Sequence comprehensions

[CeylonMigrationBot]

[@gavinking] So, it appears to me that we've moved away from the idea of Smalltalk-style arguments for invocation of higher-order functions, and that everyone seems to want a much more conventional syntax for anonymous functions inside positional argument lists (see #3160). I can live with that. But if that's the case, then I'm going to have to insist that we at least need a specialized syntax for comprehensions. There's many many possible variations of this syntax, but the two possibilities that I think would make most sense are the following:

{ (x**2+y**2)**0.5: x in 0..100 && x%10==0, y in -10..+10 && y%2==0 }

or

{ (x**2+y**2)**0.5: x in 0..100, x%10==0; y in -10..+10, y%2==0 }

I don't have an especially strong preference between these two options. I think the first is more consistent with the rest of our notation, and the second is perhaps slightly easier on the eyes. The second option is also a bit better from a grammar point of view.

Of course, there is also an argument for something like the following syntax:

{ (x**2+y**2)**0.5 for (x in 0..100) if (x%10==0) for (y in -10..+10) if (y%2==0) }

But in this case, I don't think the extra verbosity contributes to readability.

Note that since this is probably something I can desugar in the frontend, we could probably have this feature almost immediately.

Thoughts?

UPDATE:

Note that this syntax would be allowed to appear in two places:

1. where sequence enumerations appear, which would instantiate an ArraySequence, and
2. in place of a named argument list, producing an Iterable (or perhaps some kind of lazy sequence).

So you would be able to write:

HashMap { p.name->p.address: p in people }

[Migrated from ceylon/ceylon-spec#82] [Closed at 2012-05-04 01:51:59]

[CeylonMigrationBot]

[@gavinking] Y'know what, I don't think we need this feature. I mean, it seems to me that I would prefer to read:

value total = sum ( for (x in xs) x );
if ( every ( for (x in xs) x>0 ) )  { ... }
if ( any ( for (x in xs) x>0 ) ) { ... }

Than:

value total = for (y=0.0; x in xs) y+x;
if ( for (all=false; x in xs) all && x>0 ) { ... }
if ( for (any=false; x in xs) any || x>0 ) { ... }

Is there something I'm missing? Some kind of common use of fold() that can't be nicely pulled out into a utility function?

[CeylonMigrationBot]

[@RossTate] I think the

order.items.foldl(0) { next(Integer prev, Item item) = prev + item.produce.price*item.quantity }

syntax already available should be good enough for miscellaneous cases. If not, we can always try adding this feature later once we know what the demand looks like.

[CeylonMigrationBot]

[@gavinking] Closing this because the syntax is nailed down and activity has moved to #3390.