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Kawa: Compiling Scheme to Java

1.18 Continuations

Scheme continuations "capture" the current execution state. They can be implemented by copying the stack, but this requires non-portable native code. Kawa continuations are implemented using Java exceptions, and can be used to prematurely exit (throw), but not to implement co-routines (which should use threads anyway).

class callcc extends Procedure1 { public Object apply1 (Object arg1) { Procedure proc = (Procedure) arg1; Continuation cont = new Continuation (); try { return proc.apply1(cont); } catch (CalledContinuation ex) { if (ex.continuation != cont) throw ex; // Re-throw. return ex.value; } } ... }

This is the Procedure that implements call-with-current-continuation. It creates cont, which is the "current continuation", and passes it to the incoming proc. If callcc catches a CalledContinuation exception it means that proc invoked some Continuation. If it is "our" continuation, return the value passed to the continuation; otherwise re-throw it up the stack until we get a matching handler. (I have left out code to detect unsupported invocation of cont after callcc returns.)

class Continuation extends Procedure1 { public Object apply1 (Object arg1) { throw new CalledContinuation (arg1, this); } ... }

A Continuation is the actual continuation object that is passed to callcc's argument; when it is invoked, it throws a CalledContinuation that contains the continuation and the value returned.

class CalledContinuation extends RuntimeException { Object value; Continuation continuation; CalledContinuation (Object value, Continuation continuation) { super ("call/cc called"); this.value = value; this.continuation = continuation; } ... }

CalledContinuation is the exception that is thrown when the continuation is invoked.