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> =
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.