FsCheck -- a quick hack for bytes

This exceptionally trivial piece of FsCheck:

	let popbyte (a: byte array) =
	(a.[0], a.[1..])
	let prop_popbyte (array:byte array) = array.Length < 1 or popbyte array = (array.[0], array.[1..])
	quickCheck prop_popbyte

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surprised me by going Microsoft.FSharp.Core.FailureException : Geneflect: type not handled System.Byte. Especially so when the whole thing I was trying to write at the time was all about bytes, in arrays, in queues,...

So a little bit of diving into the FsCheck code later, a quick and dirty extension for handling bytes in Arbitrary.fs:

Later: now with fewer redundant parentheses and other clutter--

	///Generate arbitrary byte in 0-n
	static member Byte() =
	{ new Arbitrary < byte >() with
	override x.Arbitrary = sized <| fun n ->
	(choose(0, if n > 255 then 255 else n)).Map byte
	override x.CoArbitrary n = variant (if n >= (byte 0) then 2*(int n) else 2*( -(int n)) + 1)
	override x.Shrink n =
	let (|>|) x y = abs (int x) > abs (int y)
	seq { if n < (byte 0) then yield byte(-(int n))
	yield! Seq.map byte (Seq.unfold (fun st -> let st = st / 2 in Some (n - (byte st), st)) (int n)
	|> Seq.cons (byte 0)
	|> Seq.take_while ((|>|) n)) }
	|> Seq.distinct
	}

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which now gives Ok, passed 100 tests. which is what I had been wanting in the first place.

Instrumenting the function under test as

	let popbyte (a: byte array) =
	print_any a
	(a.[0], a.[1..])

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shows a plausibly noisy set of inputs, so any remaining flaws will be subtle ones.

This example is believed to still be correct under the Feb 2010 CTP.