A common misconception about erlang is its degree of functional-ness. It isn't completely functional. As Peyton-Jones says, a completely functional language w

A common misconception about erlang is its degree of functional-ness. It isn't completely functional. As Peyton-Jones says, a completely functional language will just sit there and make the computer run hot[1]. Erlang is a well thought out compromise between tying your hands by restricting side effects (the monad approach) and allowing a wide audience of people to be quickly productive. [1]: Changing the temperature of the environment can be considered a side effect thereby rendering the 100% functional language 99.999% functional.