Clojure: Doesn't the ability to use Java objects with state defy the whole idea of functional P?

FP is a paradigm, a concept, but not necessarily a dogma. Clojure trusts the programmer to make thoughtful decisions about where he'll depart from FP. In exchange, Clojure offers the staggering cornucopia of code that is available in the form of Java libraries.

This makes it relatively easy and painless to write a GUI app in Clojure, say, or a Web server or any of the things covered by Java library code.

You cannot be sure, apart from consulting documentation or using a java decompiler(?). This ability certainly defies the idea of pure functional programming, but the real world is not a particularly pure place and purely functional languages can't get much traction against it. Witness all the contortionism with monads in Haskell.

Besides, mutable state is very powerful computationally — many algorithms become much faster and much more economical of memory when implemented with mutable state.

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