I've heard it best as: both Ocaml and Haskell are pure functional languages. Haskell is Extra Virgin.
That said, the notion of purity is a bit BS. At some point in your program you are definitely going to invoke the world-at-large. Whether you do it through IO(), tap your nose and call it "pure" or you do it other way is all down to semantics (not in the PLT sense of the word - the PLT term to use would be "pragmatics"). The core of both Ocaml and Haskell are pure functions.
Also, Ocaml doesn't really need typeclasses. Modules. Modules are everything
OCaml is a lot less pure in spirit as well. Not only can you read and write to the outside world without having to jump through hoops, you can also write all your code with pointers, for loops and (mutable) arrays if you so desire.
That said, the notion of purity is a bit BS. At some point in your program you are definitely going to invoke the world-at-large. Whether you do it through IO(), tap your nose and call it "pure" or you do it other way is all down to semantics (not in the PLT sense of the word - the PLT term to use would be "pragmatics"). The core of both Ocaml and Haskell are pure functions.
Also, Ocaml doesn't really need typeclasses. Modules. Modules are everything