Parsing for

ruleForConstruct = rule "for construct" $ do
    symbol "for"
    list  <- maybeParens ruleExpression
    optional ruleComma
    block <- ruleBlockLiteral
    retSyn "for" [list, block]

• optional means try something out
• if it wasn't found, don't fail the current rule but don't consume anything
• skip what did parse though
• In Perl 5 regexps: /(?:,)?/