I think you may argue that some languages are in practice more or less pure by how much they encourage/enable avoiding mutability and side effects but of the 4/

I think you may argue that some languages are in practice more or less pure by how much they encourage/enable avoiding mutability and side effects but of the 4/5 languages he discusses I'm pretty sure only haskell is purely functional. Scala Scheme and SML/ocaml are all impure functional that allow mutability/side effects. (Scala is considered an object functional hybrid by some and an object oriented language dressed as functional by others - try to avoid flamewar). Some Examples: I think Twitter uses Scala(see above) for a number of their back-end services (see http://blog.redfin.com/devblog/2010/05/how_and_why_twitter_uses_scala.html http://blog.redfin.com/devblog/2010/05/how_and_why_twitter_u...). Foursquare uses scala at least on the front end http://news.ycombinator.com/item?id=1139062 http://news.ycombinator.com/item?id=1139062 . This page has a few other big success http://www.scala-lang.org/node/1658 http://www.scala-lang.org/node/1658. Haskell has pandoc(http://johnmacfarlane.net/pandoc/ http://johnmacfarlane.net/pandoc/) a cmdline tool to convert beetween markup formats(also a library with binding to several languages) I think it supports a wider range of formats/fidelity then alternatives. Ocaml: MLDonkey is an open source eDonkey2000 client Coq – a interactive theorem prover. Ocaml