[erlang-questions] Why use -record() ?

[Dmitry Kolesnikov]

Hello,

There was quite good reasoning already about records. I just want to make a quotation from my resent post about records. I hope it helps you to catch the reason why…

Type theory and a functional programming operates with [algebraic data types](https://en.wikipedia.org/wiki/Algebraic_data_type). They are known as a composition of other types. The theory defines two classes of compositions: product types (tuples, records) and co-product types (sum, enumeration or variant types). Product types are strongly expressed by [records](http://erlang.org/doc/reference_manual/records.html) in Erlang. 

```erlang
-type fullname() :: binary().
-type address()  :: binary().
-type city()     :: binary().

-record(person, {
   name    :: fullname(),
   address :: address(), 
   city    :: city()
}).
```

The beauty of Erlang records (product type) is that they definitions are only available at compile time. The compiler has complete knowledge of defined "algebra" and catches misuse errors. The usage of records in your code benefits to write correct, maintainable code and support refactoring. Use them to define your domain models!

```erlang
#person{birthday = "18810509"}.

%% Compiling src/person.erl failed
%% src/person.erl:18: field birthday undefined in record person
```

There are few other benefits of records over other data types: type testing and pattern matching. These subject has been widely covered at [official documentation](http://erlang.org/doc/programming_examples/records.html):

```erlang
%%
%% type testing
myfun(X) when is_record(X, person) -> ...

%%
%% pattern matching
myfun(#person{name = Name}) -> ...
``` 

As a summary, I would strongly advertise the usage of records to reflect your domain. Other types will work too...

Best Regards,
Dmitry

> On 2 Jun 2019, at 14.34, bengt <cean.ebengt@REDACTED> wrote:
> 
> Greetings,
> 
> It is true that it is a matter of taste but remember that some Erlang containers can not be used to pattern match. Using one of those handle_call/3 would only be one function clause. And while maps can be pattern matched the compiler will not help you with spelling mistakes, as it does with records.
> 
> 
> Best Wishes,
> bengt
> 
>> On 2 Jun 2019, at 13:25, Stefan Hellkvist <hellkvist@REDACTED <mailto:hellkvist@REDACTED>> wrote:
>> 
>> 
>>> ...
>>> handle_call({run, Args}, _From, {Limit, Spv, Refs, Queue}) when N > 0 ->
>>>   {ok, Pid} = supervisor:start_child(Spv, Args),
>>>   Ref = erlang:monitor(process, Pid),
>>>   NewRefs = gb_sets:add(Ref, Refs),
>>>   NewState = {Limit-1, Spv, NewRefs, Queue},
>>>   {reply, {ok, Pid}, NewState};
>>> 
>>> We also can use the above alternative code right?
>> 
>> Yes, you can use whatever Erlang term you prefer - record, tuple, map, a list of maps containing records of tuples...
>> 
>> What you choose is a matter of taste and depends on your requirements. A record for instance has the advantage over a tuple, that you access elements by name and the order of elements therefore becomes irrelevant. This could give advantages if you one day decide to add another field to your state. With the tuple approach this would likely break every access to the tuple everywhere in the code, but with a record where you access and match by names most of your code might still work...plus you never need to remember what meaning the third or fourth element in your tuple has l, because it has a descriptive name.
>> 
>> Stefan 
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