One-Shot Continuations and dynamic-wind

When this matters

Use ordinary if, cond, and procedures first. Reach for call/cc only when a deeply nested computation needs one explicit upward escape, and add dynamic-wind when entry and cleanup must bracket that escape.

See it run

LISPEX

(call/cc (lambda (escape) (escape 'done) 'unreachable))

Observed result

OUTPUT

done

Read the example

call/cc passes an escape procedure to the lambda. Calling it with done immediately transfers that value to the enclosing call/cc. The later symbol unreachable is never evaluated, and the escape is consumed by this transfer.

dynamic-wind takes three procedures. One for going in, one for the work in the middle, and one for coming out.

LISPEX

(dynamic-wind
  (lambda () (display "before "))
  (lambda () (display "during "))
  (lambda () (display "after")))

Observed result

OUTPUT

before during after

The last procedure still runs when an escape in the middle carries you out. That is where cleanup belongs.

How to reason about it

The first invocation atomically consumes the continuation before transfer, and every later invocation is E340.
dynamic-wind runs before, then thunk, then after. Unwinding runs pending after thunks innermost first.
A new error or escape from an after thunk replaces the signal already in flight.

Choose quickly

Need	Prefer	Why
ordinary choice	`if` or `cond`	local control stays visible
return from a helper	normal procedure result	no non-local transfer
leave several nested calls once	`call/cc` escape	one explicit upward exit
cleanup around a possible escape	`dynamic-wind`	`after` runs during unwind
resume the same point repeatedly	redesign the flow	continuations are one-shot and non-reentrant

A common mistake

Multi-shot continuation reuse and dynamic-context re-entry are deferred.

Current boundaries

Host exceptions and host continuations do not implement guest transfer.

Keep going

Conditionals covers ordinary control flow. Diagnostics explains how errors travel along those same explicit interpreter signals.

Conditionals · Diagnostics