Discussion: Change the specification of rotateL for non-finite Bits, or move rotations to FiniteBits

John Lato jwlato at gmail.com
Mon Sep 29 19:10:50 UTC 2014

Do you have a use case for this instance? I don't think I would ever want
it. Also I think the behavior is too unspecified, and trying to specify it
more precisely will inevitably lead to a much less useful instance. In this
instance I might prefer we focus on problems we have already rather make
new ones.

Also +1 for rotate acting like shift for infinite types, for the reasons
Milan gave.

John L.
On Sep 29, 2014 11:28 AM, "David Feuer" <david.feuer at gmail.com> wrote:

> Thinking about this some more, I ran into something else to chew on.
> It would seem to make a certain amount of sense to have
> instance (Bits b) => Bits (Seq b)
> with .&., .|., and xor defined using a zero-prepending zipWith variant.
> but there are a few challenges relating to size. Specifically:
> 1. The specification indicates that finiteBitSize and bitSizeMaybe are
> supposed to return a result based only on the type of their argument.
> Clearly, that will not work for this.
> 2. There is no obviously correct size to use for zeroBits and bit.
> One approach, of course, is to define something wonky like
> newtype SeqN (n::Nat) a = SeqN (Seq a)
> and then use
> instance (Bits b) => Bits (SeqN n b)
> but that gives a different, more limited type than one might expect.
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