Keep the public API small

Separate file access, parsing, and pure summary logic.

01 · Start with the whole example

Run this first

Run it once, change one input, and compare the new result.

Elixir

# Expose a small parser and one summary call
defmodule Cleaner do
  @levels %{"INFO" => :info, "WARN" => :warn, "ERROR" => :error}

  def parse_line(line) do
    case String.split(String.trim(line), ~r/\s+/, parts: 2) do
      [""] -> :skip
      [level, message] ->
        case Map.fetch(@levels, level) do
          {:ok, tag} -> {:ok, {tag, message}}
          :error -> {:error, :invalid_line}
        end
      _ -> {:error, :invalid_line}
    end
  end

  def run(records), do: {:ok, count(records)}
  defp count(records), do: Enum.frequencies_by(records, fn {level, _message} -> level end)
end

Cleaner.run([{:info, "ready"}, {:warn, "hot"}, {:info, "done"}])

Check the result first

The API returns {:ok, %{info: 2, warn: 1}}.

02 · Take the code apart

Read from the first line down

Read def run/1: Expose one project entry point.
Read defp: Keep an implementation helper private.
Read Enum.reduce/3: Build the summary in a pure fold.

03 · Meet the new symbols

Symbols are not secret signs

def run/1

Expose one project entry point.

defp

Keep an implementation helper private.

Enum.reduce/3

Build the summary in a pure fold.

04 · Ideas inside the code

Match each name to its meaning

public API

A small public API gives callers fewer contracts to learn and protects internal changes.

pure core

Pure functions transform supplied data without opening files or changing outside state.

boundary module

A boundary module performs IO and converts its errors into project results.

05 · Make the idea clear

Why these forms are useful

Run the example first. Predict one result, then change one input and run it again.

A small public API gives callers fewer contracts to learn and protects internal changes.

Pure functions transform supplied data without opening files or changing outside state.

06 · Change it yourself

Close the answer and try

Add a public total/1 that returns the number of records.

Practice starting point

def total(records), do: ____

Target result: Return the list length.

Stuck? Read one hint

Use length/1.

After you run it, see one answer

One answer

def total(records), do: length(records)

Think about it: Should callers invoke `count/1`?

No. defp keeps it private so the module can change its implementation.

Take with you

Remember these three lines

1
Expose a narrow API.
2
Keep transformations pure.
3
Put IO and error conversion at boundaries.

Lesson completeRun the code once, then mark this lesson as done.