gistfile1.txt

=begin comment
In this grammar, <expression> loops endlessly when tried with an empty string.
It first fails at <variable>, which is fine, it then fails at <function> which is also fine,
but then it keeps trying <application> recursively, even if <application> contains
at least the characters '(' and ')'.

Shouldn't Perl 6 be smart enough to understand that <application> will never match with an empty string?
=end comment

say grammar {
    rule TOP { ^^ <expression> $$ }
    rule expression { <variable> | <function> | <application> }
    rule function { sub [ '(' <variable>? ')' ]? '{' <expression>? '}' }
    rule application { <expression> '(' <expression>? ')' }
    token variable { '$'<ident>+ }
}.parse: q{ sub {} };