Proposal: Determining whether a Bits instance is FiniteBits

Edward Kmett ekmett at
Thu May 27 17:22:14 UTC 2021

This does seem like a strict improvement over the status quo.

Users can then conditionally get access to count(Trailing|Leading)Zeros by
refining the type information available to them using this combinator,
which offers a bunch of bit twiddling usecases.


On Tue, May 18, 2021 at 9:30 AM Zemyla <zemyla at> wrote:

> Every Bits instance has to know that whether it's a FiniteBits
> instance as well, due to bitSizeMaybe. Therefore, it should also be
> able to tell a program that it is in fact a FiniteBits instance. There
> should be a function added to Data.Bits.Bits:
> withFiniteBits :: Bits a => a -> (FiniteBits a => r) -> r -> r
> The default should be withFiniteBits _ _ x = x, at least for the next
> several versions. bitSizeMaybe can be defined as
> bitSizeMaybe x = withFiniteBits x (Just (finiteBitSize x)) Nothing
> once everyone is on board with properly defining the value.
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