originally proposed Option C but switched — with reuse as contextual keyword creates diagnostic ambiguity.
AI/ML B (dissent) copy names the operation at first token — no sigil overloading. LLMs trained on Rust may place ..source in wrong position or confuse with range ... Counter: ..source in struct literal is a field expression LLMs handle reliably from Rust precedent.

Result: A — 4-1 (AI/ML dissented)

Rules

  1. source must be the same nominal type as the struct being constructed
  2. ..source must appear last in the field list (hard parse rule, not style)
  3. At most one ..source per literal
  4. Explicit fields shadow source fields
  5. Desugars to field-by-field copy at typecheck — zero codegen impact
  6. Source values win over field defaults for non-explicit fields

AI-First Review

Criterion Pass/Fail Notes
Learnability Pass ..source is dual of pattern ... One concept, two directions
Consistency Pass Reuses existing .. sigil. No new keywords or grammar categories
Generability Pass Rust precedent in training data. Position error caught by parser
Debuggability Pass Type mismatch and position errors are clear and actionable
Token efficiency Pass 51-field Node update: ~15 tokens vs ~300

0 criteria fail — no reconsideration needed.