[erlang-questions] Guards syntax for multiple values

[Florent Gallaire]

> Well on the first example, there are no guards. I pretty much doubt that there is an illegal guard expression there :P

Yes excuse me for my wrong sentence. I was talking about
lists:member/2 in a guard:
is_fraction(X) when lists:member(X, "½⅓⅔¼¾⅕⅖⅗⅘⅙⅚⅐⅛⅜⅝⅞⅑⅒") -> true.
which is not possible.

is_fraction(X) -> lists:member(X, "½⅓⅔¼¾⅕⅖⅗⅘⅙⅚⅐⅛⅜⅝⅞⅑⅒").
is not good for me because you can't combine with other tests and only
can return true.

So I really think Erlang need the "in list" syntax for guards.

Florent

> But the code was poorly written, I grant you that. I fixed it (and i actually checked that it compiled this time):
>
> is_fraction(X) -> lists:member(X, "½⅓⅔¼¾⅕⅖⅗⅘⅙⅚⅐⅛⅜⅝⅞⅑⅒").
>
> is_fraction_with_guards(X) when $¼ =< X, X =< $¾ -> true;
> is_fraction_with_guards(X) when $⅐ =< X, X =< $⅞ -> true;
> is_fraction_with_guards(_) -> false.
>
> ________________________________
> Brujo Benavides
>
>
>
> On 25 Mar 2019, at 10:12, Florent Gallaire <fgallaire@REDACTED> wrote:
>
> Hello Brujo,
>
> Thanks for your answer.
>
> Why not just…
>
> is_fraction(X) -> lists:member(X, "½⅓⅔¼¾⅕⅖⅗⅘⅙⅚⅐⅛⅜⅝⅞⅑⅒”).
>
>
> Because it's not possible: "illegal guard expression".
>
> Or, if you really really want to use function clause heads, pattern-matching and guards:
>
> is_fraction(X) when $¼ =< X =< $¾ -> true;
> is_fraction(X) when $⅐ =< X <= $⅞ -> true;
> is_fraction(_) -> false.
>
> For these kinds of character manipulation things, using the fact that they’re just integers under-the-hood is not a bad idea.
>
>
> Yes you're right in this case, but it remains a trick so most of the
> time it's not applicable.
>
> Cheers
>
> Cheers!
>
> ________________________________
> Brujo Benavides
>
>
>
> On 25 Mar 2019, at 09:38, Florent Gallaire <fgallaire@REDACTED> wrote:
>
> Hello Richard,
>
> Thanks for your answer.
>
> lists:member(X, [X1,X2,X3,X4]) answers true or false.
> There is no fundamental reason that the compiler could not
> expand that in-line to (X =:= X1 orselse ... orelse X =:= X4)
> when the shape of the list is known.  So we *definitely* need
> no new syntax.
>
>
> So if there's no reason the compiler could not do it, we *really*
> should have a new syntax.
>
> We really need an actual concrete example of real code to discuss.
>
>
> The developed version of the is_fraction/1 function:
>
> is_fraction($½) -> true;
> is_fraction($⅓) -> true;
> is_fraction($⅔) -> true;
> is_fraction($¼) -> true;
> is_fraction($¾) -> true;
> is_fraction($⅕) -> true;
> is_fraction($⅖) -> true;
> is_fraction($⅗) -> true;
> is_fraction($⅘) -> true;
> is_fraction($⅙) -> true;
> is_fraction($⅚) -> true;
> is_fraction($⅐) -> true;
> is_fraction($⅛) -> true;
> is_fraction($⅜) -> true;
> is_fraction($⅝) -> true;
> is_fraction($⅞) -> true;
> is_fraction($⅑) -> true;
> is_fraction($⅒) -> true;
> is_fraction(_) -> false.
>
> The awful actual "with a guard" version:
>
> is_fraction(X) when X =:= $½; X =:= $⅓; X =:= $⅔; X =:= $¼; X =:= $¾;
> X =:= $⅕; X =:= $⅖; X =:= $⅗; X =:= $⅘; X =:= $⅙; X =:= $⅚; X =:= $⅐;
> X =:= $⅛; X =:= $⅜; X =:= $⅝; X =:= $⅞; X =:= $⅑; X =:= $⅒ -> true;
> is_fraction(_) -> false.
>
> The pretty, easy and obviously needed "with in list syntactic sugar" version :
>
> is_fraction(X) when X in "½⅓⅔¼¾⅕⅖⅗⅘⅙⅚⅐⅛⅜⅝⅞⅑⅒" -> true;
> is_fraction(_) -> false.
>
> It clearly speaks for itself.
>
> Cheers.
>
> On Mon, 25 Mar 2019 at 18:12, Florent Gallaire <fgallaire@REDACTED> wrote:
>
>
> Frank thanks for your answer.
>
> You’re probably new to Erlang.
>
>
> Yes, but...
>
> You can achieve the same with parse_transform:
> https://github.com/mad-cocktail/gin/blob/master/README.rst
>
>
> ...I can say parse_transform is not the solution Erlang needs.
>
> There’s no point to add new syntax to the language.
>
>
> Yes we need it, an easy to use built-in "in (tuple or list I'm not
> sure of the right semantic)" syntactic sugar for guards.
>
> Hope some other advices.
>
> Florent
>
> /Frank
>
> Hello everybody,
>
> I'm not very experimented in Erlang but I read carefully books and
> official documention.
>
> It seems to me that the guards syntax is not as good as it should be,
> i.e. too much verbose for multiple values.
>
> do(val1) -> val1;
> do(val2) -> val2;
> do(val3) -> val3;
> do(val4) -> val4;
> do(val5) -> val5.
>
> do(Val) when Val =:= val1; Val =:= val2; Val =:= val3; Val =:= val4;
> Val =:= val5 -> Val.
>
> It's boring and error prone to write.
>
> Has a "in tuple" syntax already be considered ? Something like :
>
> do(Val) when Val in {val1, val2, val3, val4, val5} -> Val.
>
> Cheers
>
> Florent
>
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