Scope Reuse Blocks (MVP Proposal)

Status: design-only during the feature‑pause (no implementation)

Summary

Give short, reusable logic a name within the current scope without promoting it to a top-level function.
Keep the core small: block body + postfix header sugar; desugar to local function + normal calls.
Zero runtime cost: lowers to let/if/call/ret only (no new instructions/closures).

Syntax (postfix header; Nyash style)

Block form (multi-statement):

{ /* BODY */ } scope name(arglist?) (-> Ret)?
// call within the same scope
name(args)

Expression form (one-liner):

=> EXPR  scope name(arglist?) (-> Ret)?

Semantics

Visibility: name is local to the defining scope; not exported.
Capture: by reference by default. Mutating captured vars requires explicit mut on those bindings.
Recursion: disallowed in MVP (can be lifted later).
Errors/exits: same as regular functions (return/cleanup/catch apply at the function boundary).

Lowering (desugaring)

Transform into a local function plus a local binding for convenience calls.

// { BODY } scope check(a:Int)->Str
// ↓ (conceptual)
let __cap_me = me; let __cap_locals = { /* needed refs */ };
method __scope_check__(a:Int)->Str {
  return BODY
}
let check = (x) => __scope_check__(x)

Captures are passed via hidden arguments or an environment box; no new VM opcodes.

Examples

{ if x % 2 == 0 { return "even" } return "odd" } scope parity(x:Int)->StringBox

for i in range(0,10) {
  print(parity(i))
}

Safety rules (MVP)

Capture: read-only by default; writes allowed only when the captured binding is declared mut.
Name uniqueness: scope name must be unique within the scope.
No cross-scope escape: values may be returned but the function reference itself is not exported.

Observability & Tooling

Add trace toggles (design only):
- NYASH_SCOPE_TRACE=1|json to emit enter/exit and capture lists as JSONL.
- Example: { "ev":"enter","sid":42,"caps":["me","cfg","mut total"] }.
Lints (design only):
- Single-use scope → suggest inline.
- Excess captures → suggest narrowing.

Interactions

Works with guard/with/await sugars (it’s just a call).
Compatible with ASI and postfix aesthetics; no new top-level keywords beyond scope suffix.

Tests (syntax-only smokes; design)

scope_basic: called twice → same result.
scope_capture_read: reads me/foo.
scope_capture_mut: mutation only allowed when mut is present.
scope_with_catch_cleanup: postfix catch/cleanup applied at local-function boundary.

Pause note

This is documentation and design intent only. Implementation is deferred until after the feature‑pause (post‑bootstrap).

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Scope Reuse Blocks (MVP Proposal)

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