Functional Patterns & Metaprogramming

Enum and Stream

Enum processes collections eagerly — all at once. Stream processes lazily — one element at a time, on demand. Enum.map/filter/reduce/sort/group_by cover most data processing needs. For large collections or infinite streams, Stream.map/filter/take work the same way but only pull elements when needed. 'Stream.cycle([1,2,3]) |> Enum.take(10)' produces [1,2,3,1,2,3,1,2,3,1] without ever creating an infinite list in memory.

Protocols: Polymorphism

A Protocol defines an interface that different data types implement. Protocol.derive/2 can auto-derive implementations. Built-in protocols: Enumerable (makes a type work with Enum), Collectable (can receive elements), Inspect (custom iex display), String.Chars (to_string conversion). Define: 'defprotocol Serializable do; def serialize(term); end'. Implement: 'defimpl Serializable, for: MyStruct do...'.

Macros

Elixir macros run at compile time and generate AST nodes. defmacro defines a macro. quote/2 captures code as AST. unquote/1 injects values into quoted expressions. Macros are why Elixir can implement features like defmodule, def, if, and use as library code, not built-in syntax. Rule: use functions when possible; reach for macros only when you need to transform syntax or generate repetitive code.

# Enum vs Stream comparison
# Enum: eager, processes all at once
1..1_000_000
|> Enum.map(&(&1 * 2))
|> Enum.filter(&(rem(&1, 3) == 0))
|> Enum.take(5)
# Creates 1M intermediate list

# Stream: lazy, only computes what's needed
1..1_000_000
|> Stream.map(&(&1 * 2))
|> Stream.filter(&(rem(&1, 3) == 0))
|> Enum.take(5)  # Triggers evaluation
# [6, 12, 18, 24, 30] -- O(n) not O(n^2)

# Protocol example
defprotocol Area do
  def calculate(shape)
end

defimpl Area, for: %{type: :circle} do
  def calculate(%{r: r}), do: :math.pi() * r * r
end

# Macro example: unless (inverse of if)
defmacro unless(condition, do: block) do
  quote do
    if !unquote(condition), do: unquote(block)
  end
end

unless 1 == 2 do
  IO.puts "1 is not 2"
end