Lykn

The IR: Markdown as type Variants

Start with the data. What does a parsed Markdown document look like in memory?

(type Block
  (Heading :number level :string text)
  (Paragraph :string text)
  (CodeBlock :string language :string code)
  (ListBlock :array items)
  (Blockquote :string text)
  (Empty))

A document is a list of Block values. Each variant represents one structural element. Inline formatting — bold, italic, code, links — happens during rendering, not parsing.

This is the type-driven design from Chapter 10. The IR is a closed set of variants. match on a Block is exhaustive — the compiler tells you if you forgot a case. Add a Table variant later and every match in the codebase must handle it before the code compiles.

This is why Lykn has type and match. A real program has data with known shapes. Making those shapes explicit lets the compiler help.