[Python-ideas] Match statement brainstorm

[Guido van Rossum]

No, what I'm saying is that seeing it as sugar for exception handing
is the wrong way to look at it.

On Mon, May 23, 2016 at 4:52 PM, Michael Selik <michael.selik at gmail.com> wrote:
>
>
> On Mon, May 23, 2016 at 7:13 PM Guido van Rossum <guido at python.org> wrote:
>>
>> On Mon, May 23, 2016 at 3:49 PM, Michael Selik <michael.selik at gmail.com>
>> wrote:
>> >
>> >
>> > On Mon, May 23, 2016 at 3:58 PM Guido van Rossum <guido at python.org>
>> > wrote:
>> >>
>> >> On Mon, May 23, 2016 at 11:45 AM, Joao S. O. Bueno
>> >> <jsbueno at python.org.br> wrote:
>> >> > I still fail to see what justifies violating The One Obvious Way to
>> >> > Do
>> >> > It which uses an if/elif sequence
>> >>
>> >> Honestly I'm not at all convinced either! If it was easy to do all
>> >> this with a sequence of if/elif clauses we wouldn't need it. The
>> >> problem is that there are some types of matches that aren't so easy to
>> >> write that way, e.g. combining an attempted tuple unpack with a guard,
>> >> or "instance unpack" (check whether specific attributes exist)
>> >> possibly combined with a guard. (The tricky thing is that the guard
>> >> expression often needs to reference to the result of the unpacking.)
>> >
>> > I figure it's better to solve the category of problems --
>> > exception-catching
>> > expressions -- rather than the single problem of catching exceptions in
>> > an if/elif/else chain.
>>
>> People are likely either going to put in exceptions that don't catch
>> enough (e.g. IndexError isn't the only exception that example can
>> throw) or, overreacting to that problm, that catch everything ("except
>> Exception:" is an anti-pattern that's hard to fight).
>
>
> It sounds like the justification for a switch/match syntax is to provide a
> special situation where generic Exception-catching assignment expressions
> are acceptable, because they're useful in a long elif chain but too
> dangerous for widespread use.
>
> Clearly it's beneficial enough to have appeared in other languages. However,
> languages like Haskell have already accepted the danger of assignment (and
> more) being an expression. Is there a language that makes the
> expression/statement distinction that has as powerful a matching syntax?



-- 
--Guido van Rossum (python.org/~guido)