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 Functionality
• The Lexer AST

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:

• Var: Variable identifiers.
• Ref: Referrent identifiers.
• Opr: Operator identifiers.
• Number: Numbers.
• Text: Text.
• Invalid: Invalid constructs that cannot be lexed.
• Block: Syntactic blocks in the language.

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.