compile/dyn-wind

Symbol: compile/dyn-wind

Syntax:

• Positional: (dyn-wind BEFORE BODY AFTER) – three elements in fixed order; BEFORE and/or AFTER may be nil.

• Keyword: (dyn-wind [before: BEFOREBODY...] [body: BODY...] [after: AFTERBODY...]) – top-level lists with marker keywords; omitted segments default to nil; order is arbitrary.

Use-case: Dynamic-wind extent: run before thunk on entry, body in the middle, after thunk on exit. Both thunks run on continuation transfer (multi-shot). Before and after are implicitly wrapped in null-arity lambdas; body is compiled as (begin ...).

Behaviour: The form accepts either positional or keyword syntax. Detection: if any top-level element is a pair whose car is keyword before:, body:, or after:, keyword mode is used; otherwise positional mode. In keyword mode, segments are identified by their marker; missing segments are nil. Body is required; at least one of before or after is required. Emits: push before thunk (or CONST nil), push after thunk (or CONST nil), PUSH_WIND (arg 1 or 2), compile body, POP_WIND. Stack layout for PUSH_WIND 2: [..., before, after] (before below, after above).

Examples:

;; Positional: before, body, after
(dyn-wind
  (begin (setup))
  (begin (work) result)
  (begin (teardown)))

;; Positional: after only (before = nil)
(dyn-wind nil
  (begin 42)
  (begin (cleanup)))

;; Keyword: before, body, after
(dyn-wind
  (before: (setup))
  (body: (work) result)
  (after: (teardown)))

;; Keyword: body and after only (before omitted)
(dyn-wind
  (body: 42)
  (after: (cleanup)))

Also: layout handler dyn-wind.