Today I Learned

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4 posts about #erlang

Erlang records are just tuples

Erlang has a Record type which helps you write clearer code by having data structures that are self descriptive.

One thing that is interesting about a Record is that a record is just a tuple underneath the hood.

Erlang/OTP 22 [erts-10.4.4] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]

Eshell V10.4.4  (abort with ^G)
1> rd(fruit, {name, color}).
fruit
2> #fruit{name="Apple", color="Red"}.
#fruit{name = "Apple",color = "Red"}
3> #fruit{name="Apple", color="Red"} == {fruit, "Apple", "Red"}.
true

As you can see, that internal representation is exposed when comparing a record to a tuple:

#fruit{name="Apple", color="Red"} == {fruit, "Apple", "Red"}.

You can even use pattern matching with the two data structures:

7> #fruit{name="Apple", color=NewColor} = {fruit, "Apple", "Green"}.
#fruit{name = "Aplle",color = "Green"}
8> NewColor.
"Green"

Declaring Erlang records in a shell

Erlang records are typically declared in a header file. If you want to experiment with records at the command line, you’ll have to use a shell command.

rd is an erl shell command that you can remember as standing for record definition

Let’s try that in the erlang shell tool, erl.

Erlang/OTP 21 [erts-10.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe] [dtrace]

Eshell V10.1  (abort with ^G)
> rd(fruit, {name, color}).
fruit

And then you can declare a record the same way you would in Erlang.

> Apple = fruit#{name="Apple", color="Grren"}.
#fruit{name = "Apple",color = "green"}

And to inspect that variable in erl, just declare the variable and put a . after it to conclude the expression.

> Apple.
#fruit{name = "Apple",color = "green"}

You can find other shell commands that deal with records and other things here.