006: elm/parser

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 parser

Indications 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