A quite decent syntax and an excellent domain name!

How does $person get set?

This looks very similar to The Ultimate Conditional Syntax, although that's for ML so it doesn't have the nice syntax for chaining method calls.

Isn't match already such a unified expression? Especially once you extend matches with guards, it seems to me like this is a solved problem. E.g.,

if x == 1.0 then "a" else "x"

is

match x with | 1.0 -> "a" | _ -> "b"

and

if x ==
  1.0 then "a"
  2.0 then "b"
      else "z"

is (and IMO reads much clearer this way):

match x with
| 1.0 -> "a"
| 2.0 -> "b"
| _ -> "z"

and

if xs
  .isEmpty then "e"
  .contains(0,0) then "n"
  else "z"

is

match () with
| _ when x.isEmpty -> "e"
| _ when x.contains(0,0) then "n"
| _ -> "z"

and

if person
  .age < 18                 then 18
  is Person("Alice", _)     then person.age
  is Person("Bob", let age) then age
                            else -1

is

match person with
| _ when person.age < 10 -> 18
| Person("Alice", _) -> person.age
| Person("bob", age) -> age
| _ -> -1

.

Finally,

if person is Person("Alice", let age) then age else -1

Would be the simple

match person with
| Person("Alice", age) -> age
| _ -> -1

Seems to me this reads more clear in general and has less magic. Plus, it's already implemented in a bunch of languages.