return more than one value from a for-comprehension

[@FroMage] I know we can do that with something that flattens the resulting comprehension afterwards, but it could be useful to support “returning” more than one value from a comprehension:

assert({1, 10, 2, 20, 3, 30}, {for (i in 1..3) *{i, i*10}});

Where I reuse the * operator to spread more than one value in the comprehension.

[Migrated from ceylon/ceylon-spec#860]

[zamfofex]

So, I’ve been thinking about this since I first found Ceylon, and I think I’ve figured out a pretty good, simple, intuitive, and regular syntax for this.

The idea is to have a syntax for both “expressions” and “multi-expressions”. Consider:

comma-list:

• (multi-expression ("," multi-expression)*)?

multi-parentheses:

• "(" multi-expression ")"

spread:

• "*" expression

for-comprehension:

• "for" for-iterator multi-expression

if-comprehension:

• "if" condition-list multi-expression

multi-expression:

• multi-parentheses
• expression
• comma-list
• spread
• for-comprehension
• if-comprehension

With this approach, you would be able to have a comprehension that returns multiple values by either using the spread syntax or by using parentheses. For example:

function expand<Element>({{Element*}*} streams) => {for(value stream in streams) *stream};

value div = Div{for(section in sections) (Title(section.title), Text(section.text))};

Now, as an addendum to the idea, something interesting that could be done is to allow someone to define condition lists in terms of multi-expressions. That is, consider the following extension to the syntax of multi-expression:

multi-expression:

• is-condition
• exists-condition
• nonempty-condition
• "!" multi-expression

And now look at how simple the syntax for condition-list becomes:

condition-list:

• "(" multi-expression ")"

This also helps fix the problem posed in #4540, by simply allowing people to write the following:

if(!(exists foo, exists bar, hours < 24))
{
	return(null);
}

return(foo + bar + hours);

It’s interesting to note that those conditions would still be only allowed inside if, assert, and while, but this restriction would be done by the typechecker, not the parser. That is, using prefix exists where an expression is expected would be syntactically incorrect, while using them where a multi-expression is expected (but outside a condition list) would be semantically incorrect.

value foo = exists bar; // syntax error

value foo = [exists bar]; // semantical error

[jvasileff]

+1 for figuring this out. When I get a chance, I’ll link to real world code examples where this would help.