Logos for Crystal

A fast lexer generator for Crystal, ported from Rust's Logos library.

Overview

Logos allows you to create fast lexers by defining tokens in a Logos.define block. The library generates an optimized lexer at compile time with zero runtime overhead.

This is a Crystal port of the Rust Logos library, aiming to provide similar functionality and performance characteristics while following Crystal idioms.

Features

• Zero-copy parsing - Work with slices of your input
• Built-in error recovery - Skip invalid tokens and continue parsing
• Token disambiguation - Automatic priority resolution for overlapping patterns
• UTF-8 safe - Proper handling of Unicode boundaries
• No runtime dependencies - Pure Crystal implementation
• Fast compile times - Minimal macro overhead

Installation

Add this to your shard.yml:

dependencies:
  logos:
    github: dsisnero/logos

Then run:

shards install

Quick Start

require "logos"

Logos.define Token do
  error_type Nil

  token "fn", :KeywordFn
  token "let", :KeywordLet

  regex "[a-zA-Z_][a-zA-Z0-9_]*", :Identifier
  regex "[0-9]+", :Number

  token "+", :Plus
  token "-", :Minus

  skip_regex "\\s+", :Whitespace
end

lexer = Token.lexer("fn hello = 42")

loop do
  result = lexer.next
  break if result.is_a?(Iterator::Stop)
  result = result.as(Logos::Result(Token, Nil))
  puts "#{result.unwrap}: #{lexer.slice}"
end

Context-Dependent Lexing

Use Lexer#morph to switch token modes while preserving cursor position.

require "logos"

Logos.define OuterToken do
  token "{", :Open
  regex "[^\\{]+", :Text
end

Logos.define InnerToken do
  token "}", :Close
  regex "[^\\}]+", :Body
end

outer = OuterToken.lexer("prefix{inside}suffix")

token = outer.next.as(Logos::Result(OuterToken, Nil))
token.unwrap # => :Text

token = outer.next.as(Logos::Result(OuterToken, Nil))
if token.unwrap == OuterToken::Open
  inner = outer.morph(InnerToken)
  inner_token = inner.next.as(Logos::Result(InnerToken, Nil))
  puts inner_token.unwrap # => InnerToken::Body
end

Use #spanned when you need (token, span) tuples:

lexer = OuterToken.lexer("ab{cd}")
lexer.spanned.each do |result, span|
  puts "#{result.unwrap} @ #{span}"
end

Token Disambiguation and Priority

When multiple patterns can match at the same position:

• Logos prefers the longest match.
• If multiple matches have the same length, higher priority wins.
• If same-length and same-priority patterns overlap, Logos raises a compile-time diagnostic.

Example:

Logos.define Token do
  token "===", :StrictEq
  token "==", :Eq
  token "=", :Assign
  regex "[a-zA-Z_][a-zA-Z0-9_]*", :Ident
end

Explicit priorities:

Logos.define Token do
  regex "[a-z]+", :Word, priority: 10
  token "if", :If, priority: 50
end

If you hit ambiguity diagnostics, either:

• Raise priority for the intended winner, or
• Refine regex patterns so they no longer overlap at equal priority.

Annotation-based API

For a Rust-style attribute-driven setup, use type-level annotations and logos_derive:

require "logos"

@[Logos::Options(skip: "\\s+", error: Nil)]
@[Logos::Subpattern("xdigit", "[0-9a-fA-F]")]
@[Logos::Token(:KeywordLet, "let")]
@[Logos::Regex(:Hex, "0x(?&xdigit)+")]
@[Logos::Regex(:Number, "[0-9]+")]
enum Token
  KeywordLet
  Hex
  Number
end

logos_derive(Token)

lexer = Token.lexer("let 0x10 42")

Notes:

• Crystal cannot introspect per-enum-variant annotations the same way Rust proc-macros do, so mappings are declared at the enum type level.
• Logos::Token and Logos::Regex support both (:Variant, "pattern") and (pattern, variant: :Variant) forms.

Subpatterns

Logos.define Token do
  error_type Nil

  subpattern :xdigit, "[0-9a-fA-F]"
  regex "0[xX](?&xdigit)+", :Hex
end

Examples

Crystal ports of the Rust Logos examples are available in examples/:

• examples/brainfuck.cr
• examples/calculator.cr
• examples/custom_error.cr
• examples/extras.cr
• examples/json.cr
• examples/json_borrowed.cr
• examples/string_interpolation.cr
• examples/token_values.cr

Rust Handbook Parity Index

Reference mapping from Rust handbook topics to Crystal docs/spec coverage:

• Getting started: this README (Quick Start) and spec/logos/simple_spec.cr
• Attributes / derive: this README (Annotation-based API) and spec/logos/derive_spec.cr
• Callbacks: spec/logos/callbacks_spec.cr
• Extras: examples/extras.cr and spec/logos/custom_error_spec.cr
• Common regex patterns: spec/logos/advanced_spec.cr, spec/logos/properties_spec.cr
• Context-dependent lexing: this README (Context-Dependent Lexing) and spec/logos/lexer_modes_spec.cr
• Token disambiguation: this README (Token Disambiguation and Priority) and spec/logos/old_logos_bugs_spec.cr
• Unicode support: spec/logos/unicode_dot_spec.cr, spec/logos/ignore_case_spec.cr
• Source and spans: spec/logos/source_spec.cr, spec/logos/lexer_spec.cr

Enum Payload Parity Plan (logos-04d)

Rust uses enum variants with associated payloads. Crystal enums do not, so the port currently uses Lexer#callback_value_as(T) as a side-channel.

Planned parity path:

• logos-nxw (done): Design union-backed typed payload API.
• logos-15n: Implement typed payload extraction helpers on Lexer/Result.
• logos-pk9: Port Rust payload-oriented examples to concise Crystal equivalents.

Rust UI Error Parity (tests/ui/err)

Status

✅ Port Complete: The Crystal port is feature-complete and the full spec suite passes.

Current Progress

• Source abstraction (String and Slice(UInt8))
• Basic lexer structure and state machine
• Pattern AST and parsing
• Result types and error handling
• Regex pattern compilation
• NFA/DFA construction
• Code generation
• Full test suite

Dependencies

The project includes two companion shards ported from Rust:

• regex-syntax - Regular expression parser
• regex-automata - Automata construction library

Hybrid Automaton Status

• Regex::Automata::Hybrid::LazyDFA now performs lazy determinization and caches transitions on demand.
• It supports anchored/unanchored start states and reuses the same look-around semantics as the DFA builder.
• This hybrid path is suitable for large regex sets where eager DFA construction has higher startup cost.

Development

# Install dependencies
make install

# Run tests
make test

# Format code
crystal tool format --check

# Lint code
make lint

Contributing

1. Fork it (https://github.com/dsisnero/logos/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

License

MIT - see LICENSE file

Acknowledgments

• maciejhirsz/logos - The original Rust implementation
• BurntSushi/regex-automata - Rust regex engine used as reference
• rust-lang/regex - Rust regex library used as reference