Erlang function options

Posted on November 25, 2013

In working on a small Erlang project I found myself writing a few functions that take one or more required parameters and then possibly a few optional ones. A common idiom in Erlang is take a list of tuples and/or atoms for optional parameters. For example, gen_tcp:connect has a signature like this: gen_tcp:connect(Host, Port, Options), where Options can be all kinds of stuff, such as:

{ok, Socket} = gen_tcp:connect("localhost", 12345, [binary, {packet, 0}, {active, false}]).

In this case, the host and port are required, and the third is a list of various kinds of meta data, in various forms. binary is a single atom; packet takes an integer, and according to the docs (which are actually part of inet:setopts) the value can be either 0 or raw, or 1, 2 or 4. The value of active can be true, false or once.

I’m using a function to build up a record that will then be passed on to another function for further processing. The user should only have to pass in one required parameter, and then a list of zero or more optional ones, so I want the call would look something like this:

{ok, Result} = my_func(Arg2, Arg2, Options).

where Options looks like [foo, {bar, 20}, {baz, "hello"}].

In this case no options are required, so I also added in a 1 parameter version of the function:

Pattern matching and recursion make handling this kind of thing pretty clean. If I need to add an option or change how one is handled, its clear where to do it and its easy to test those changes.