# [erlang-questions] Guards syntax for multiple values

Florent Gallaire fgallaire@REDACTED
Mon Mar 25 15:00:54 CET 2019

```> 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,
>
>
> 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,
>
>
> 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:
>
>
>
> You’re probably new to Erlang.
>
>
> Yes, but...
>
> You can achieve the same with parse_transform:
>
>
> ...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.
>
>
> 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
>
> --
> FLOSS Engineer & Lawyer
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
>
>
>
>
> --
> FLOSS Engineer & Lawyer
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
>
>
>
>
> --
> FLOSS Engineer & Lawyer
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
>
>
>
>
> --
> FLOSS Engineer & Lawyer
>
>

--
FLOSS Engineer & Lawyer

```