Case expressions

[lrhn]

(This is basically reopening #2181 as a new feature.)

Allow <expression> 'case' <pattern> ('when' <expression>)? as an expression which:

• Evaluates to a bool result.
• Introduces variables that are available on the true path out of that bool, if it occurs in a condition position.

Since patterns look like expressions, and can end in ?, and the start and end of case...when are expressions, we must know where a case expression ends.
That means it must likely be a production of <expression>, which means it cannot be combined with any operators without being parenthesized, and it will have to be delimited in some way in all uses.

Proposed:

  <expression>  ::= ...
                               | <caseExpression>
  <caseExpression> ::=
        <expressionWithoutCascade> 'case' <pattern> ('when' <expressionWithoutCascade>)?

(Not absolutely sure about the placement in the grammar, but likely close to this. At least you definitely don't want an unparenthesized case-expression as the value or when clause of another case-expression.)

If the pattern introduces variables, then those variables are in scope in the when clause as usual.
Further, if the case-expression (or parenthesized expression, or !-negated expression) is use in a condition position, which means any position where its value directly affects control flow (has a "true" branch and a "false" branch), then the variables are available on the true path, until the end of the current constructor/block.
(This is not intended to be something new, it's exactly the same continuation where a promoting test would promote, except that the variables are also limited to the scope they are declared in.)

Example:

var x = input case Box(:var value) ? value : input;

and

{
   input case Box(:var value) || throw "nope";
   print(value);
}

(I know this is bad style. It does show that a variable is available in the rest of the block, as long as it's dominated by the true test.)

If a case expression is used as the condition of an if statement/element, a for statement/element or a while statement, then the variables are available in the body (if that's the true-branch), but not outside of the satement. It is as if the if/loop structure introduces a scope block containing the condition itself, and then the body is another nested scope.

This is consistent with how if/case works today, but this allows doing multiple independent cases:

if ((ex case Foo(:var x) when x > 0) && (ex2 case Bar(:var y) when y > 0)) { 
  both `x` and `y` in scope.
}

[munificent]

This looks really nice. In particular, I have sometimes wanted to have an if condition that is a chain of case and/or other conditional tests like:

if (thing case Foo(:var bar) && someCondition && bar case Blah() && otherCondition) { ... }

It looks like this would address that.

I'm not as sold on allowing variable declarations outside of if conditions. I think this is probably fine:

input case Box(:var value) ? value : input;

But:

{
   input case Box(:var value) || throw "nope";
   print(value);
}

Feels pretty subtle and unintuitive to me. I do grant that it effectively offers a solution for #2537. I just wonder if it's too subtle and we should have a more explicit guard-like statement.

But aside from that, this looks great.

[FMorschel]

I'd love this! It would also allow us to do:

if (input != other && (input case MyType(:var value))) {
  print(value);
}

Which reads the != test first and could short-circuit before the pattern without the need of a full outer if or weird workarounds like:

if ((input == other ? null : input) case MyType(:var value)) {
  print(value);
}