#light
let g = new System.Random()
let s = Seq.init_infinite (fun _ -> g.NextDouble())
/// You thought 'a seq is Haskell [a]? Not quite! It does not memoize:
printfn "%A" (Seq.hd s, Seq.hd s) (* Surprise! fst and snd differ! *)
type X =
static member X = g.NextDouble()
/// You thought X is a value? Not quite! It is a property (think function):
printfn "%A" (X.X, X.X) (* Surprise! fst and snd differ! *)
Friday, March 6, 2009
F#: Haskellers beware!
Equational reasoning may fail you. F# land is the land where nuclear missiles are launched all the time, and puny gnomes carry wands of death.
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The first example can be brought back to the land of the lazy with a cache call:
ReplyDeleteopen System
let g = Random()
let s = Seq.init_infinite (fun _ -> g.NextDouble()) |> Seq.cache
printfn "%d, %d" (Seq.hd s) (Seq.hd s) (* performs like a Haskell lazy list, at least in this example *)
The PowerPack type LazyList<'a> I'm sure would also work but I'm not as familiar with using it.
As for the second example, I agree that behavior is peculiar. However it's apparently another feature :)
From the 1.9.6 Draft Language spec, section 8.4.1 Property Members:
Furthermore
[static] member [self-ident .] ident = expr
is equivalent to:
[static] member [self-ident .] ident with get () = expr
Oh well. Let's just hope the AreNukesLaunched property doesn't accidentally press the button.
Right :)
ReplyDeleteOne problem I used to have with Seq.cache is its type (it's not returning a seq).
But now that you mention it, I gave it a second look, and the return type can be easily mended:
let cache<'a> : 'a seq -> 'a seq = fun x -> upcast (Seq.cache x)
Concerning properties, it's just, well, one of those .NET things :)
I wish .NET designers read the "Lambda Papers" from the 1970s. One concept, lambda, is quite enough!