Given a Source object, creates a Lexer for that source. A Lexer is a stateful stream generator in that every time it is advanced, it returns the next token in the Source. Assuming the source lexes, the final Token emitted by the lexer will be of kind EOF, after which the lexer will repeatedly return the same EOF token whenever called.

A helper function to describe a token as a string for debugging

A helper function to describe a token kind as a string for debugging