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

[Milan Straka]

Hi all,

I agree with Edward that although rotate{,L,R} should really live in
FiniteBits, it is probably too costly to move them there.

When staying in Bits, I find the "rotate is shift for infinite"
semantics the most appealing -- rotations do not make really sense for
infinite sequences, so I would not pretend they are rotating the
sequence.

Cheers,
Milan

> -----Original message-----
> From: Edward Kmett <ekmett at gmail.com>
> Sent: 29 Sep 2014, 11:41
>
> It does seem like it should rotate in 1s, but if it does so, what happens
> when it rotates out something that isn't all 1s into a negative number or
> all 0s into a positive number?
> 
> We'd expect `rotateL n . rotateR n = id = rotateR n . rotateL n` for the
> finite cases.
> 
> but you can't rotate out the digits "up at infinity".
> 
> So even this is messy. =/
> 
> Ideally we *would* move rotate/rotateL/rotateR to FiniteBits, but there is
> a huge upgrade/migration cost associated with that. All user specified
> instances break in a way that requires CPP and it diverges from the
> standard further.
> 
> 
> On Mon, Sep 29, 2014 at 11:33 AM, David Feuer <david.feuer at gmail.com> wrote:
> 
> > Problem:
> >
> > The documentation currently specifies that for non-finite Bits instances,
> > rotate is the same as shift. This seems reasonable for Integer rotateR, but
> > very strange for Integer rotateL. In particular:
> >
> > rotateL 5 (-1 :: Int) = -1
> > rotateL 5 (-1 :: Integer) = 16
> >
> > Based on the concept that Integers represent two's complement out to
> > infinity, rotating a negative integer left should fill with 1s, but it
> > fills with 0s instead.
> >
> > Solutions:
> >
> > As stated in the subject, I think one reasonable solution is just to make
> > rotateL fill with 1s for negative non-finite instances, and the other is to
> > move rotations to FiniteBits, as was done with bitSize.
> >
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> >
> >

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