A "pure" functional programming language has the problem you have identified and that is why haskel for a very long time was considered an academic language sin

A "pure" functional programming language has the problem you have identified and that is why haskel for a very long time was considered an academic language since it couldnt do anything useful as anything useful requires side effects like IO. The above is how haskel was introduced when i took it in CS at Syracuse University. The practicalities of haskel as a useful and practical language are realized through monads[1]. Use of monads to deal with side effects and impure world automatically make haskel "none pure". With the existence of monads,the purity of the language is a matter of opinion or degrees since it largely based on "idiomatic" usage. To say haskel is an impure language because of monads will be akin to say C# is an unsafe language because it allows pointer managements through the usage of "unsafe" keyword. [1] http://en.wikipedia.org/wiki/Monad_%28functional_programming%29 http://en.wikipedia.org/wiki/Monad_%28functional_programming...