do record_info and tuple_to_list return the same k

问题:

I.e, if I have a record

-record(one, {frag, left}).

• Is record_info(fields, one) going to always return [frag, left]?
• Is tl(tuple_to_list(#one{frag = "Frag", left = "Left"})) always gonna be ["Frag", "Left"]?

Is this an implementation detail? Thanks a lot!

回答1:

The short answer is: yes, as of this writing it will work. The better answer is: it may not work that way in the future, and the nature of the question concerns me.

It's safe to use record_info/2, although relying on the order may be risky and frankly I can't think of a situation where doing so makes sense which implies that you are solving a problem the wrong way. Can you share more details about what exactly you are trying to accomplish so we can help you choose a better method? It could be that simple pattern matching is all you need.

As for the example with tuple_to_list/1, I'll quote from "Erlang Programming" by Cesarini and Thompson:

"... whatever you do, never, ever use the tuple representations of records in your programs. If you do, the authors of this book will disown you and deny any involvement in helping you learn Erlang."

There are several good reasons why, including:

1. Your code will become brittle - if you later change the number of fields or their order, your code will break.
2. There is no guarantee that the internal representation of records will continue to work this way in future versions of erlang.

回答2:

Yes, order is always the same because records represented by tuples for which order is an essential property. Look also on my other answer about records with examples: Syntax Error while accessing a field in a record

回答3:

Yes, in both cases Erlang will retain the 'original' order. And yes it's implementation as it's not specifically addressed in the function spec or documentation, though it's a pretty safe bet it will stay like that.