<div dir="ltr"><div class="gmail_default" style="font-family:monospace,monospace">I can't recall everything I said against this EEP,</div><div class="gmail_default" style="font-family:monospace,monospace">but there are two issues I can think of right now,</div><div class="gmail_default" style="font-family:monospace,monospace">and a better alternative.</div><div class="gmail_default" style="font-family:monospace,monospace">(A) Inconsistency.</div><div class="gmail_default" style="font-family:monospace,monospace"> Suppose a variable occurs multiple times</div><div class="gmail_default" style="font-family:monospace,monospace"> in a single pattern. What does it mean if</div><div class="gmail_default" style="font-family:monospace,monospace"> it is "pinned" in some of those occurrences</div><div class="gmail_default" style="font-family:monospace,monospace"> but not others? Are there two different</div><div class="gmail_default" style="font-family:monospace,monospace"> variables?</div><div class="gmail_default" style="font-family:monospace,monospace"> Suppose a construct has multiple clauses,</div><div class="gmail_default" style="font-family:monospace,monospace"> and a variable is "pinned" in some of those</div><div class="gmail_default" style="font-family:monospace,monospace"> clauses but not others. Are there two</div><div class="gmail_default" style="font-family:monospace,monospace"> different variables?</div><div class="gmail_default" style="font-family:monospace,monospace">(2) There are other problems with scope that we</div><div class="gmail_default" style="font-family:monospace,monospace"> want to address, but "pinning" does nothing</div><div class="gmail_default" style="font-family:monospace,monospace"> for them. (The classic one is people</div><div class="gmail_default" style="font-family:monospace,monospace"> expecting variables to be local to a 'case'</div><div class="gmail_default" style="font-family:monospace,monospace"> but leaking out.)</div><div class="gmail_default" style="font-family:monospace,monospace"><br></div><div class="gmail_default" style="font-family:monospace,monospace">The counter-proposal is quite simple.</div><div class="gmail_default" style="font-family:monospace,monospace">(A) Fix the original problem. Erlang had a very</div><div class="gmail_default" style="font-family:monospace,monospace"> simple scope rule: the scope of a variable is</div><div class="gmail_default" style="font-family:monospace,monospace"> the *entire* top-level function clause it</div><div class="gmail_default" style="font-family:monospace,monospace"> occurs in. Reinstate that rule.</div><div class="gmail_default" style="font-family:monospace,monospace"> This is a breaking change, so it needs some</div><div class="gmail_default" style="font-family:monospace,monospace"> sort of -simple_scopes declaration to enable it.</div><div class="gmail_default" style="font-family:monospace,monospace"> We now have a feature declaration that can be</div><div class="gmail_default" style="font-family:monospace,monospace"> used for this.</div><div class="gmail_default" style="font-family:monospace,monospace"><br></div><div class="gmail_default" style="font-family:monospace,monospace">(B) Fix the locality problem. The right way.</div><div class="gmail_default" style="font-family:monospace,monospace"> By declaring which variables are *local*,</div><div class="gmail_default" style="font-family:monospace,monospace"> not which variables are *not* local.</div><div class="gmail_default" style="font-family:monospace,monospace"> Borrowing some syntax from Prolog (as Erlang</div><div class="gmail_default" style="font-family:monospace,monospace"> originally got its syntax thence), let D be<br></div><div class="gmail_default" style="font-family:monospace,monospace"> a syntactic form made of tuples, lists, and</div><div class="gmail_default" style="font-family:monospace,monospace"> variables, and C be a pattern or expression.</div><div class="gmail_default" style="font-family:monospace,monospace"> Then D ^ C means that each of the variables</div><div class="gmail_default" style="font-family:monospace,monospace"> in D represents a new variable in C, not</div><div class="gmail_default" style="font-family:monospace,monospace"> provided, visible, or in any way accessible</div><div class="gmail_default" style="font-family:monospace,monospace"> in C.</div><div class="gmail_default" style="font-family:monospace,monospace"><br></div><div class="gmail_default" style="font-family:monospace,monospace">With this approach, local scope is available for</div><div class="gmail_default" style="font-family:monospace,monospace">*every* construct. With the scope marker being</div><div class="gmail_default" style="font-family:monospace,monospace">located at the *beginning* of a construct, instead</div><div class="gmail_default" style="font-family:monospace,monospace">of being buried possibly deeply inside, it should</div><div class="gmail_default" style="font-family:monospace,monospace">be easier to read.</div><div class="gmail_default" style="font-family:monospace,monospace"><br></div><div class="gmail_default" style="font-family:monospace,monospace"><br></div><div class="gmail_default" style="font-family:monospace,monospace"><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, 28 Apr 2022 at 14:52, Dmytro Lytovchenko <<a href="mailto:dmytro.lytovchenko@gmail.com">dmytro.lytovchenko@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Would
it provide a solution
to just assume that the variable shadowing is never wanted?</div><div><br></div><div>What if code like:<br></div><div> f(X) -> fun(X) -> X end.</div><div> g(Y) -> case h() of Y -> %code% end.<br></div><div><br></div><div>1. Could create an error due to X being shadowed.</div><div>2. Could compile with inner X shadowed but a guard is added by the Erlang compiler, comparing inner to the outer:</div><div> f(X) -> fun(X2) when X2 =:= X -> X2 end.</div><div> g(Y) -> case h() of Y2 when Y2 =:= Y -> %code% end.</div><div></div>3. Current behaviour: Inner X is shadowed and is an entirely new variable (leads to naming and scoping confusion)<div><br></div><div>Solution 2 does not introduce new syntax and matches expectations for many developers. The compiler fixes the confusion during the compile time. Also should not break the existing code.<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, 27 Apr 2022 at 21:11, Attila Rajmund Nohl <<a href="mailto:attila.r.nohl@gmail.com" target="_blank">attila.r.nohl@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Fred Dushin <<a href="mailto:fred@dushin.net" target="_blank">fred@dushin.net</a>> ezt írta (időpont: 2022. ápr. 27., Sze, 15:14):<br>
><br>
[...]<br>
> But as noted later in the EEP, this can be written as<br>
><br>
> f(X, Y) -><br>
> F = fun ({a, Z}) when Z =:= Y -> {ok, Y};<br>
> (_) -> error<br>
> end,<br>
> F(X).<br>
[...]<br>
> And not to be too critical, but I am having a hard time understanding parts of the Rationale section. The author(s) suggest(s) that in current Erlang temporary variables are needed to achieve the same as the proposed glyph, but then provide an example that uses a guard (as above) but doesn't use a temporary variable, after all (?)<br>
<br>
Z is the temporary variable in the above example.<br>
<br>
> I am not sure what the problem is with temporary variables. The compiler has registers at its disposal, so I don't think it's a performance argument, but more an issue of readability, which again, I think is a purely aesthetic question, and has no bearing on the features of the language, per se. In other words, the proposal is not suggesting anything that cannot already be achieved in current Erlang, with effectively the same compiled BEAM ASM.<br>
<br>
Naming is hard, and if you have to come up with two meaningful<br>
variable names instead of one, it's even harder. This leads to code<br>
like this:<br>
<br>
F = fun ({a, TmpMeaningfulName}) when TmpMeaningfulName =:=<br>
MeaningfulName -> {ok, MeaningfulName};<br>
<br>
The current solution in the language easily leads to too long lines or<br>
overly verbose code.<br>
</blockquote></div>
</blockquote></div>