Bug: parallel assignment with a side effect.

[dblock]

Let me demonstrate:

type = [Hash]
type, value_type = type.class, type.first
=> [Array, Hash] 

after auto-correct

type = [Hash]
type = type.class
value_type = type.first
=> Undefined method `first` for Array:Class

I think a generic fix just changes the order of assignment to do right-to-left, but that will seem odd most of the time.

[jonas054]

A generic change of order will not be enough. The problem is that parrallel assignment evaluates all expressions on the right hand side before doing all the assignments. So with right-to-left order, you'd still have the same problem with

type = [Hash]
value_type, type = type.first, type.class

We need to check if any variable used on the left hand side appears on the right hand side. If it does, we can either skip auto-correction (easy way) or change the order (hard way).

[dblock]

Oh, interesting, how would you figure out the right order without temporary variables every time?

I think skipping auto-correction is more viable.

[jonas054]

I agree. There are probably cases when there is no correct order without introducing temporary variables.

[alexdowad]

There are definitely cases where there is no correct order. Here's one:

a, b = b, a

[alexdowad]

Aha, I see that ParallelAssignment doesn't try to correct in that case.

[alexdowad]

Just opened a PR which still needs a little work, but (when finished) will provide a robust fix to this problem.

Oh, interesting, how would you figure out the right order without temporary variables every time?

You can figure out the right order using a topological sort.