5. Elixir: Immutable Data

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Immutable Data in Elixir

Notes

Accessing Map Values with Atoms vs. Strings

Suppose we have the following map defined in an iex session:

iex> conv = %{ method: "GET", path: "/wildthings" }

The keys are atoms, so to get the values associated with the keys, we can use the square-bracket syntax:

iex> conv[:method]
"GET"
 
iex>; conv[:path]
"/wildthings"

The square-bracket syntax works with all maps. However, when the keys are atoms, you can also use the dot notation:

iex> conv.method
"GET"
 
iex> conv.path
"/wildthings"

Now suppose we use strings instead of atoms as the map keys:

iex> conv = %{ "method" => "GET", "path" => "/wildthings" }

To access the same values as before, we must use strings with the square-bracket syntax:

iex> conv["method"]
"GET"
 
iex> conv["path"]
"/wildthings"

Trying to use an atom returns nil because no such key exists:

iex> conv[:method]
nil

It’s also worth noting that the dot notation only works for keys that are atoms:

iex> conv.method
** (KeyError) key :method not found in: %{"method" => "GET", "path" => "/wildthings"}

Gotcha: String Lengths

A sequence of bytes in Elixir is referred to as a binary. You rarely need to work at the byte level, but it’s important to understand that double-quoted Elixir strings are represented internally as a sequence of bytes. Thus, double-quoted strings are binaries. Here’s one reason that’s important…

In an iex session, suppose we have a string variable representing a response body:

iex> resp_body = "Bears, Lions, Tigers"
"Bears, Lions, Tigers"

In the video, we got the length of the string using the String.length/1 function:

iex> String.length(resp_body)
20

Since the string is a binary, we can get the number of bytes in the string using the built-in byte_size/1 function:

iex> byte_size(resp_body)
20

No surprise, both functions return the same number: 20.

But now suppose we change “Lions” to “Liöns” (notice the umlaut):

iex> resp_body = "Bears, Liöns, Tigers"
"Bears, Liöns, Tigers"

The length of the string is still 20:

iex> String.length(resp_body)
20

But the number of bytes has increased from 20 to 21:

iex> byte_size(resp_body)
21

Why? Well, a string is a UTF-8 encoded binary, and the character “ö” takes 2 bytes to be represented in UTF-8. So even though the string has 20 characters, the number of bytes in that string is 21.

Now, this isn’t a problem for the response body strings we’ll use in the videos. However, using String.length/1 to set the value of the Content-Length header in the response won’t be correct for all possible UTF-8 encoded strings. The Content-Length header must indicate the size of the body in bytes. Therefore, it’s better to use the byte_size/1 function to get the number of bytes:

"""
Content-Length: #{byte_size(conv.resp_body)}
"""

wööt!

Code So Far

The code for this section is in the route-response directory found within the video-code directory of the code bundle

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