Hi Hugo What is a POSTags and how big do you expect it to be? Generally I'd recommend you first try to calculate the size of your data rather than try to strictify things, see Johan Tibell's very useful posts: http://blog.johantibell.com/2011/06/memory-footprints-of-some-common-data.ht... http://blog.johantibell.com/2011/06/computing-size-of-hashmap.html Once you know the size of your data - you can decide if it is too big to comfortably work with in memory. If it is too big you need to make sure you're are streaming[*] it rather than forcing it into memory. If POSTags is large, I'd be very concerned about the top line of updateState - reversing lists (or sorting them) simply doesn't play well with streaming. [*] Even in a lazy language like Haskell, streaming data isn't necessarily automatic.