CODEWITHSUNDEEP
erlangrecorddefault-value
Stratus3D
Stratus3D

Reputation: 4916

Get default value from Erlang record definition?

Is there an easy way to get a default value from an Erlang record definition? Suppose I have something like this:

-record(specialfield, {
    raw = <<"default">> :: string()
}).

I would like to have some way to retrieve the default value of the raw field. Something like this would be very simple:

 #specialfield.raw % => <<"default">>

This is not possible. I would need to instantiate a record in order to get the default value:

 Afield = #specialfield{}
 DefaultValue = Afeild#specialfield.raw
 DefaultValue % => <<"default">>

Is there an easier way of doing this? I seems like there should be some way to retrieve the default value without having to create an instance of the record.

Upvotes: 0

Views: 1207

Answers (3)

Marc Paradise
Marc Paradise

Reputation: 1939

Take a look at erlang - records, search section "11.8".

There's not much special about records - they're just a tuple at runtime. So to get the field raw from the tuple of default values that is the internal representation of #specialfield{} you would use: 

element(#specialfield.raw, #specialfield{}).

In this case, #specialfield.raw is the index of the value for raw in the #specialfield tuple. When you pass in specialfield that resolves to a tuple in the form {specialfield, <<"default">>}.

Upvotes: 1

Stratus3D
Stratus3D

Reputation: 4916

Constructing an empty record and accessing one field can be done on one line:

(#specialfield{})#specialfield.raw.

Upvotes: 1

I GIVE CRAP ANSWERS
I GIVE CRAP ANSWERS

Reputation: 18879

How about:

raw_default() -> <<"default">>.

-record(specialfield, { raw = raw_default() }).

And now you have a function with the default in it. This will be extremely fast since it is a function call to a constant value. If this is also too slow, enable inlining.

Upvotes: 3

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