Elixir basics · Lesson 07 (open contents)

01 · Let the code speak 02 · Meet six kinds of values 03 · Put values together 04 · Build a new list 05 · Make the shapes match 06 · Choose with case 07 · Parse a number safely 08 · Unpack &1 and the pipe 09 · Put code in a project 10 · Ask precise true-or-false questions 11 · Read and update nested data 12 · Count visible text and bytes 13 · Let function clauses choose 14 · Solve a list twice 15 · Choose the smallest clear control form 16 · Give domain data a shape 17 · Put one tested module in Mix 18 · Make success and failure predictable 19 · Read and write one real file 20 · Take only the data you need 21 · Give different data one shared action 22 · Write a callback contract 23 · Write down the data contract 24 · Test behavior and boundaries 25 · Turn a project into a command 26 · Write the project promise 27 · Parse one real line 28 · Keep the public API small 29 · Make the promise executable 30 · Package the edge and leave clues 31 · Prove the project is done

Home/From Scratch/Elixir/Lesson 07

ELIXIR · SCRATCH · LESSON 0730 minutes

Parse a number safely

Trim text, turn it into an integer, and return a clear error for invalid input.

01 · Start with the whole example

Parse valid text and catch invalid text

String.trim/1 names a function in documentation. String.trim(text) calls it with one argument.

Elixir

# Bind a complete parser to parse_integer
parse_integer = fn text ->
  try do
    clean = String.trim(text)
    {:ok, String.to_integer(clean)}
  rescue
    ArgumentError -> {:error, :not_an_integer}
  end
end

# Run both paths
{parse_integer.(" 42 "), parse_integer.("not-a-number")}

Check the result first

The valid input is trimmed to "42", parsed, and returned as {:ok, 42}.
The invalid input raises ArgumentError inside the try block.
The rescue clause turns that exception into {:error, :not_an_integer}.

02 · Take the code apart

Read from the first line down

String.trim/1 and String.to_integer/1 are documentation names. Calls use parentheses and do not include /1.
String.to_integer(clean) returns an integer for valid text and raises ArgumentError for invalid text.
The rescue clause handles that expected exception and leaves unrelated exceptions visible.
Both paths return tagged tuples: {:ok, integer} or {:error, :not_an_integer}.

03 · Meet the new symbols

Symbols are not secret signs

String.trim/1

The name-and-arity form used in documentation: module String, function trim, one argument.

String.to_integer/1

A one-argument function that parses integer text or raises ArgumentError.

fn text -> ... end

Create a full anonymous function whose argument is called text.

rescue ArgumentError ->

Handle only an ArgumentError raised inside the try block.

04 · Ideas inside the code

Match each name to its meaning

arity

The number of arguments a function takes. /1 says that both String.trim/1 and String.to_integer/1 take one argument.

anonymous function

A function written with fn ... end and bound to a name such as parse_integer.

try/rescue

Run code that may raise, then turn a named exception into a normal result.

05 · Make the idea clear

Why these forms are useful

String.trim/1 removes whitespace from both ends of a string. String.to_integer/1 turns clean integer text into an integer, but raises ArgumentError when the text is invalid.

An anonymous function lets us keep those steps together. Bind it to parse_integer, then call it with a dot: parse_integer.(text).

try/rescue handles the expected conversion error. The parser returns a tagged tuple for both paths, so the caller gets data instead of a crash.

DESIGN DECISION

Why this shape?

Why put `/1` after a function name?

Choice

BEAM identifies an entry point by module, function name, and argument count. trim/1 means the trim function that takes one argument.

Cost and boundary

The same name with another argument count is another function. Default arguments generate arities at compile time; they do not make calls loosely sized.

A FAMILIAR POINT OF VIEW

Coming from Java, Python, or JavaScript

Java

Familiar starting point: Methods can overload by parameter types and count.
What BEAM changes: BEAM first finds a name and arity, then clauses can match data shapes.
False friend: run/1 and run/2 are distinct entry points.

Python

Familiar starting point: One function often uses defaults or *args.
What BEAM changes: Arity is part of a function's identity and appears in exports and docs.
False friend: Leaving out an argument does not discover another arity automatically.

JavaScript

Familiar starting point: Functions often tolerate missing or extra arguments.
What BEAM changes: A BEAM call must find the exact arity.
False friend: &1 is a capture placeholder; it is not another spelling of /1.

06 · Change it yourself

Close the answer and try

Complete safe_number so " 7 " returns {:ok, 7} and "seven" returns {:error, :not_an_integer}.

Practice starting point

safe_number = fn text ->
  try do
    clean = String.____(text)
    {:ok, String.____(clean)}
  rescue
    ____ -> {:error, :not_an_integer}
  end
end

{safe_number.(" 7 "), safe_number.("seven")}

Target result: The result should be {{:ok, 7}, {:error, :not_an_integer}}.

Stuck? Read one hint

Use trim, to_integer, and the exception name ArgumentError.

After you run it, see one answer

One answer

# Handle only the conversion error we expect
safe_number = fn text ->
  try do
    clean = String.trim(text)
    {:ok, String.to_integer(clean)}
  rescue
    ArgumentError -> {:error, :not_an_integer}
  end
end

{safe_number.(" 7 "), safe_number.("seven")}

Think about it: Why does the rescue clause name `ArgumentError` instead of catching every exception?

The parser expects one conversion error. Handling only ArgumentError keeps unrelated mistakes visible so they can be fixed.

Take with you

Remember these three lines

1
Module.function/arity is a documentation name; Module.function(argument) is a call.
2
Bind a full anonymous function with fn ... end and call it with .().
3
Rescue the expected conversion error and return a tagged result.

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