I agree completely that it's a tradeoff. I also only bring up purity to suggest that other strongly typed, impure languages could solve this with less trouble.

I agree completely that it's a tradeoff. I also only bring up purity to suggest that other strongly typed, impure languages could solve this with less trouble. NEL is a particularly interesting point of compromise. I don't think it carries its weight very frequently at all, though I have used it very effectively in a few cases. I also think the burden of type hierarchies as you're describing isn't much felt in Haskell. You typically don't build large taxonomies, or taxonomies at all, in a language without subtyping. The only place where I see taxonomies forming at all is in `lens` and there they are used to great effect.