Proposal: Determining whether a Bits instance is FiniteBits

[Edward Kmett]

Destroying GND for Bits _is_ a pretty huge downside. =/

On Thu, May 27, 2021 at 10:45 AM Oleg Grenrus <oleg.grenrus at iki.fi> wrote:

> 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> 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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>
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