Elixir Core Usage Rules

Pattern Matching

• •Use pattern matching over conditional logic when possible
• •Prefer to match on function heads instead of using if/else or case in function bodies
• •%{} matches ANY map, not just empty maps. Use map_size(map) == 0 guard to check for truly empty maps

Error Handling

• •Use {:ok, result} and {:error, reason} tuples for operations that can fail
• •Avoid raising exceptions for control flow
• •Use with for chaining operations that return {:ok, _} or {:error, _}

Common Mistakes to Avoid

• •Elixir has no return statement, nor early returns. The last expression in a block is always returned.
• •Don't use Enum functions on large collections when Stream is more appropriate
• •Avoid nested case statements - refactor to a single case, with or separate functions
• •Don't use String.to_atom/1 on user input (memory leak risk)
• •Lists and enumerables cannot be indexed with brackets. Use pattern matching or Enum functions
• •Prefer Enum functions like Enum.reduce over recursion
• •When recursion is necessary, prefer to use pattern matching in function heads for base case detection
• •Using the process dictionary is typically a sign of unidiomatic code
• •Only use macros if explicitly requested
• •There are many useful standard library functions, prefer to use them where possible

Function Design

• •Use guard clauses: when is_binary(name) and byte_size(name) > 0
• •Prefer multiple function clauses over complex conditional logic
• •Name functions descriptively: calculate_total_price/2 not calc/2
• •Predicate function names should not start with is and should end in a question mark.
• •Names like is_thing should be reserved for guards

Data Structures

• •Use structs over maps when the shape is known: defstruct [:name, :age]
• •Prefer keyword lists for options: [timeout: 5000, retries: 3]
• •Use maps for dynamic key-value data
• •Prefer to prepend to lists [new | list] not list ++ [new]

Mix Tasks

• •Use mix help to list available mix tasks
• •Use mix help task_name to get docs for an individual task
• •Read the docs and options fully before using tasks

Testing

• •Run tests in a specific file with mix test test/my_test.exs and a specific test with the line number mix test path/to/test.exs:123
• •Limit the number of failed tests with mix test --max-failures n
• •Use @tag to tag specific tests, and mix test --only tag to run only those tests
• •Use assert_raise for testing expected exceptions: assert_raise ArgumentError, fn -> invalid_function() end
• •Use mix help test to for full documentation on running tests

Debugging

• •Use dbg/1 to print values while debugging. This will display the formatted value and other relevant information in the console.