Got it. Thanks.
On Mon, Jul 14, 2014 at 11:44 AM, Kacper Gutowski <mwgam...@gmail.com> wrote: > Whoops, this thread was about the message rather than return value. > About that, standard states that event-message is local to a context and > that ⎕ES should create an exception in caller's context, but I don't > think it's properly specified what does it exactly mean. > > But the point is, that ⎕ES _is_ a part of the standard, not just APL2. > > > Anyway, regarding the result. > > On 2014-07-14 10:09:02, Blake McBride wrote: > > That "return nil" is clearly a typo and a further reflection of the poor > > quality of that spec. > > Admittedly standard is hard to read and underspecified in many places > but this is not the case here. > > Look at 5.3.4.1: > >> – Result: A metaclass containing *nil* and the members of *exception* > >> and *value*. > > Actually "return nil" is not a typo, but *nil* is not a *value*. > Every function that terminates returns _something_ in standard's > parlance — be it a value or an exception, or just nil — but it has > a result. > > Returning nil is having the same result as that of a defined function > whose header doesn't start with Z←, i.e. without return value. > > > I offer the following as proof that that comment is a > > typo: > > > > 1. Look at the signature of ⎕ES at the beginning of 11.5.7. It says: > > > > ⎕ES B > > > > it does not say: > > > > Z←⎕ES B > > > > like they do everywhere else when a quad function returns something. > > Form Z←Something is consistently used when Something returns a value. > But ⎕ES never returns any *value* according to the standard so it would > be incorrect to write Z←⎕ES B. In fact, it should result in value error. > > > 2. In IBM APL 2 ⎕ES never returns a value. > > > > 3. Having ⎕ES return a value did cause me an unnecessary problem. > > Yes, and the standard is on your and APL2's side here. > No value shall be returned, result shall be nil. > > > > > Where they day "return nil", they should have said "return". > > Look at 5.4.1 which justifies that wording: > >> – Return X: An evaluation sequence phrase used to specify that > >> evaluation of this evaluation sequence is to stop and that a *token* > >> is to be returned to the caller of the evaluation sequence. If X is > >> a *token*, then X is returned; if X is an *array*, a *token* of > >> class *constant* and content X is returned. > > Nil is not an array; nil is a token. The standard doesn't say what would > "return" without an object mean. > > The standard consistently uses verb "to return" for all kinds of results, > not only values; for example *signaling* an error also means that error > is _returned_: > > >> – Signal X: An evaluation sequence phrase used to specify that > >> evaluation of this evaluation sequence is to stop and that a *token* > >> whose class is X, where X is an error, is to be returned. > > > -k >
