Exactly. Parametricity is the killer feature of statically typed functional languages. This why it saddens me when Haskell and OCaml add features that weaken parametricity, like GADTs and type families.
Without either GADTs or type families, two types `Foo` and `Bar` with mappings `fw :: Foo -> Bar` and `bw :: Bar -> Foo` that compose in both directions to the identity, are “effectively indistinguishable” from one another in a precise sense. If you have a definition `qux :: T Foo`, for any type function `T` not containing abstract type constructors, you can construct `justAsQuxxy :: T Bar` by applying `fw` and `bw` in the right places.
With either GADTs or type families, this nice property is lost.
However, GADTs and TFs are completely opt-in, so it seems a bit of a stretch to construe this as a generally bad thing. IME it's not as if library authors are arbitrarily (i.e. for no good reason) using GADTs or TFs instead of plain old type parameters in their APIs.
Reflection, downcasts and assigning `null` to pointers are completely opt-in in Java too.
With respect to type families, I'm probably being a little bit unfair. Personally, I don't have much against associated type families. (Although I think Rust handles them much more gracefully than GHC.) But very important libraries in the GHC ecosystem like vector and lens make extensive use of free-floating type families, which I find... ugh... I don't want to get angry.
> Reflection, downcasts and assigning `null` to pointers are completely opt-in in Java too.
No, they're not -- not in the same sense, at least. A GADT/TypeFamily is going to be visible in the API. None of the things you mentioned are visible in the API.
> A GADT/TypeFamily is going to be visible in the API.
Only works if you're never going to make abstract types. Which I guess is technically true in Haskell - the most you can do is hide the constructors of a concrete type. But the ability to make abstract types is very useful.
Don't get me wrong, I love Haskell. It's precisely because I love Haskell that I hate it when they add features that make it as hard to reason about as C++. (Yes, there I said it - type families are morally C++ template specialization.)
It's something weaker. Consider the groupoid of Haskell types and isomorphisms. Without GADTs and type families, all type constructors of kind `* -> *` are endofunctors on this groupoid.
Note 1: And there are higher-kinded analogues, but I hope you get the idea from this.
Note 2: There are also exceptions, like `IORef` and friends.
Sorry, for some reason the “reply” link didn't appear below your post until after I had written my reply to Peaker. My reply to you is exactly the same: