[erlang-questions] Guards syntax for multiple values

[Florent Gallaire]

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
>> >>
>> >> --
>> >> FLOSS Engineer & Lawyer
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>>
>>
>>
>> --
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