Erlang Thursday – user_default

Today’s Erlang Thursday takes a look at user_default.

I stumbled across this when trying to verify what version of c/1 and l/1 was used when called in the shell, and came across shell_default documentation.

It mentioned that if there are functions that we would like to have available in the shell, we can have a module user_default and specify its location in our .erlang file in our home directory.

Since I have a fizzbuzz example in Erlang handy, lets start out with that for our user_default module as proof that we can get it working. And we’ll put it in tmp under our home directory.

-module(user_default).

-export([fizzbuzz/1]).

fizzbuzz(N) ->
    Translations = lists:map(fun translate/1, lists:seq(1, N)),
    lists:foreach(fun(Item) -> io:format("~s~n", [Item]) end, Translations).

translate(N) when N rem 3 =:= 0 andalso N rem 5 =:= 0 ->
   'FizzBuzz';
translate(N) when N rem 3 =:= 0 ->
   'Fizz';
translate(N) when N rem 5 =:= 0 ->
   'Buzz';
translate(N) ->
   integer_to_list(N).

Let’s open up a new .erlang file in our home directory, and add the following line, pointing to the location of the user_default file we just created.

code:load_abs("tmp/user_default").

If you already have a .erlang file, then the call to code:load_abs/1 for the user_default module must be the first line in the file.

Time to ensure both files are saved, and open up the erlang shell, and try calling fizzbuzz/1.

$ erl
Erlang/OTP 17 [erts-6.2.1]  [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

Eshell V6.2.1  (abort with ^G)
1> fizzbuzz(20).
** exception error: undefined shell command fizzbuzz/1
2> q().
ok

Doesn’t seem to be working as advertised. Realizing that this is load_abs, and user_default is a new module, the error is probably because there is no file to load. Let’s go compile the module using erlc, and try again.

$ cd tmp/
$ erlc user_default.erl
$ ls user_default.*
user_default.beam user_default.erl
$ cd ..

We now have a BEAM file there, so let’s startup the Erlang shell again and try to call fizzbuzz/1.

$ erl
Erlang/OTP 17 [erts-6.2.1]  [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

Eshell V6.2.1  (abort with ^G)
1> fizzbuzz(20).
1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz
ok
2>

And it works! We now have fizzbuzz/1 available for use in the shell without needing to specify a module.

For more information on the .erlang configuration file, check out the Configuration section of the Erlang Getting Started documentation.

–Proctor

2 thoughts on “Erlang Thursday – user_default

    1. Proctor Post author

      That was an awesome post. Thanks for putting it on my radar. I have enjoyed inaka’s posts around Erlang, and this just makes me realize I need to go through the backlog and catch up on all of the older ones as well. 😀

      Reply

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