Stream: beginners

Topic: Comparison of List U8

Luke Boswell (Jul 21 2023 at 06:43):

Random question, but how do we do equality for a List U8?

If I have a large List U8 and then take lots of smaller sublists, is it super efficient to compare these smaller List U8's against each other if I pass them into functions etc? I guess I'm asking if these would be just a comparison of the slice or reference, or is it something deeper?

Luke Boswell (Jul 21 2023 at 06:44):

Would it be inefficient to use a List U8 as a key to a Dict?

Luke Boswell (Jul 21 2023 at 07:07):

Or could I use something like Box (List U8) if I want it to be a reference?

Folkert de Vries (Jul 21 2023 at 07:49):

dicts use a hash, which means the whole list is traversed. That might be a problem for very large lists

Folkert de Vries (Jul 21 2023 at 07:50):

really depends on the specifics how good/bad of an idea this is

Luke Boswell (Jul 21 2023 at 08:43):

I'm thinking of a use case when decoding JSON, of using a List U8 input bytes as an index into another data structure.

Luke Boswell (Jul 21 2023 at 08:44):

I feel like if the List U8 is actually stored on the heap and is only a reference, then equality may be very fast.

Folkert de Vries (Jul 21 2023 at 08:52):

well yes but not with hashing.

Folkert de Vries (Jul 21 2023 at 08:53):

for equality we first compare the lengths, then the pointers, and if the pointers are not the same we compare elements

Folkert de Vries (Jul 21 2023 at 08:53):

but a hash will always look at all the elements

Luke Boswell (Jul 23 2023 at 00:06):

If I have a record such as {field : List U8, value : List U8} and the List U8 are slices from another larger List U8 is this record a fixed size? I guess I am concerned that putting these lists in record fields will make unnecessary allocations.

Brendan Hansknecht (Jul 23 2023 at 00:32):

The record is always fixed sized, the data being allocation free depends on how you use it

Brendan Hansknecht (Jul 23 2023 at 00:32):

If they are always referencing a larger list and never edited, they would be seamless slices and never allocate

Luke Boswell (Jul 23 2023 at 00:34):

Thanks Brendan that makes sense

Last updated: Jul 06 2025 at 12:14 UTC