Evaluation of both primitive and programmed procedures should be alike upon domain errors and similar mal-invocations. I consider the primitive procedures first. When a primitive procedure is to signal an error it invokes the procedure locally bound to the symbol 'error. (MzScheme already has a binding for 'error in the top scope. I will define my own 'error here but I would certainly choose a different symbol to avoid problems that might arise.) 'error is invoked with two arguments:

• The procedure call, in the form of a Scheme list. The first in the list is the symbol conventionally bound to the reporting primitive procedure, the rest of the values in the list are the (evaluated) arguments passed to the primitive procedure.
• The continuation to which the unavailable result value was to have been provided.

There are many names which a program can use without defining such as write and sin. We propose here a name which the program can define without using.

Now we try to make it so that a programmed procedure can report errors this way. The body of the reporting procedure does not have access to the procedure bound to 'error at the call site. The continuation produced there is available however. I propose a new primitive procedure re that takes such a continuation c and one other argument and invokes the procedure bound to 'error at the call site as if (error c x) had been evaluated in the context of the call site. x is the continuation for the expression whose failure this is a report of. Perhaps the file name string is passed as the first argument.

An alternative to re of the above paragraph would merely be a primitive procedure, called erx here, that takes one argument and invokes the procedure named error in the caller’s caller. This call to error would pass no continuation or merely a primitive error reporting continuation.

None of these proposals allow sending arbitrary messages to the procedure named error. I shall try to explain why I think that this a good idea. An insufficient reason is that it allows easier coding or such routines.

Other errors: Some variation is needed for the missing file error in the procedure open-input-file. Also to be distinguished:
storing into an immutable object
to reference an unbound variable

We must consider if procedure re can be abused. This proposal sneaks up on exception returns and should be coordinated with any such design. I am not anxious to design exception returns for Scheme.

It the purpose of the language designers was to avoid requiring implementations to check for pair-ness of the argument of car for reasons of performance then this proposal fails to address that issue.

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