Hacker News Reader

Actually, today's hardware, with multiple cores/CPUs clashing over shared memory through non-shared cache, is less friendly to mutability than the old systems.

Immutability does require different data structures; a (singly-linked) list is easy to make immutable in most cases -- many lists with the same tail part can share that tail, without copying when a new list starts sharing that tail. Arrays, not so much.

View on HN

Though to be fair, singly-linked lists are not useful for parallelism.

Unless the list elements can be thunks that can be evaluated when necessary or appropriate. See for instance Control.Parallel.Strategies in Haskell, which provides amongst others 'parMap' (parallel mapping over lists).

Even then it's not enough. The problem is that the structure of a list is itself linear. Guy Steele had a nice talk about that.

(This only bites you when your lists get long.)

I think that's a too blanket statement. What do you want to do with the list, a map or a fold? In the former case, what is the cost of the function application?

Please have a look at Guy Steele's talk (http://www.vimeo.com/6624203). Basically he suggests using tree-like data structures, instead of linear data structures. You can also map, fold and filter over trees, but it parallelizes better.

Reply on news.ycombinator.com