Proposal: Determining whether a Bits instance is FiniteBits

[Oleg Grenrus]

I don't think this as good idea, as that definition doesn't work with GND.

    {-# LANGUAGE RankNTypes, GeneralizedNewtypeDeriving,
ConstrainedClassMethods #-}
   
    import Data.Bits
    import Data.Word
   
    class Bits a => Bits' a where
        withFiniteBits :: Bits a => a -> (FiniteBits a => r) -> r -> r
   
    instance Bits' Word8 where
        withFiniteBits _ x _ = x
   
    newtype W = W Word8 deriving (Eq, Show, Bits, Bits', FiniteBits)

fails with

Bi.hs:12:47: error:
    • Couldn't match type ‘Word8’ with ‘W’
        arising from the coercion of the method ‘withFiniteBits’
          from type ‘forall r.
                     Bits Word8 =>
                     Word8 -> (FiniteBits Word8 => r) -> r -> r’
            to type ‘forall r. Bits W => W -> (FiniteBits W => r) -> r -> r’
    • When deriving the instance for (Bits' W)
   |
12 | newtype W = W Word8 deriving (Eq, Show, Bits, Bits', FiniteBits)


Note, FiniteBits has nominal role, so cannot be coerced to FiniteBits W.


If CLC decides this is still fine, then I'd suggest to not have
any migration period as adding method is breaking change for GND users,
so rather break everyone at once, maybe even by moving bitSizeMaybe out
of the class.

- Oleg

On 27.5.2021 20.22, Edward Kmett wrote:
> 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.
>
> -Edward
>
> On Tue, May 18, 2021 at 9:30 AM Zemyla <zemyla at gmail.com
> <mailto:zemyla at gmail.com>> 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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