Matthias, On Fri, Jul 12, 2013 at 7:36 PM, Greg Hendershott <greghendersh...@gmail.com > wrote:
> >> Then why not return lists instead of values? > > Error checking. > Well, expecting a number and receiving a list (instead of several values) is not very different from expecting a number and receiving a string. In both cases the user didn't read the specs correctly. But sure, the more checks we can have, the better. I suspect it has more to do with optimization (once again), than error checking. > See Dybvig's paper, Thanks for the reference, this is very informative and well written. The performance checks seem to be quite tailored toward `values', and there are cases where what you really need to do is to handle the returned values as a list. I read it quickly (for that first time) but the "error handling" part does not seem to be the main reason. Greg, > Although nowadays we have match-define and friends. > > (define-values (a b c) (return-3-values)) > (match-define (list a b c) (return-3-element-list)) > > Both would error appropriately. > Probably those cases are not the ones with problems. I guess one of the worst cases would be if you call: (foo (bar 3)) where (bar 3) returns a list, but at some point its implementation changes and returns both a list and a string, encapsulated as multiple values in a list. Then foo will not error correctly and it may be non-trivial to find the bug. But of course this is a backward-incompatible change so that's the kind of problem you can expect from such a change. A related problem is if you misread the docs and consider that `bar' returns `(list 3 3 3)' whereas it actually returns multiple values `(list "something" (list 3 3 3))'. An intermediate solution would be that `values' returns indeed a `list', but (somehow) tagged with 'values, so that one can now it's suppose to mean a multiple return value but can still be handled with all the tools from `list'. Thanks to all other repliers for their interesting comments. Laurent
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