I think the response gets right to the point!

Using something like Haskell for ROOT is ridiculous for a lot of obvious reasons. A simple and dismissive "no" invites the cautious reader to discover them on their own rather than waste engaging in a protracted debate. Maybe it's better to reject the idea out of hand and spend our time elsewhere.

There's a number of reasons for this. The first is that the quant physics community has never really adopted functional programming. It's not particularly obvious to scientists, who typically want to express their computation the way they want to- something that C, C++, and Fortran are all long-established at doing. The second is that much of physics depends on old libraries written over the last 30-40 years, and it's easiest to use them from a language that the library is written in, or one that has a highly similar interface (for example, Python is similar enough to C++ that many foreign function interfaces are literally just direct wrappers). The third is that types (other than simple scalars, arrays, and trees/graphs) have never been a high priority in quant physics. The fourth is that undergrad education outside CS rarely teaches students Haskell, while most undergrads in a quant field graduate knowing some amount of Python.

It's much more likely the physics community would adopt Julia, or maybe Rust, and even that has been pretty slow.

(nothing I said above should be construed as taking a position about the suitability of any specific language or lack thereof for doing scientific computing. I have opinions, but I am attempting to explain the reason factually with a minimum of bias)