Module

lexers._state_machine

Base class for hand-written state machine lexers.

Thread-safe, O(n) guaranteed, zero regex.

Design Philosophy:

Rosettes lexers are hand-written state machines rather than regex-based.

This design provides:

  1. Security: No ReDoS vulnerability. Crafted input cannot cause exponential backtracking because there IS no backtracking.

  2. Performance: O(n) guaranteed. Single pass, character by character. Predictable performance regardless of input.

  3. Thread-Safety: Each tokenize() call uses only local variables. No shared mutable state means true parallelism on Python 3.14t.

  4. Debuggability: Explicit state transitions are easier to trace than regex match failures.

Architecture:

StateMachineLexer provides:

  • Base class with shared character sets (DIGITS, IDENT_START, etc.)
  • Defaulttokenize_fast()implementation
  • Protocol-compatible interface

Helper functions provide common patterns:

  • scan_while(): Advance while chars match set
  • scan_until(): Advance until char in set
  • scan_string(): Handle quoted strings with escapes
  • scan_triple_string(): Handle triple-quoted strings
  • scan_line_comment(): Scan to end of line
  • scan_block_comment(): Scan to end marker

Adding New Languages:

To add a new language lexer:

  1. Createrosettes/lexers/{language}_sm.py
  2. Subclass StateMachineLexer
  3. Set name, aliases, filenames, mimetypes class attributes
  4. Implementtokenize()method with character-by-character logic
  5. Add entry to_LEXER_SPECSinrosettes/_registry.py
  6. Add tests intests/lexers/test_{language}_sm.py

Example skeleton:

class MyLangStateMachineLexer(StateMachineLexer):
    name = "mylang"
    aliases = ("ml",)
    filenames = ("*.ml",)
    mimetypes = ("text/x-mylang",)

    # Language-specific character sets
    KEYWORDS = frozenset({"if", "else", "while"})

    def tokenize(self, code, config=None, start=0, end=None):
        # Your tokenization logic here
        ...

Key rules:

  • Use only local variables (noself.statemutations)
  • Yield tokens as you find them (streaming)
  • Handle all characters (emit TEXT for unknown)
  • Use helper functions for common patterns

Performance Tips:

  • Use frozenset for keyword/operator lookups (O(1))
  • Usescan_while/scan_untilhelpers for common patterns
  • Avoid string slicing in hot loops (use start/end indices)
  • Pre-compute character sets as class attributes

See Also:

  • rosettes/_protocol.Lexer: Protocol that all lexers must satisfy
  • rosettes/_registry: How lexers are registered and looked up
  • rosettes/lexers/python_sm.py: Reference implementation
1Class6Functions

Classes

StateMachineLexer 14 ▼
Base class for hand-written state machine lexers. Thread-safe: `tokenize()` uses only local variab…

Base class for hand-written state machine lexers.

Thread-safe:tokenize()uses only local variables. O(n) guaranteed: single pass, no backtracking.

Subclasses implement language-specific tokenization by overriding thetokenize()method with character-by-character logic.

Design Principles:

  1. No regex — character matching only
  2. Explicit state — no hidden backtracking
  3. Local variables only — thread-safe by design
  4. Single pass — O(n) guaranteed

Class Attributes:

  • name: Canonical language name (e.g., 'python')
  • aliases: Alternative names for registry lookup (e.g., ('py', 'python3'))
  • filenames: Glob patterns for file detection (e.g., ('*.py',))
  • mimetypes: MIME types for content detection

Shared Character Sets:

  • DIGITS: '0'-'9'
  • HEX_DIGITS: '0'-'9', 'a'-'f', 'A'-'F'
  • LETTERS: 'a'-'z', 'A'-'Z'
  • IDENT_START: Letters + '_'
  • IDENT_CONT: IDENT_START + digits
  • WHITESPACE: Space, tab, newline, etc.

Example Implementation:

class MyLangLexer(StateMachineLexer):
    name = "mylang"
    aliases = ("ml",)
    KEYWORDS = frozenset({"if", "else"})

    def tokenize(self, code, config=None, start=0, end=None):
        pos = start
        end = end or len(code)
        line, col = 1, 1

        while pos < end:
            char = code[pos]
            # ... tokenization logic ...
            yield Token(TokenType.TEXT, char, line, col)
            pos += 1
            col += 1

Common Mistakes:

# ❌ WRONG: Storing state in instance variables
self.current_line = 1  # NOT thread-safe!

# ✅ CORRECT: Use local variables
line = 1

# ❌ WRONG: Using regex for matching
match = re.match(r'\d+', code[pos:])  # ReDoS vulnerable!

# ✅ CORRECT: Use scan_while helper
end_pos = scan_while(code, pos, self.DIGITS)

Attributes

Name Type Description
name str —
aliases tuple[str, ...] —
filenames tuple[str, ...] —
mimetypes tuple[str, ...] —
DIGITS frozenset[str] —
HEX_DIGITS frozenset[str] —
OCTAL_DIGITS frozenset[str] —
BINARY_DIGITS frozenset[str] —
LETTERS frozenset[str] —
IDENT_START frozenset[str] —
IDENT_CONT frozenset[str] —
WHITESPACE frozenset[str] —

Methods

tokenize 4 Iterator[Token] ▼
Tokenize source code. Subclasses override this with language-specific logic.
def tokenize(self, code: str, config: LexerConfig | None = None, start: int = 0, end: int | None = None) -> Iterator[Token]
Parameters
Name Type Description
code —

The source code to tokenize.
config —

Optional lexer configuration.

Default:None
start —

Starting index in the source string.

Default:0
end —

Optional ending index in the source string.

Default:None
Returns
Iterator[Token]
tokenize_fast 3 Iterator[tuple[TokenType… ▼
Fast tokenization without position tracking. Default implementation strips pos…
def tokenize_fast(self, code: str, start: int = 0, end: int | None = None) -> Iterator[tuple[TokenType, str]]

Fast tokenization without position tracking.

Default implementation strips position info from tokenize(). Subclasses may override for further optimization.

Parameters
Name Type Description
code —

The source code to tokenize.
start —

Starting index in the source string.

Default:0
end —

Optional ending index in the source string.

Default:None
Returns
Iterator[tuple[TokenType, str]]

Functions

scan_while 3 int ▼
Advance position while characters are in char_set.
def scan_while(code: str, pos: int, char_set: frozenset[str]) -> int
Parameters
Name Type Description
code str

Source code string.
pos int

Starting position.
char_set frozenset[str]

Set of characters to match.
Returns
int
scan_until 3 int ▼
Advance position until a character in char_set is found.
def scan_until(code: str, pos: int, char_set: frozenset[str]) -> int
Parameters
Name Type Description
code str

Source code string.
pos int

Starting position.
char_set frozenset[str]

Set of characters to stop at.
Returns
int
scan_string 3 int ▼
Scan a string literal, handling escapes.
def scan_string(code: str, pos: int, quote: str) -> int
Parameters
Name Type Description
code str

Source code.
pos int

Position after opening quote.
quote str

The quote character (' or ").
Returns
int
scan_triple_string 3 int ▼
Scan a triple-quoted string.
def scan_triple_string(code: str, pos: int, quote: str) -> int
Parameters
Name Type Description
code str

Source code.
pos int

Position after opening triple quote.
quote str

The quote character (' or ").
Returns
int
scan_line_comment 2 int ▼
Scan to end of line (for line comments).
def scan_line_comment(code: str, pos: int) -> int
Parameters
Name Type Description
code str

Source code.
pos int

Starting position (after comment marker).
Returns
int
scan_block_comment 3 int ▼
Scan a block comment until end marker.
def scan_block_comment(code: str, pos: int, end_marker: str) -> int
Parameters
Name Type Description
code str

Source code.
pos int

Position after opening marker.
end_marker str

The closing marker (e.g., "*/" or "-->").
Returns
int