I can't remember the last time I implemented a data-structure in a web framework. You just use the built in dictionary/hash table and get on with life.
Well, whenever you write non-trivial code, your code is going to have invariants. These invariants can be partially tested or they can be fully type-checked. The latter is better and cheaper, when available. Haskell makes the latter available far more often, so you don't have to pay for the former. This is not just useful for data structures, but code in general.
This post kind of sums it up for me. No, I'm not thinking about invariants. We're talking about writing a web app here. Typing is pretty meaningless when 99.9% of your data is just strings.
