[erlang-questions] Guards syntax for multiple values

[Florent Gallaire]

> is_fraction(($½ | $⅓ | $⅔ | $¼ | $¾ | $⅕ | $⅖ | $⅗ | $⅘ | $⅙ | $⅚ | $⅐ |
>                    $⅛ | $⅜ | $⅝ | $⅞ | $⅑ | $⅒ )) -> true;
> is_fraction(_) -> false.
>
> doesn't look too bad, does it?  No new operator.  No new reserved
> word.  It _is_ new syntax, but it is strictly more general than the
> rather ugly 'in' special case.

Yes it doesn't look bad, as I prefer function header patten matching
to anything else.
But your "|" syntax is already used in pattern matching;

div8([Char | Tail]) when is_member("⅛⅜⅝⅞", Char) -> {divisor8, Tail};

div8([Char | Tail]) when Char in "⅛⅜⅝⅞" -> {divisor8, Tail};

is_member/2 and "in list" operator are really working examples.

div8([$⅛ | $⅜ | $⅝ | $⅞ | Tail]) -> {divisor8, Tail};

"|" as OR pattern operator doesn't work, you need a new
operator/reserved keyword.

The "in list" operator and is_member/2 guard BIF are quite equivalent.
I prefer the "in list" syntax, probably because of my Python roots,
but the is_member/2 guard
is good enough to make me happy !

Cheers

Florent

> On Tue, 26 Mar 2019 at 01:38, 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
>> >> >>
>> >> >> --
>> >> >> FLOSS Engineer & Lawyer
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>> >>
>> >>
>> >>
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>>
>>
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