[erlang-questions] Guards syntax for multiple values

[Dániel Szoboszlay]

You are right that a parser could easily distinguish "X in Y" from
"M:in(...)" or even "fun M:in/N" based on the context. The problem, as I
see it, is that "X in Y" introduced a new infix operator to the language,
while "is_member(X, Y)" does not, it is syntactically just an ordinary
function call.

The first problem with introducing a new operator is that there are some
conventions for operators in Erlang. Like every alphabetic operator is
currently a reserved word. So we either break this convention with "in", or
we make "in" a reserved word that would break all the existing "M:in(...)"
use cases. I like neither of these options.

The second problem I see with a new operator is that it would require an
update of every tool that parses Erlang code. An Erlang parser is very
likely to have a hard-coded list of operators (together with e.g. the
precedence order of them), and wouldn't just try to parse any atom as a
potential infix operator. On the other hand, a parser may have a hard-coded
list of BIF-s that are allowed in a guard, but that's not likely. Most
probably it would just accept any function call in a guard position, so
going with "is_member/2" or "lists_member/2" would not break a typical
parser.

Cheers,
Daniel

On Tue, 26 Mar 2019 at 19:25 Eckard Brauer <eckard.brauer@REDACTED> wrote:

> Am Tue, 26 Mar 2019 11:15:28 -0400
> schrieb Fred Hebert <mononcqc@REDACTED>:
>
> > On Tue, Mar 26, 2019 at 10:51 AM Florent Gallaire
> > <fgallaire@REDACTED> wrote:
> >
> > > It's also a good opportunity to make Erlang more modern with a sexy
> > > syntactic sugar...
> > >
> > >
> >
> > this should be done very carefully. For example, adding the 'in'
> > keyword for 'X in [A,B,C]' syntax will break all existing code out
> > there that uses the word 'in' for a function, including the
> > `queue:in/2` function call in the standard library. That could ripple
> > into inconsiderate breakage in the whole ecosystem.
>
> I don't really prefer lists_member/2 or is_member/2 over 'X in [A,B,C]'
> or the other way, but could you please enlight me (rather erlang
> beginner) about where/why this would break existing code? If we talk
> about a list as the second/right side arg of 'in', I could think of
> either having an opening bracket '[' or a variable bound to that list
> ("when X in List") after the keyword when to be used as a guard, or of
> '(' when to be used as the abovementioned function call. So for me,
> that looks rather simple, but as already written, I'd like to further
> improve knowledge.
>
> Thanks in advance
> Eckard
>
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