Data.Bits instances should use default shift instead of shiftL/shiftR

The documentation notes that the second arg of shiftL/shiftR must be positive, yet many definitions of "instance Bits …" rely on the class defaults implementation of shift.

This patch defines shiftL and shiftR explicitly in terms of the corresponding prim ops, while falling back to the class default for shift. This produces the same output for shift, but much more readable core for shiftL and shiftR.

For "shiftR 1 i", contrast

case i of _ { I# i# -> case >=# i# 64 of _ {
    False -> uncheckedIShiftRA# 1 i#
    True -> 0 } }

with

case i of _ { I# i# -> let { n# = negateInt# i# } in case >=# n# 0 of _ {
    False -> let { nn# = negateInt# n# } in case >=# nn# 64 of _ {
        False -> uncheckedIShiftRA# 1 nn#
        True -> 0 };
    True -> case >=# n# 64 of _ {
        False -> uncheckedIShiftL# 1 n#
        True -> 0 } } }

Cheers,

/Liyang

Trac metadata

Trac field	Value
Version	7.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture