Binary, List and Tuple Inequalities (Paradox?)

Matthias Lang matthias@REDACTED
Sun Nov 24 00:51:53 CET 2019

```Jumping into a thread that's a few weeks old.

My first thought was to see what the 'Barklund' draft says:

| An empty list precedes a cons ... and conses are ordered first by
| their heads, then by their tails. (Thus a longer list may preced a
| shorter list even though a shorter list tuple always precedes a
| longer tuple.) For example, [] < [a|2] <[a|b] < [a] < [a, a], [b].
|

So far no surprises, but then it says

| Binaries are ordered first by their size, then according to their
| elements lexicographically. (That is, the same as the order
| between tuples of integers.

So I think the Barklund draft doesn't agree with the implementation.

I went back to OTP R8B. Even that doesn't agree with the Barklund draft:

1> init:script_id().
{"OTP  APN 181 01","R8B"}    // I think R8B is from about 2002
2> <<0,0,1>> < <<1,0>>.
true

I'm not sure if the Barklund draft never agreed with the implementation,
or if the implementation changed some time before 2003.

Also, it seems that the current reference manual defines what happens
for lists, but not binaries and bit strings:

http://erlang.org/doc/reference_manual/expressions.html#term-comparisons

Matthias

----------------------------------------------------------------------
Date: 28. October 2019 From: Raimo Niskanen
<raimo.niskanen@REDACTED> To erlang-questions@REDACTED Subject:
Re: Binary, List and Tuple Inequalities (Paradox?)

> I'd like to defend the current term order of lists, tuples and
> binaries.
>
> Lists are compared a'la string comparison, which avoids having to
> traverse the whole list when comparing.  Binaries are compared as lists
> since both are used for handling strings, and also to be compared like
> memcmp does it.
>
> Tuples are used for general complex terms.  They can be compared size
> first whithout having to be fully traversed, as opposed to lists, which
> is useful in other contexts.  You can thereby choose your data
> structure to select comparision.
> --
> Raimo Niskanen
>

```