if-let binding

[kentosugama]

In addition to let-else #3817, if-let would let users recover from pattern match failure.

Most likely implemented as direct desugaring in the parse to a switch with a wildcard case.

https://doc.rust-lang.org/rust-by-example/flow_control/if_let.html

[ggreif]

Since if let ... { } else { } is already valid syntax (but different semantics), we should consider a different keyword. Maybe
case let <pat> = <exp> <then> else <else> would fit the bill?

[rvanasa]

Since if let ... { } else { } is already valid syntax (but different semantics), we should consider a different keyword.

We should be okay to use if let because parentheses are required around the let / let-else binding with the current grammar (if I understand correctly).

[rossberg]

I really don't see how

if let <pat> = <exp1> { <exp2> } else { <exp3> }

is substantially shorter or clearer than the equivalent

switch <exp1> { case <pat> { <exp2> } case _ { <exp3> } }

It saves about one pair of braces. Does that justify piling up more redundant syntax that everybody needs to learn?

Don't be too quick to throw out new syntax unless it addresses a major pain point and reduces structural complexity. Being small is a virtue for a language in itself. As is adhering to this principle from the Zen of Python:

"There should be one – and preferably only one – obvious way to do it."

[nomeata]

For if (I assume) the else can be omitted? Just this morning I was writing some code (using switch) where I’d leave out the else if I had if let, but with switch I had to write a rather noisy and unhelpful case _ ().

(But not feeling strongly about iflet.)

[rossberg]

Yeah, but that's still just minor convenience, probably still not worth the syntax budget.

If you truly want to go that route, you should at least come up with a syntax that scales, i.e., works naturally in similar other places, such as for or while. It's also preferable to have a stronger signal that this intentionally allows for partial patterns (unlike let). Something like if (case pat = exp) would hence be much better IMO, along with for (case pat in exp) and while (case pat = exp).

[chenyan-dfinity]

Just my 2 cents. I don't feel strongly about having if let at the beginning. But since we already have let else, where the else can only be None, we kinda missing a more general sugar such as if let, which allows us to return a non-None value in the else branch.

[rossberg]

Well, yes, that's sort of making my point above. Unsurprisingly, I felt similarly about let-else. There is no coherent design concept behind either. And once you go there, the sweet seduction is towards piling on more ad-hoc sugar and become an overweight language.