[erlang-questions] Guards syntax for multiple values

[Brujo Benavides]

Hey Florent,

	Why not just…

	is_fraction(X) -> lists:member(X, "½⅓⅔¼¾⅕⅖⅗⅘⅙⅚⅐⅛⅜⅝⅞⅑⅒”).

	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.

	Cheers!

Brujo Benavides <http://about.me/elbrujohalcon>



> 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
>>>>> 
>>>>> --
>>>>> 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
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