On Dec 4, 2007 11:51 AM, Don Stewart wrote:

Awesome. We can use this in Data.Bits, if you've got some QuickChecks for it.

Hear hear. But is there any way to just make the compiler use fastTestBit in place of testBit :: (Bits a) => a -> Int -> Bool when a = Integer? (That is, without having to introduce a new function to the public interface of Data.Bits.) Some kind of SPECIALIZE pragma, perhaps? I've attached a program with two QuickCheck properties. Unfortunately they fail on negative Integers. I can't figure out why.

There's a bit of work required to make this code good enough for general consumption, and I don't know much about Haskell internals. (1) What is the "official" way to find the size of a word? A quick look at 6.8.1's base/GHC/Num.lhs reveals that it uses a #defined symbol. (2) Is it safe to assume an underlying implementation based on GMP? (In Num.lhs there is an alternative definition for .NET. Is that ever used?) Is it safe to assume the size of a GMP "limb" is the same as the word size? (I'm assuming it is for now.) David said:

I've attached a program with two QuickCheck properties. Unfortunately they fail on negative Integers. I can't figure out why.

Judging by Num.lhs, the sign of the Integer is stored as the sign of the length of the Integer in "limbs". The code needs to deal with that explicitly, as well as doing some bounds checking on the index. Anyway, if someone can help with those questions I can try to write a bulletproof version of the code. -- Dan

Dan Piponi wrote:

There's a bit of work required to make this code good enough for general consumption, and I don't know much about Haskell internals.

(1) What is the "official" way to find the size of a word? A quick look at 6.8.1's base/GHC/Num.lhs reveals that it uses a #defined symbol.

I'm not 100% sure what you mean by "word" in this context, but assuming you mean the same thing as we do in GHC when we say "word" (a pointer-sized integral type), then one way is Foreign.sizeOf (undefined :: Int) or Foreign.sizeOf (undefined :: Ptr ()) (in GHC these are guaranteed to be the same). There's also Data.Bits.bitSize (undefined :: Int) which might be more appropriate since you're using Data.Bits anyway. Cheers, Simon