servy

An HTTP Server using Elixir + Erlang/OTP

Question:

• Is 'world' same as "world"? Seems like it when passing it to hello method but passing it to String.reverse/1 throws an error 🤔.

Notes

Installation & Setup

• brew update && brew install erlang && brew install elixir
• check version: elixir --version
• IntelliJ plugin - https://github.com/KronicDeth/intellij-elixir

mix

• create a new mix project: mix new servy

iex

• To start an iex session: iex
• To compile a file in memory within an iex session: c lib/servy.ex
• To start an iex session with just a file: iex lib/servy.ex
• To start an iex in the context of a mix project: iex -S mix
• To recompile a module within an iex session: r Servy
• To get help within an iex session: h <enter> you can also do h String. to get help on String module

Shell History

• iex doesn't remember commands from the previous sessions by default unless enabled. Add the following line to your .zshrc or .bashrc: export ERL_AFLAGS="-kernel shell_history enabled"

Pattern Matching

= operator is not an assignment operator like in other programming languages but is a match operator 🤯

a = 1
> 1
a
> 1
1 = a
> 1 🤯

Elixir tries to match left with right with the = operator, if it does then it returns the matching value. So when we do 1 = a the value of a was already assigned to 1 previously, 1 = a matches both values, hence it returns 1. If you do 2 = a then it returns no match error as value of a is 1 not 2.

You can reassign a by doing a = 2 and now if you do 2 = a then it returns 2. If you only want to check for a match instead of assigning, you can use a pen ^ operator like so ^a=3 This returns no match error as a's value is currently 2 not 3.

Destructure using Pattern Matching

[first, 2, last] = [1,2,3]
first
> 1
last
> 3

Destructuring should match the values on left hand side with term on the right unless the values on the left hand side are replaced with variables in which case variables are mapped to values instead of matching (given the whole thing) matches in the first place. Also, the arrity on the both sides should match. So, [first, last] = [1, 2, 3] will return no match error.

If you want to match with anything but don't want to assign it to a variable, you can use an underscore _: [first, _, last] = [1, 2, 3] In this case first is assigned 1, last is assigned 3. You cannot use _ as a variable.

• A term is a value of any data type: a string, an atom, a map, a list, etc.

• Elixir atoms are prefixed by a colon. Here's a map with atoms as keys: %{:method => "GET", :path => "/wildthings"

Which can be rewritten in a more concise way by: %{method: "GET", path: "/wildthings"}

This concise way is only allowed if keys are atoms otherwise you have to use the general => form: %{ "method" => "GET", "path" => "/wildthings" }

Map

Given conv = %{method: "GET", path: "/wildthings", resp_body: ""}, you can get the values method or path like so: conv[:method] or conv[:path]. You can also use .dot syntax such as conv.method or conv.path Dot syntax is more strict throwing an error if key doesn't exist whereas [] returns nil.

Elixir data structures is immutable so conv[:resp_body] = "check" won't work. Even without immutability, if you think about it, this doesn't make that much sense as = operator is a match operator. You can instead need to: Map.put(conv, "resp_body", "Bears") this creates a new map.

More concise way: conv = %{ conv | resp_body: "Bears, Lions, Tigers" } Using the concise way, you can only modify a value cannot add a new one

Strings

Double-quoted strings are binaries. String.length returns the length and for ascii characters byte_size returns the length as well but it is actually the number of bytes. So, for non-ascii characters, it will be different:

• String.length("Lions") > 5
• byte_size("Lions") > 5
• String.length("Liöns") > 5
• byte_size("Liöns") > 6