On Sun, Sep 28, 2014 at 11:06 PM, Gershom B
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
The problem is that Bool is the odd duck with respect to the Data.Bits operations, so the only way your intuition would serve you is if you were sure you would only be working with Bool, in which case the question is why you were reaching for Data.Bits. Anthony
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
_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries