Allow if(!exists someVariable)

[lucaswerkmeister]

The following code is illegal:

variable Type? cached = cache[query];
if (!exists cached) {
    // calculate
    cache[query] = calculated;
    cached = calculated;
}
// use

Instead, you have to write

if (!is Object cached)

or

if (is Null cached)

I don't see any point in this, since exists is just a shortcut for is Object anyways.

[lucaswerkmeister]

See section 5.4.5 of The Ceylon Language Specification.

Should this also apply to if (!nonempy something)? Seems a bit redundant to have two negations in there...

[lucaswerkmeister]

In the grammar, isCondition begins with an optional NOT_OP, while existsCondition doesn't. In the nodes list, IS_CONDITION has a boolean not, while EXISTS_CONDITION doesn't.

[lucaswerkmeister]

Well, my example is a bit weak - I ended up using if (exists cached = cache[query]), which is probably nicer. I still think that if (!exists should be allowed, though.

[gavinking]

The only reason we don't support this is that it's a bit useless, at least until we get #74. I mean, there's just no value in narrowing the type of something to Empty or Null. OTOH, when we have #74, this will obviously be useful and we will need it then.

[lucaswerkmeister]

Oh, I didn't see #74 and #170. You're totally right, it makes a lot more sense with that.

Should we close this issue then? if (!exists was proposed in the other issues as well, so this one's a bit of a duplicate.

[gavinking]

Nono, leave it open, it's a distinct issue.

[sgalles]

This feature would also be useful for #891

[lucaswerkmeister]

Done in #74, particularly d024d99.