So recently I’ve been working on automating some interactions with a website.  Elixir’s Hound library is a great help, of course.  But I also ran across an interesting and unexpected benefit to pattern matching that greatly simplified some code.

I was trying to dynamically build a URL to pass parameters. The URL would be in the form:

http://www.exampledomain.com?param1=p&param2=q&param3=r

Whenever I see code like this, it’s always a little tricky because you want to add the & at the end of each parameter except the last.  This always used to entail writing a special case in the loop to test if the index of the current element was the maximum index.  But with pattern matching and recursion there’s a much simpler and cleaner solution.  Like this:

defp add_params_to_url(url,[%{:name => name, :value => value}]) when is_binary(url) do     #1
 "#{url}#{name}=#{value}"
end
defp add_params_to_url(url,[%{:name => name, :value => value} | t]) when is_binary(url) do   #2
 url = "#{url}#{name}=#{value}&"
 add_params_to_url(url,t)
end
defp add_params_to_url(url,[]) when is_binary(url), do: url   #3

This is Elixir for those unfamiliar with it.  Basically it’s three function clauses.  The first clause (#1) is hit if only one element is in the list passed into the function. The second clause (#2) is hit if multiple elements are present in the list and the third clause (#3) is hit if the list is empty.

So this is how this works.  When I call add_params_to_url, Elixir will automatically try to match the correct function call dependent on which list I pass.  It will also stop at the first function clause which matches so that other function clauses will not be evaluated.

If I pass a list of URL params which has only one element, clause 1 is matched and the parameter and value are appended and I’m done.  The URL already has the ? on the end, of course.  If I pass a list of URL params with mutliple elements, clause 1 doesn’t match so Elixir jumps down to clause 2 and that matches.  Clause 2 takes the first element from the list tacks it on to the list of parameters with a & at the end and then recursively calls itself with the tail of the list.  If the tail of the list contains more than one parameter, clause  1 will once again fail to match and clause 2 will match again.  If it contains only one parameter, clause 1 is matched.  Because clause 1 doesn’t call itself recursively, clause 3 should never be matched; it’s actually more of a guard case in case someone calls the function with an empty parameter list by mistake.

This, to me, is a small bit of genius.  Much simpler to get exactly the effect I want, that is not having an extraneous character tacked to the end of the string, and it’s nice and simple.

By the way, it occurs to me that I might be able to use a reduce or a fold function to achieve this same effect.  The issue there is that it’s less apparent what it is that I’m trying to do and I also run into the same problem–that is, how do I deal with that last element in the list so I don’t get my separator appended?