[WX-963] Unzip Engine Function

[THWiseman]

Implemented the unzip() engine function, new with WDL 1.1.

As a new WDL 1.1 feature, it was implemented for Biscayne and Cascades, but not earlier versions (e.g. draft-2, draft-3).

[aednichols]

Nice!

[jgainerdewar]

Looks great!

Reviewing this made me think about how we don't seem to have much unit testing of failure modes (my suffix PR suffered from the same issue) - what if I run a WDL with unzip([("banana", 5), (6, "apple"])? Is that valid, or does it fail because we can't find good types for the output arrays?

[jgainerdewar]

There's another test in the Biscayne version, should it be added here?

[jgainerdewar]

I think we're still missing a test here that is is Biscayne, no? Once that's in I think this is good to go!

[THWiseman]

Looks great!

Reviewing this made me think about how we don't seem to have much unit testing of failure modes (my suffix PR suffered from the same issue) - what if I run a WDL with unzip([("banana", 5), (6, "apple")])? Is that valid, or does it fail because we can't find good types for the output arrays?

Agree with this sentiment! I added a few unit tests for basic failure modes.

To your more specific question: unzip([("banana", 5), (6, "apple")]) will succeed, because putting stuff into arrays homogenizes the type, and the integers are coercible to strings. For that to fail, I think we would want arrays to throw errors instead of attempting to homogenize types, which is a big and breaking change that we probably shouldn't make.

However, unzip ([(1, 2), ([1,2,3], 3)]) will fail because there's no way to coerce between int and array[int]. Added a test case to assert this.