Hi,
I just read a blog post [1] talking about Elixir pattern matching. I was
thoroughly impressed with the way its handled in Elixir. I am posting this
here cuz I got rather excited and wanted to discuss this with you all.
My experience with pattern matching is limited to the basics of F# and
reading the docs of core.match. I think its a great idea, but I also feel
unless its supported by language at the fundamental level, it remains as
syntactic sugar.
All pattern matching code I had read previously, involve matching on a
specific argument, often inside a function. From what I see in the blog
post, in Elixir its taken one step further and pattern matching is done at
the function declaration/invocation level. Normally one would create one
outer public function and multiple private (?) functions to handle each
branch. The outer function would only have the pattern matching code. At
the very least, I find the Elixir version easier to read. It seems to be
idiomatic for Elixir libraries, functions to return a tuple whose first
(few?) elements are purely used for pattern matching.
Consider the following code samples from that blog post,
def to_registration_result({:ok, res}) do
{:ok, %Membership.RegistrationResult{
success: res["success"],
message: res["message"],
new_id: res["new_id"],
validation_token: res["validation_token"],
authentication_token: res["authentication_token"]
}}
end
def to_registration_result({:error, err}) do
{:error, err}
end
Normally, I would have one function with an if condition to check for an
error value in the args and then call respective functions to handle each
branch. I could also do
(defmulti to-registration-result first)
(defmethod to-registration-result :ok
[[_ email password]]
(println email password))
(defmethod to-registration-result :err
[[_ err]]
(println "err: " err))
But this wouldn't be idiomatic Clojure. This would also require all
libraries and functions to return data in a certain form, one where in the
first element is some kind of status.
He has another example where pattern matching is used with recursive
functions to handle transition from one state/step to another and handle
terminating conditions. This specific example is a very poor choice, but it
does demonstrate the possibilities.
def map_single({:ok, res}) do
cols = res.columns
[first_row | _] = res.rows
map_single {:cols_and_first, cols, first_row}
end
def map_single({:cols_and_first, cols, first_row}) do
zipped = List.zip([cols,first_row])
map_single {:zipped, zipped}
end
def map_single({:zipped, list}) do
{:ok, Enum.into(list, %{})}
end
def map_single({:error, err}) do
{:error, err}
end
Interesting stuff.
[1] -
http://rob.conery.io/2015/09/04/using-recursion-in-elixir-to-break-your-oo-brain/
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