[GHC] #14839: Bits typeclass law for LSB

[GHC]

#14839: Bits typeclass law for LSB
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           Reporter:  andrewthad     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:  base           |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The documentation for the `Bits` typeclass claims:

 > Bits are numbered from 0 with bit 0 being the least significant bit.

 However, there's no law specified in the typeclass that enforces this. I
 realized this recently because I've been adding the laws for `Bits` to a
 library of property tests I maintain: http://hackage.haskell.org/package
 /quickcheck-classes-0.3.3/docs/Test-QuickCheck-Classes.html#v:bitsLaws

 In another package of mine, someone requested to add a `Bits` instance for
 a type but with the MSB considered bit 0. (https://github.com/andrewthad
 /haskell-ip/issues/29) I thought this would fail to satisfy a law of
 `Bits`, but it doesn't. So at the least, I was thinking we could add the
 following laws to `FiniteBits`:

 {{{
 countTrailingZeros (bit 0) = 0
 countLeadingZeros (bit 0) = finiteBitSize undefined - 1
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14839>
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