As an Elixir novice, I think one of the hardest things about getting started in Elixir is figuring out what kind of data structures to use. My team has recently started going all in on Elixir, so I've been trying to brush up in earnest. Often, however, I'll be reading over some of my team's code and have a hard time deciphering what I'm even looking at. The syntax looks very similar to Ruby (which I know pretty well), but the patterns, conventions, and data structures are just slightly different. In my mind, that makes sense since it's a functional rather than object-oriented language: where you would use objects in Ruby, you're probably spawning processes instead in Elixir.
But anyway, since I'm learning Elixir now, I thought it'd be useful to provide a cheat sheet or overview of the differences in data structures I've noticed as a Rubyist exploring Elixir.
Data Types
If you're coming from Ruby (or most other programming languages), integers, floating-point numbers, ranges, and regular expressions are all probably familiar to you. Fortunately, those all exist in Elixir too. There are a few differences, but I haven't dealt too much with them yet.
Atoms are like symbols in Ruby. They begin with colons and their names are their values. For example, :hello is a valid atom in Elixir. They're often used to tag values.
There are also strings in Elixir. Strings always have double quotation marks, while charlists are in single quotations marks. Strings are binaries, and charlists are actually just lists of code points. I have rarely used charlists so far.
Here's a quick glance at what those types look like
iex> 2 # integer
iex> 2.0 # floating point
iex> false # boolean
iex> 1..4 # range
iex> ~r/hello/ # regular expression
iex> :hello # atom
iex> "world" # string
iex> 'world' # charlist
Elixir has the additional data types, Port and PID, which are used in process communication. They are entities that are made available through the Erlang VM.
Port
A Port is used to communicate (read/write) to resources outside your application. They are great for starting operating system processes and communicating with them. For example, you might want to open a port to run an
OS command, like echo.
You could open a port and send it a message, like this:
iex> port = Port.open({:spawn, "echo sup"}, [:binary])
#Port<0.1305>
Then, you can use the flush() IEx helper to print the messages from the port.
iex> port = Port.open({:spawn, "echo sup"}, [:binary])
#Port<0.1305>
iex> flush()
iex> {#Port<0.1305>, {:data, "sup\n"}}
iex> :ok
You can send a port any name of a binary you want to execute. For example, from the directory of my jekyll blog, I opened up an iex session, opened a port, and then sent the bundle install command, which installed all the Ruby gem dependencies. Here's a snippet of the output.
iex> port = Port.open({:spawn, "bundle install"}, [:binary])
#Port<0.1306>
iex> flush()
{#Port<0.1306>, {:data, "Using concurrent-ruby 1.0.5\n"}}
{#Port<0.1306>, {:data, "Using i18n 0.9.5\n"}}
{#Port<0.1306>, {:data, "Using minitest 5.11.3\n"}}
{#Port<0.1306>, {:data, "Using thread_safe 0.3.6\n"}}
{#Port<0.1306>, {:data, "Using tzinfo 1.2.5\n"}}
{#Port<0.1306>, {:data, "Using activesupport 4.2.10\n"}}
{#Port<0.1306>, {:data, "Using public_suffix 2.0.5\n"}}
{#Port<0.1306>, {:data, "Using addressable 2.5.2\n"}}
{#Port<0.1306>, {:data, "Using bundler 1.16.2\n"}}
{#Port<0.1306>, {:data, "Using coffee-script-source 1.11.1\n"}}
{#Port<0.1306>, {:data, "Using execjs 2.7.0\n"}}
{#Port<0.1306>,
{:data,
"Bundle complete! 4 Gemfile dependencies, 85 gems now installed.\nUse `bundle info [gemname]` to see where a bundled gem is installed.\n"}}
:ok
PID
A PID is a reference to a process. Whenever you spawn a new process, you'll get a new PID. Expect to talk a lot about PIDs. You'll probably need to hold onto PIDs so you can send different processes messages.
Here's an example of spawning a process and getting the PID back.
iex> pid = spawn fn -> IO.puts("hello world") end
iex> hello world
iex> #PID<0.123.0>
The process dies after it has done its job. PIDs and Ports warrant their own standalone post, but for now, I think it's sufficient to just be aware that they exist.
So, now that we've added our new types, this is our basic cheat sheet.
The Elixir Data Type Cheat Sheet
iex> 2 # integer
iex> 2.0 # floating point
iex> false # boolean
iex> 1..4 # range
iex> ~r/hello/ # regular expression
iex> :hello # atom
iex> "world" # string
iex> 'world' # charlist
iex> #Port<0.1306> # port
iex> #PID<0.123.0> # pid
The real challenge with Elixir in my opinion, though, is figuring out how to organize these basic data types into structures you can use. So let's take a look at the various collection types and why you would use each.
Collection Types
Here are the collection types you'll likely encounter:
- Tuples
- Lists
- Keyword Lists
- Maps
- Structs
You've probably heard those words before, at least in passing, but if you're accustomed to Ruby, you're probably wondering why you need all those extra types of collections. Let's investigate.
Tuples
Tuples are ordered collections of values. They look like this:
iex> {:hello, "world"}
iex> {1, 2}
iex> {:ok, "this is amazing!", 2}
# You can check if it's really a tuple
iex> tuple = { "hello", "world"}
iex> is_tuple tuple
iex> true
# and then you can get an element from a tuple by index
iex> elem(tuple, 1)
iex> "world"
I think tuples are a little wild. I mean, they look like they should be hashes, but they sort of behave like Ruby arrays. And then they're called tuples! It'll pay off to get familiar with them though, which is what I keep telling myself when I get confused for the hundredth time.
Tuples crop up all over the place in Elixir. Return values of functions are often tuples that you can pattern match on, so it makes sense to start seeing the world through tuples. Tuples usually have two to four elements, and at this point, they're my go-to data structure. When you're dealing with data structures that have more than four elements, that's probably a good case for using a map or struct instead.
Lists
Lists are linked data structures. They look like this:
iex> [1, 2, 3, 4]
iex> ["hello", "world"]
In Ruby, you'd think that was an array, but in Elixir, it's a list! Because lists are implemented as linked data structures, they're good for recursion, but bad for randomly retrieving an element or even figuring out the length because you'd need to traverse the whole list to figure out the size. To date, I've mostly been using tuples instead of lists. If you had to choose between them, I suppose you'd need to consider the expected size of the collection and what kind of operations you'll be performing on it.
Keyword Lists
To further complicate matters, there are also such things as keyword lists in Elixir. In essence, this is a list of two-value tuples.
# keyword list
iex> [ phrase: "oh hello", name: "tracy" ]
# is actually two-value tuples
iex> [ {:phrase, "oh hello"}, {:name, "tracy"} ]
This continues to baffle me, even though I am aware of its general existence.The cool thing about keyword lists is that you can have two of the same keys in a keyword list.
iex> keyword_list = [food: "peanut butter", food: "ice cream", flavor: "chocolate"] # a valid keyword list
Keyword lists are good for command-line parameters and options.
Maps
Next up are maps. If you wanted a real key-value store, not a list of key-values, this is what you're looking for. They look a bit like hashes in Ruby.
iex> %{"greeting" => "hello", "noun" => "world"}
iex> %{:greeting => "hello", :noun => "world"}
iex> %{greeting: "hello", noun: "world"} # if the keys are atoms, you can skip the hash rockets.
iex> greeting = %{spanish: "hola", chinese: "ni hao", english: "hello"}
iex> greeting[:spanish]
iex> "hola"
iex> greeting.chinese
iex> "ni hao"
Maps are good for passing associative data around, and pretty much everything else that is bigger than tuple size.
Structs
Structs are like enhanced maps. They permit only certain keys and those keys must be atoms. Structs need to be defined in modules with reasonable default values. They're maps with rules.
iex> defmodule IceCream do
.... defstruct flavor: "", quantity: 0
.... end
iex> chocolate = %IceCream{flavor: "chocolate"}
iex> chocolate.flavor
iex> "chocolate"
You'll see that the struct is defined with the same % percent symbol as a map, but it's followed by the name of the module. That's how I remind myself that they're just stricter maps.
Older versions of Elixir used to also include the HashDict to handle maps with more than a couple hundred values, but that module has been deprecated in favor of the good ol'fashioned Map.
That brings us to the end of the common data types and collection types you'll see in Elixir. Although there are a number of differences between the two languages, there are some similarities. Of course, there's plenty more to learn about Elixir, the conventions, and the cool things you can do, but this (I think) is a good start to getting familiar with the language. Hopefully this'll serve as a decent guide to deciphering any Elixir you might encounter soon!
The Elixir Collection Cheat Sheet
iex> {:ok, "this is amazing!", 2} # tuple
iex> ["hello", "world"] # list
iex> [ phrase: "oh hello", name: "tracy" ] # keyword list
iex> greeting = %{spanish: "hola", chinese: "ni hao", english: "hello"} # map
iex> chocolate = %IceCream{flavor: "chocolate"} # struct
Resources
- Basic Types on Elixir Lang
- Programming Elixir by Dave Thomas
- Hex Docs
Top comments (8)
Let me qualify my statement with semantically they’re equivalent. The fact that they’re not compiled to the same byte code doesn’t mean that mathematically a list is just a tuple of tuples.
I also clearly said most FP languages do this with syntax sugar. I’m honestly surprised Elixir doesn’t make this optimiztation. I’d be curious if putting the
nilfirst changes your results.I nearly had a heart attack when you declared lists and tuples basically the same thing.
In the Erlang VM, a tuple is always stored as a contiguous segment of memory – the item at index
nis stored at the memory address right after wheren-1ended. As such, it's generally very bad form to do a lot of modifying of tuple content and structure, as a good number of those operations will result in data being copied and collated at a new memory address.Contrast this to lists, which the Erlang VM keeps track of by storing a reference to the previous data point's address alongside the current data point. As such, not only is the length of a list dynamic, its addresses within RAM are, too (and the newest data is, paradoxically, usually at the leftmost side of the list).
tl;dr, Erlang doesn't make lists out of syntactic tuple sugar, because that would be madness
"Tuples are ordered collections of values." So how is this different to lists?
Essentially, they’re not different. Lists are actually just tupples of tupples.
[1,2,3]is really{1, { 2, {3, nil}}}Most, of not all FP languages represent lists this way, usually with a bit of syntactic sugar. This all comes from λ calculus, which is worthwhile to learn if you’re into FP.
Man, you know what I find really hard? No joke -- it's the syntax/writing differences between maps and structs and things with atom keys or string keys and accessing with a dot or brackets or whatever. I swear I always pick the wrong one and I just cannot keep straight in my head which is which and how to tell what I should be using.
Its easy, if the keys are atoms than with . :) If they are with String you cannot access with . In my head the two dots that are used for atom declaration are reminding me that i must use . while trying to access. One more thing worth mention is that if you try to access something with . and there is no such key it will give you error - key ... not found in .. but if you use strings as keys even the key is not there, it will just return nil it will not give you error
Like you article alot, thx for sharing !
Another very handy collection type, when you try to organize the basic data types into structures, are MapSet's.
Loved the way you elegantly mapped things back to Ruby. Will definitely recommend this as great reading material for Elixir-curious Rubyists!