Proposal: Determining whether a Bits instance is FiniteBits

[Carter Schonwald]

Yeah.
Is there any other way we could make it work or
Does it turn into the same issue of coercions in ghc don’t have the right
fancy to support this improvement nicely?

On Fri, May 28, 2021 at 2:45 AM Edward Kmett <ekmett at gmail.com> wrote:

> 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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