Did I miss in this thread an actual use-case for strict and/or for Bool such that it might occur in a tight inner loop? Absent such a use case, I’m -1 on this proposal. If I wanted to use something that I felt had strict bitwise behaviour, I’d never reach for a bool, no matter what operator set I was working with. My instinct would be to reach for a Word8 or the like, since on any real implementation it would take up the same space regardless? Essentially, to my laziness-infected brain, the current instance obeys the principle of least surprise, and the proposed instance violates the sort of wall of “abstraction intuitions” I’ve built up regarding when I should and shouldn’t expect lazy behaviour. -g On September 27, 2014 at 1:59:43 PM, David Feuer (david.feuer@gmail.com) wrote:
Currently, the (.&.) and (.|.) methods for Bool are short-circuiting, defined like this:
instance Bits Bool where (.&.) = (&&)
(.|.) = (||)
Unlike the instances for Int, Word, etc., this gives short-circuiting behavior (conditionally lazy in the second operand). Unfortunately, this requires a conditional branch to implement, which can sometimes be bad. Since (&&) and (||) are readily available to anyone who wants short-circuiting, I propose that we use the following instead. Note that the Bits class does not specify anything about this aspect of instance behavior.
x .&. y = tagToEnum# (dataToTag# x `andI#` dataToTag# y)
x .|. y = tagToEnum# (dataToTag# x `orI#` dataToTag# y)
The rest of the operations look like this:
x `xor` y = tagToEnum# (dataToTag# x `xorI#` dataToTag# y)
complement x = tagToEnum# (dataToTag# x `xorI#` 1#)
shift x s = testBit x s
rotate x _ = x
-- I don't think we gain anything changing this one. bit 0 = True bit _ = False
testBit x b = tagToEnum# (dataToTag# x `andI#` (dataToTag# b ==# 0#))
bitSizeMaybe _ = Just 1
bitSize _ = 1
isSigned _ = False
popCount x = I# (dataToTag# x)
instance FiniteBits Bool where finiteBitSize _ = 1 countTrailingZeros x = I# (dataToTag# x `xorI#` 1#) countLeadingZeros x = I# (dataToTag# x `xorI#` 1#) _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries