I agree and should note that pure functional languages more suited for writing libraries than languages with some first-class functional features. This is so because pureness encourages composability of various parts of program.

The state allows parts of program to communicate through implicit information channel. One part of program could signal another to increment a variable by deleting a file. Or vice versa.

That's why I think Haskell is the best for libraries, Scala less so and Lisp even lesser, even taking into account macro system, code-as-data, etc.