On immutable data structures, a language like Scala makes it really practical to use only immutable data structures in daily programming. My own code is full of

On immutable data structures, a language like Scala makes it really practical to use only immutable data structures in daily programming. My own code is full of immutable data-structures, except in instances in which I have to process stuff returned by Java libraries and even in those cases I wrap them in Scala's immutable interfaces (note, I'm talking about Scala and not Haskell only because it's the language I know). The only problem is that many times you need mutable references to immutable data-structures, with no easy way out and this is where you can still have problems with side-effects and multi-threading. But dealing with such references is easier, because (a) you tend to do it less and less often and (b) you can use STM or just plain atomic references or @volatiles, as in many instances you need to just ensure the visibility of updates and you only really need locks only on writes. Also, I tend to encapsulate and hide really well pieces of code that deal with mutability. A good standard library of immutable data-structures is very practical for day to day use. I wasn't talking about immutable data-structures though, as the data-structures themselves are only a small part of the puzzle. Much more difficult is to deal with inter-modules communications, processing streams of data, I/O and stuff like that. And that's also where things break and where I feel the urge to look for better techniques. On lazy data structures, Python is an awful example.