Variables changing type is not actually all that big a problem – we just haven’t optimized it at all until recently. On the flip side, it’s also not nearly sufficient to ensure type stability since one can easily write programs with unpredictable types without any variables that change type. What would be required to enforce type stability would be having a set of rules by which a computable type is assigned to every expression in a program, especially method definitions. Just making variables not change type is neither sufficient nor even particularly helpful. Having that as a blanket rule would mostly have the effect of forcing some high-level programs with particularly dynamic behavior to be written in much more annoying ways (e.g. with loosely typed Refs).
2 Likes