Elixir provides multiple ways to handle key-value data structures, with two of the most common being Maps and Keyword Lists. While they may seem similar at first glance, they serve different purposes and have distinct characteristics that make them suitable for different use cases.

Maps (Map Module)

Key Characteristics:

• Defined using %{}.
• Keys can be any data type (atoms, strings, integers, etc.).
• Keys are unique—each key can only appear once.
• Provides fast key lookups, making it efficient for large datasets.
• Implements strict ordering when pattern matching.

Example Usage:

map = %{:a => 1, "b" => 2, 3 => "three"}
IO.inspect(Map.get(map, :a)) # Output: 1

Maps are ideal for structured data where each key should be unique and retrieval speed matters.

Keyword Lists (Keyword Module)

Key Characteristics:

• Defined using [key: value, key2: value2] syntax (or [{:key, value}] tuple format).
• Keys must be atoms.
• Keys are not unique—a keyword list can have duplicate keys.
• Maintains insertion order, making it useful for ordered parameters.
• Slower lookups compared to maps since it is implemented as a list.

Example Usage:

kw = [a: 1, b: 2, a: 3] # Duplicate :a key
IO.inspect(Keyword.get(kw, :a)) # Output: 1 (returns first occurrence)
IO.inspect(Keyword.get_values(kw, :a)) # Output: [1, 3]

Keyword lists are commonly used for passing options to functions in Elixir due to their ordering and ability to handle multiple entries for the same key.

When to Use Each?

Feature	Maps	Keyword Lists
Key Type	Any data type	Atoms only
Unique Keys	Yes	No
Lookup Speed	Fast (hash map)	Slower (list traversal)
Maintains Order	No	Yes
Best Use Case	General-purpose key-value storage	Function options, ordered data

Conclusion

• Use Maps when you need fast lookups, unique keys, and flexible key types.
• Use Keyword Lists when passing options to functions, preserving order, or handling duplicate keys.