bad

fn mk_appender(suffix: ~str) -> @fn(~str) -> ~str {
    return @fn(s: ~str) -> ~str { s + suffix };  // error: mismatched types: expected `fn@(~str) -> ~str` but found `@fn&(~str) -> ~str` (expected fn but found @-ptr)
}

fn main() {
    let shout = mk_appender(~"!");
    io::println(shout(~"hey ho, let's go"));
}

good

fn mk_appender(suffix: ~str) -> fn@(~str) -> ~str {
    return fn@(s: ~str) -> ~str { s + suffix };
}

fn main() {
    let shout = mk_appender(~"!");
    io::println(shout(~"hey ho, let's go"));
}