enso/docs/parser/lexer.md

layout	title	category	tags	order
developer-doc	Lexer	syntax	parser lexer	4

Lexer

The lexer is the code generated by the flexer that is actually responsible for lexing Enso source code. It chunks the character stream into a (structured) token stream in order to make later processing faster, and to identify blocks

• Lexer Architecture
  • Libraries in the Lexer Definition
• Lexer Functionality
• The Lexer AST

Lexer Architecture

The structure of the flexer's code generation forces the lexer to be split into two parts: the definition, and the generation. As the latter is the point from which the lexer will be used, the second subproject is the one that is graced with the name lexer.

Libraries in the Lexer Definition

The lexer generation subproject needs to be able to make the assumption that all imports will be in the same place (relative to the crate root). To this end, the definition subproject exports public modules library and prelude. These are re-imported and used in the generation subproject to ensure that all components are found at the same paths relative to the crate root.

This does mean, however, that all imports from within the current crate in the definition subproject must be imported from the library module, not from their paths directly from the crate root.

Lexer Functionality

The lexer needs to provide the following functionality as part of the parser.

• It consumes the source lazily, character by character, and produces a structured token stream consisting of the lexer ast.
• It must succeed on any input, even if there are invalid constructs in the token stream, represented by Invalid tokens.

The Lexer AST

In contrast to the full parser ast, the lexer operates on a simplified AST that we call a 'structured token stream'. While most lexers output a linear token stream, it is very important in Enso that we encode the nature of blocks into the token stream, hence giving it structure.

This encoding of blocks is crucial to the functionality of Enso as it ensures that no later stages of the parser can ignore blocks, and hence maintains them for use by the GUI.

It contains the following constructs:

• Referent: Referrent identifiers (e.g. Some_Ref_Ident).
• Variable: Variable identifiers (e.g. some_var_ident).
• External: External identifiers (e.g. someJavaName).
• Blank: The blank name _.
• Operator: Operator identifiers (e.g. -->>).
• Modifier: Modifier operators (e.g. +=).
• Number: Numbers (16_FFFF).
• DanglingBase: An explicit base without an associated number (e.g. 16_).
• Text: Text (e.g. "Some text goes here.").
• Line: A line in a block that contains tokens.
• BlankLine: A line in a block that contains only whitespace.
• Block: Syntactic blocks in the language.
• InvalidSuffix: Invalid tokens when in a given state that would otherwise be valid.
• Unrecognized: Tokens that the lexer doesn't recognise.

The distinction is made here between the various kinds of identifiers in order to keep lexing fast, but also in order to allow macros to switch on the kinds of identifiers.

The actionables for this section are:

• Determine if we want to have separate ASTs for the lexer and the parser, or not.