# Quick basic list questions. Not a test just checking what I believe to be correct

**URL:** <https://swi-prolog.discourse.group/t/quick-basic-list-questions-not-a-test-just-checking-what-i-believe-to-be-correct/2621>\
**Category:** Data Structure\
**Tags:** list, difference-list\
**Created:** [July 9, 2020, 8:27pm UTC](https://swi-prolog.discourse.group/t/quick-basic-list-questions-not-a-test-just-checking-what-i-believe-to-be-correct/2621 "2020-07-09T20:27:29Z")\
**Posts on this page:** 1\
**Showing post:** 11

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**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\
**Post date:** [July 11, 2020, 1:55pm UTC](https://swi-prolog.discourse.group/t/quick-basic-list-questions-not-a-test-just-checking-what-i-believe-to-be-correct/2621/11 "2020-07-11T13:55:09Z")

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> [@EricGT](#):
>
> Three ways I use to check if something is a difference list

OK, I now understand maybe what is going on. I just assumed that you (the programmer) know when you have a difference list. And you are probably doing more complicated things, where you actually want to take a pair of variables and determine whether they form a difference list? This now finally makes sense. I re-read [the other thread](https://swi-prolog.discourse.group/t/is-there-a-has-type-2-for-difference-list/2622) more carefully, this is all discussed there apparently.

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