Am Mo., 4. Dez. 2023 um 12:11 Uhr schrieb Sergei Egorov <e...@acm.org>:
> > I would like to repeat the argument that seems to be lost in the > discussion. After giving it some thought, now I can see that the two SRFIs > are not entirely incompatible. > > What if we explicitly mark situations where there is an identifier that > would refer to an outside scope in SRFI-251 semantics while referring to > "over-the-run-of-commands" definition according to SRFI-245 semantics, and > only those situations, as "it is an error"? Would it still leave enough > useful programs "in the clear"? If this is so, both SRFIs become different > implementation strategies for the same common idea; all current > implementations of both STFIs would stil be compliant, stepping into "is an > error" territory in different ways, but giving the compliant bodies the > same meaning. > The problem with this approach is that - unless it becomes mandatory to raise an exception when in the "it is an error situation" - Schemes and programs will start to diverge. > There's still a need to insert an occasional command in the middle of a > run of mutually recursive definitions, and it can be addressed by > introducing a *define! *form (any other name would do), such that > (define! <command>) is a definition, so it does not interrupt the "run" of > definitions. It can be just a macro expanding into R7RS (define-values () > (begin <command> (values)) or something implementation-dependent with the > same semantics. In fact, just this form alone can solve all the problems > mentioned in rationale, albeit a little more clumsily. > This approach has the advantage that no change to any expander is necessary and that it already works in R5RS, R6RS and R7RS-small. The name "define!" should be bike-shedded, of course. :) I would also make it a form taking zero, one or more commands. A similar approach is to extend define-values: (define-values <formals> <definition> ... <command> ... <expression>) will expand into (define-values <formals> (let () <definition> ... <command> ... <expression>)) This will already cover a lot of use cases, namely to allow long calculations for local variables appearing before the point of definition. For the other use cases, (define-values #f ...) will expand into (define-values <tmp> ... #f) One restriction of RnRS would still apply, namely that every form is either an expression or a definition. One cannot write a general `check-argument' macro that expands into the correct form in each context; but this is possibly a good thing. Marc
