Erlang language issues
Wed Apr 17 08:48:48 CEST 2002
> Thomas Lindgren:
> [new and old type tests]
> >The old type tests must of course remain for backwards
> >compatibility, but they are now being renamed to the "is_" forms
> >very early in the compilation.
> I think my basic question is whether one can rename guard tests
> into the new operations directly. This way, one could get rid
> of having both type(X) and is_type(X). It would seem to be possible
> for most cases, perhaps excepting the dual mode of float(X) that
> you mention, or any similar cases.
> (Though looking at the context, it should be possible to deduce
> what version is intended: off the top of my head, on the toplevel
> one uses is_float(X), inside arithmetic one uses to_float(X) or
> whatever the operation may be called. Any questionable cases are
> flagged with a warning.)
> > /.../ the reason for the extension is to make guards more like
> > other expressions. A complication is that failure in the first
> > argument of a top-level guard 'or' (semicolon) goes on to try the
> > other alternative as if there were two clauses, while at the
> > expression-level the whole 'or'-operation fails immediately.
> I think making guards behave like expressions is a Good Thing,
> generally speaking, because it is more convenient to have full
> boolean expressions. As you say, the problem is nailing down the details.
> I suppose the behaviours above should be unified. If nothing else, for
> the sanity of programmers :-)
I have always thought of Erlang as a make-it-easy-for-the-programmer type of
language, especially with respect to understandability and ease-of-use; only
one or two ways of doing the same thing, clear language constructs etc. But
lately I find that Erlang is drifting away from that, my thought, with
things like: andalso, the MatchSpec used in ets/mnesia, etc.
Please do not misinterpret my intension here, some or most of these changes
are very useful; it is the syntax of them that concerns me. For instance, I
cannot see why 'and', 'or' and 'not' could not be allowed in guards. This
syntax change(?) could still coexists with (;.), and have the same rules
applied to them. With 'and' as syntactic sugar for ',', 'or' as syntactic
sugar for ';' and 'not' as suntactic sugar for 'false == ' (or something)
and possibly warning if (;,) is used, would I as a programmer only have one
boolean syntax to use.
> Thomas Lindgren:
> [boolean expressions]
> >For some reason, the original and/or were defined to evaluate both
> >arguments. Personally, I would love to give and/or the semantics
> >of andalso/orelse, but previous experience suggests that any
> >semantic changes in Erlang, however obscure, are politically
> >impossible. Could this be an exception to the rule?
> One might hope so, if only for the reason that most people might
> think 'and' really works like 'andalso'. And that is why I
> asked the question: do you people out there really want 'and' to evaluate
> both arguments?
> Thomas Lindgren:
> (Personally, I tend to curse the fact that it does :-)
> [deep guards]
> >The main problem with allowing any function in guards is not
> >really the time (after all, length/1 is allowed), but the side
> >effects (such as sending messages or updating ETS tables). The
> >assumption when a guard fails is that the state of the world did
> >not change. Also, guards are used in 'receive': sending a message
> >from a guard expression in a receive-clause, or evaluating a
> >receive within a receive, could cause major inconsistencies (it
> >would be very hard to specify what the semantics actually should
> >be, if this was allowed).
> To handle this, I once proposed that execution would be in 'normal mode'
> 'guard mode'. The latter would trigger an un-catchable exception
> (i.e., fail) when a side-effect was attempted. That way, remote
> calls still work. (The originating guard would catch the failure.)
> (Roughly speaking, you would only switch out of guard mode when you
> exit the guard that set the computation into guard mode.)
> I'm a bit ambivalent about permitting "deep guards", however.
> What do we define as a side-effect, for instance? _Reading_ the database?
> Permitting long-running guards means we must be able to context-switch
> inside the guard, which means the database can change while we run the
> guard, which might be construed as an impurity. (And what about read
> And so on.
> I believe the original restriction on guards comes from concurrent logic
> programming, where "deep guards" had severe problems and ultimately
> led to disallowing them.
> Well, the Real World intrudes so I have to leave off here. Basically,
> I'd like to reduce the number of constructs when their meanings are
> very close. And yes, legacy code is a problem.
Could one way to go be to add a 'guard' construct for writing
guard-functions, that only may use other guard-functions?
guard my_guard(Arg1, Arg2) -> true | false.
Maybe with the extension of considering the guard construct as a catch - to
guard from exceptions?
Also maybe to consider all guards to belong to the same module 'guard', but
allow new guards to be defined in other modules (these should then be
updated when re-loading the original module):
guard:my_guard(Arg1, Arg2) -> true | false.
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