[pjmlp]

That is only true since people started equating FP with Haskell.

OCaml as the discussion subject on this thread, allows for mutable data structures, and I am old enough to have been taught Lisp as one possible avenue for FP.

[nrlucas]

They have mutability, but it's generally not encouraged. The default is immutability.

[pjmlp]

It is available, that is all that matters.