> On 24 Jun 2020, at 19:14, Guido van Rossum <[email protected]> wrote:
>
> (Jakub, next time please trim the original post from your quote to what's
> necessary.)
>
Apologies, unintentional, I was replying from the web interface since I wasn’t
subscribed to this gorup when the thread was started and I clicked some wrong
buttons.
>
> So, now for the one thing that makes me unhappy: the rejected idea to make it
> an expression. In my short experience with pattern matching, mainly in Rust,
> roughly half (very vague estimate) of its usefulness came from it being an
> expression. It's even small things like
>
> let i = match i {
> 9 => 10,
> 10 => 9,
> _ => i,
> };
>
> and
>
> let mut file: Box<Write> = match filename.as_ref() {
> "-" => Box::new(io::stdout()),
> _ => Box::new(File::create(filename).expect("Cannot open file for
> writing")),
> };
>
> and it adds up. I'm not sure how to approach this with Python syntax and I'll
> think about this, but I feel that it'd be a huge missed opportunity to not
> have this.
>
> We considered it, but it simply doesn't work, for the same reason that we
> haven't been able to find a suitable multi-line lambda expression. Since
> Python fundamentally is not an expression language, this is no great loss --
> you simply write a match statement that assigns a value to the variable in
> each branch. Alternatively, the match could be inside a function and each
> block could return a value.
>
> --
> --Guido van Rossum (python.org/~guido)
> Pronouns: he/him (why is my pronoun here?)
That’s fair, but there’s a way in which this doesn’t have to be equivalent to
multi-line lambda expressions. Granted, I should’ve clarified that I thought
about it being an expression in a very limited, special way.
Let’s take one example from the PEP text:
match greeting:
case "":
print("Hello!")
case name:
print(f"Hi {name}!”)
Let’s say we allow prepending “match …” with an assignment and the value of the
assignment is the value of the last statement/expression in the block that’s
selected, this allows for the following hypothetical code:
message = match greeting:
case "":
"Hello!"
case name:
f"Hi {name}!”
So I didn’t express this clearly – it’s not a bout full-blown match expressions
but rather an optional "assigning form” of match statements. This seems like it
wouldn’t affect parsing massively.
Jakub
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