Pattern variables

[kyoyama-kazusa]

Hello.

Can we extract a pattern into a variable to store, and then use it multiple times? Such as:

expr equalOrNotEquals = not ((int)SyntaxKind.EqualsExpression or (int)SyntaxKind.NotEqualsExpression);

if (
    node is BinaryExpression
    {
        RawKind: (int)SyntaxKind.LogicalAndExpression,
        Left: { RawKind: not equalOrNotEquals },
        Right: { RawKind: equalOrNotEquals }
    }
)
{
    // ...
}

[333fred]

Replace expr with const int and yes, you can.

When you answer things later at night, sometimes you misread.

[kyoyama-kazusa]

Hmmm... What about other patterns, such as property pattern? A property pattern isn't a constant, I think.

[333fred]

Hmm. I entirely misread your question, ignore my answer.

[alrz]

Does #4131 address your question?

[kyoyama-kazusa]

That proposal does make sense to me. Maybe that proposal has already covered the idea here. But... as a beginner for learning the pattern matching, using method invocation as a pattern is a little bit weird, especially combined with other pattern types (only in my opinion).

Anyway, thanks for your reply.

[WhiteBlackGoose]

I would have syntax like
pattern {TYPE TO BE EXAMINED} {NAME OF PATTERN} {PATTERN}
For example,

public static class MyClass
{
    pattern int Positive > 0;
    pattern int Even % 2 == 0;
    pattern Complex Real { RealPart: > 0 };
}

example similar to what the OP posted:

if (c is Real re) ...
if (a is Even) ...
if (student is { Attitude: Positive }) ...

(the patterns will be colored differently from types by IDEs to make it clear)
@333fred what do you think?

[WhiteBlackGoose]

Here's how we could handle cases with extracting variables:

pattern Person ValidPerson(name) { Age: > 0, Name: { } name }

Use case:

if (person is ValidPerson(var name)) ...

[333fred]

I think @alrz's proposal or Neal's original one are the next steps to go in.