Question 1

How do I use Expat to simplify Phoenix controller pattern matching?

Accepted Answer

Define named patterns for large parameter maps or structs in a separate module using defpat, then import them into your controllers to match and extract data in function heads, keeping actions concise. For example, create a pattern for user parameters and use it directly in action definitions.

Question 2

What's the difference between Expat and plain Elixir pattern matching?

Accepted Answer

Plain Elixir pattern matching is inline and static, while Expat allows you to abstract patterns into reusable, named components that can be composed and guarded. This improves code organization but adds a layer of macro-based abstraction that may affect compilation.

Question 3

How to add conditional logic with guards in Expat patterns?

Accepted Answer

Use the 'when' clause in defpat definitions, like 'defpat adult(%{age: age}) when is_integer(age) and age >= 18'. Guards are expanded at call-sites, ensuring conditional matching works correctly in functions or case statements.

Question 4

Can Expat patterns be used in Elixir's case statements?

Accepted Answer

Yes, Expat patterns integrate seamlessly with case, fn, and other constructs by using the expat keyword to expand patterns within these expressions. The README shows examples in the Voting module where patterns are used in function definitions and case-like logic.

Question 5

What are union patterns and when should I use them?

Accepted Answer

Union patterns let you compose multiple named patterns into a single pattern, enabling algebraic data type-like constructs such as Maybe or Either. Use them to match over varied data shapes with a unified interface, as demonstrated in the test files for natural numbers or result types.

Question 6

How does documentation for Expat patterns work in IEx?

Accepted Answer

Pattern macros generated by Expat include built-in documentation detailing variables and expansions. In IEx, you can view this by running commands like 'h MyPatterns.ok', which helps understand pattern usage without manual lookup.

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