Language Design: Unified Condition Expressions – Introduction
soc.me
Idea

A design that subsumes the various syntactic forms of

  • if statements/expressions
  • switch on values
  • match on patterns and pattern guards
  • if-let constructs

and scales from simple one-liners to complex pattern matches.

  • considerealization
    1·
    3 days ago

    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.

    • soc@programming.devOP
      2·
      8 days ago

      Good catch, that should have been if person in the first line.

      It’s been a left-over from when syntax looked like this:

        is Person("Alice", _)$person then "{$person.age}"
  is Person("Bob", $age)       then "$age"
    • soc@programming.devOPEnglish
      1·
      8 days ago

      The author of that paper hung around in the lang design forum were I originally presented this.