Re: Proposal: AS assertions

Hi Rowan On Tue, Mar 19, 2024 at 8:39 PM Rowan Tommins [IMSoP] <imsop.php@rwec.co.uk> wrote: > > As well pattern matching, which Ilija mentioned, another adjacent feature is a richer set of > casting operators. Currently, we can assert that something is an int; or we can force it to be an > int; but we can't easily say "make this an int if safe, but throw otherwise" or > "make this an int if safe, but substitute null/$someValue otherwise". > > I've been considering how we can improve that for a while, but not settled on a firm > proposal - there's a lot of different versions we *could* support, so choosing a minimal set is > hard. I've thought about this in the context of pattern matching a while back. I was thinking about something like $x is ~int, where the pattern match is successful iff $x is coercible to int without loss of information. Given that patterns may be nested, array<~int> could check that all elements of an array are coercible to int. The same could work for literal patterns, e.g. ~5, where 5, 5.0 and '5' are all accepted. This can potentially be combined with the variable binding pattern, $var @ pattern. The syntax looks a bit confusing at first, but it basically makes sure that the matched value conforms to pattern, and then binds it to $var. Hence, something like `$foo as Foo { $bar @ ~int } would 1. make sure $foo is an instance of Foo`, 2. make sure $foo->bar is coercible to int, and then assigned the coerced value to $bar. (It gets more complicated, because the assignment must be delayed until the entire pattern matches.) If the pattern matching fails at any point, it throws. This is just an idea, neither the as operator nor the ~ pattern have been implemented. I don't know whether they are feasible. Anyway, we're probably going off-topic. :) Ilija