Parsing Python's indent rules

suite     ::= stmt_list NEWLINE
                | NEWLINE INDENT statement+ DEDENT

token suite {
    <stmt_list> <newline>
      | <newline> <indent> <statement>
        [ <indent_same> <statement> ]* <dedent>
}

• <indent> measures horizontal whitespace, pushes new level onto stack
• <indent_same> verifies same level of nesting as previous line
• <dedent> pops stack to previous indentation level

• These are all just custom Parrot subs
• manage the stack
• fail or throw exceptions if indentation errors arise