What is a parser?* yacc/lex * AST (Abstract Syntax Tree) vs. CST (Concrete Syntax Tree) * JSON decoding vs. parsing * JSON decoding is validating a data structure that has already been parsed. Assumes a valid structure. * elm/parser * Haskell parsec library - initially used for the Elm compiler, now uses custom parser

What is a parser?* One character at a time * Takes input string, turns it into structued data (or error)

Comitting* Backtrackable parsers * chompIf and chompWhile * Parser.oneOf

Benchmarking* elm-explorations/benchmark * Benchmark before making assumptions about where performance bottlenecks are * Write unit tests for your parser * GFM table parsing live stream * Parser.succeed

Elm regex vs elm parserIndications that you might be better off with parser

• Lots of regex capture groups
• Want very precise error messages

Getting source code locations* Parser.getRow and getCol * Parser.getSource

Parser.loop* Loop docs in elm/parser * Looping allows you to track state and parse groups of expressions * Loop over repeated expression type, tell it termination condition with Step type (Loop and Done)

Error Messages* You can fail with Parser.problem * Parser.Advanced module is designed to give you more precise context and error messages on failure * Parser.Advanced.inContext

Getting Started with a Parser Project* Write lots of unit tests with elm-test!

There's likely a specification doc if you're parsing a language or formal syntax

• CommonMark Spec
• GitHub-Flavored Markdown Spec
• dillonkearns/elm-markdown test results directory from executing spec examples

Look at examples of parser projects

• dillonkearns/elm-markdown
• elm-in-elm parser
• elm-in-elm conference talk
• mdgriffith/elm-markup - good reference for parsing, fault-tolerant parsing, and giving nice error messages
• Tereza's YAML parser
• Tereza's elm conf talk "Demystifying Parsers"
• Jeroen's elm/parser Ellie
• "It's not hacking if you have tests."

Look at elm/parser docs and resources

• elm/parser project's semantics document describes backtrackable behavior in detail