haael writes:
> Loop invariant is such a statement that if it was true before the loop
> iteration, it will also be true after the iteration. It can be
> implemented as an assertion of an implication.
>
> now_value = False
> while running_condition(...):
> prev_value = now_value
> now_value = invariant_condition(...)
> assert now_value if prev_value else True
Can you do this with a context manager? The syntax would be
now_values = None
while running_condition(...):
with invariant_conditions(now_values, invariant1, ...):
# loop body goes here
The idea is that the __enter__ of the invariant_conditions context
manager would initialize 'now_values' to [False] * invariants_count if
now_values == None. The __exit__ would loop over zip(now_values,
invariants), doing
prev_value = now_value
now_value = invariant(...)
assert now_value if prev_value else True
for each. An alternative definition of loop invariant from Wikipedia
(https://en.wikipedia.org/wiki/Loop_invariant) suggest the simpler
syntax (and implementation)
while running_condition(...):
with invariant_conditions(invariant1, ...):
# loop body goes here
where the invariants are simply asserted in __enter__, stored in an
attribute of the context manager object, and asserted again in
__exit__.
In both cases, the evaluations would be controlled by a global
variable, which would have minimal performance impact.
You may also want to look at MacroPy, which allows you to implement
syntax like you've proposed.
Steve
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