Lambda the Ultimate

A rather amusing thread on the plt-scheme mailing list, reminded me of Wadler's paper Why Calculating is Better than Scheming from 1987, which wasn't discussed here as far as I recall.

It's a bit of a blast from the past, but still worth reading.

A while back I began tinkering with the idea of continuation-carrying exceptions as an approach to divide error handling policy from the mechanism. Of course, I later discovered I was putting old wine into new bottles. Common Lisp follows a similar approach with its Condition/Restart mechanism.

Anyhow, at that time a friend pointed me towards A Modular Verifiable Exception-Handling Mechanism by S.Yemini and D.Berry (1985).

The following varieties of handler responses to an exception can be identified in the literature:

1. Resume the signaller: Do something, then resume the operation where it left off.
2. Terminate the signaller: Do something, then return a substitute result of the required type for the signalling operation; if the operation is not a value returning operation, this reduces to doing something and returning to the construct following the invocation of the operation. This includes using alternative resources, alternative algorithms, and so on.
3. Retry the signaller: Do something, then invoke the signaller again.
4. Propagate the exception: Do something, then allow the invoker of the invoker of the signalling operation to respond to the detection of the exception.
5. Transfer control: Do something, then transfer control to another location in the program. This includes doing something and then terminating a closed construct containing the invocation.

This paper presents a new model for exception handling, called the replacement model. The replacement model, in contrast to other exception-handling proposals, supports all the handler responses of resumption, termination, retry, and exception propagation, within both statements and expressions, in a modular, simple, and uniform fashion. The model can be embedded in any expression-oriented language and can also be adapted to languages which are not expression oriented with almost all the above advantages. This paper presents the syntactic extensions for embedding the replacement model into Algol 68 and its operational semantics.

Without [an exception-handling mechanism], too much information is not hidden and coupling is high. Either the signaler has to be told more about what the invoker is doing, so that the signaler can do what the invoker would want done, or else the invoker has to be given more implementation details so that it can do the exception checking.

Whether the exception-handling mechanism be continuations or something else, I'd really love to see the modern stack-based languages (Java, C++, etc.) implement something much more along these lines. I have been bitten far too often by the high coupling that comes from the inability to separate error handling policy from the exception handling mechanism.

What does the F stand for in System F?

Thanks,
Chris