Strange Confluence: An Immutable Queue in F#

Jomo Fisher — Wed, 12 Dec 2007 19:09:00 GMT

Jomo Fisher--Reading one of my favorite blogs this morning--Eric Lippert's Fabulous Adventures in Coding--I came across his article on implementing an immutable queue in C#. The funny thing is that just yesterday I wrote exactly the same structure in F#. Here it is for your reading pleasure:

type Fifo<'a> =

new()={xs=[];rxs=[]}

new(xs,rxs)={xs=xs;rxs=rxs}

val xs: 'a list;

val rxs: 'a list;

static member Empty() = new Fifo<'a>()

member q.IsEmpty = (q.xs = []) && (q.rxs = [])

member q.Enqueue(x) = Fifo(q.xs,x::q.rxs)

member q.Take() =

if q.IsEmpty then failwith "fifo.Take: empty queue"

else match q.xs with

| [] -> (Fifo(List.rev q.rxs,[])).Take()

| y::ys -> (Fifo(ys, q.rxs)),y

This is code modified from a mutable structure that I found in the F# distribution. The queues are pretty similar except for a few minor details:

F# version doesn't implement IEnumerable<T>.
F# version relies on the built-in list class which is semantically identical to an immutable stack.
C# version carries the "popped" instance of T along with the queue itself whereas the F# version returns this as part of a tuple from Take(). I think I prefer my way on this point--it seems like information leakage for the queue have a memory of the last thing popped.

That's all for now, and as always...

This posting is provided "AS IS" with no warranties, and confers no rights.

Jomo Fisher -- Sharp Things : Data Structures

Strange Confluence: An Immutable Queue in F#