Twenty years of Pandoc

Sources: primary, discussion
Summary: John MacFarlane traces Pandoc from roughly 3000 lines of Haskell to 51 input and 76 output formats, crediting the central AST with readers and writers on either side. The post also explains why the author of the CommonMark spec still ships a pre-CommonMark Markdown parser.
Why it matters: The design rationale is a concrete case for an intermediate representation over pairwise converters, which is the same choice any format-conversion tool faces.