[brudgers]

The turtle at the bottom of Haskell is machine code...mutable non functional machine code that is isomorphic with the Haskell source.

[mhh__]

The machine code can still be basically functional. GHC doesn't use any mutable data unless you tell it to, it generates and then immediately collects garbage rather than reuse mutable memory.

You can have mutable data in Haskell, obviously.