On 17 November 2015 at 15:42, Nathaniel Smith <n...@pobox.com> wrote:
>> So for clarity:
>> True and -> right hand side evaluated stand alone
>> False and -> False
>> True or -> True
>> False or -> right hand side evaluated stand alone
>>
>> We can roll that up using the parse tree:
>> (('posix', '==', 'dud'), [('and', ('posix', '==', 'odd')), ('or',
>> ('posix', '==', 'fred'))]))
>> evaluate the start to get a 'result'
>> pop an expression from the beginning of the list giving opcode, next.
>> lookup (result, opcode) in the above truth table, giving one of True,
>> False, None
>> on None, result = next and loop.
>> on True or False, return that.
>
> Not sure if we're communicating or not? The case I'm concerned about is
>
> a or b and c
>
> which Python parses as (a or (b and c)) because 'and' has higher
> precedence than 'or'. So for example in Python,
>
> True or True and False
>
> returns True, but if you use a left-to-right evaluation rule than it
> returns False.
True or True and False
->
lookup = {
(True, 'and'): None,
(False, 'and'): False,
(True, 'or'): True,
(False, 'or'): None}
def evaluate(expr):
result = expr[0]
remainder = expr[1]
while remainder:
opcode, next = remainder.pop(0)
branch = lookup[(result, opcode)]
if branch is not None:
return branch
result = next
return result
expr = (True, [('or', True), ('and', False)])
evaluate(expr)
-> True
So, I think what I describe does what you want it to.
-Rob
--
Robert Collins <rbtcoll...@hp.com>
Distinguished Technologist
HP Converged Cloud
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