alt Hacker News> Deterministic evaluation - Executing the same code twice will give the same results.
What’s this all about? Don’t most languages?
Replies
phyrex • 12/09/2024
You don't get access to randomness or time functions or anything that could change the output of a function with the same input
lann • 12/09/2024
One specific example: many languages use randomized seeds in builtin dict/map types, leading to randomized iteration order.
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zellyn • 12/09/2024
It can be very valuable to know that if you put the same files in, you get _exactly_ byte-for-byte identical artifacts out of your build system. Even letting your language access the actual current date/time can break that.
(IIRC, I don't believe Bazel actually has fully deterministic builds yet, though.)
neuroelectron • 12/09/2024
Apparently not. Wow. No wonder China is hacking into everything.
Try this in Python:
id("ab") # not deterministic
hash("ab") # not deterministic
def foo(): pass
str(foo) # not deterministic
Another sneaky one is the `is` operator in Python, where the Python documentation says:
> Due to automatic garbage-collection, free lists, and the dynamic nature of descriptors, you may notice seemingly unusual behaviour in certain uses of the `is` operator
Related to that is the `__del__` method: when exactly is it called?
It's quite easy to get non-deterministic code in Python and in many languages. And of course, there are lots of non-deterministic functions in the standard library (Starlark doesn't provide them).