# Let's say we want to parse payloads from a TCP connection:
my Supply:D $connection = supply {
    emit 'fo';
    emit 'obarb';
    emit 'azf';
    emit 'ooba';
    emit 'rbazfooba';
    emit 'rbaz';
    done;
};

# It's not guaranteed that these payloads will be complete messages, so our
# parser needs to keep context for where it is in the stream of payloads...
grammar Connection::Grammar {
    token TOP { ( foo | bar | baz )+ }
}
class Connection::Actions {
    method TOP($/) { make $0».Str }
}

# ...but this is a problem! We don't want locks slowing down communication with
# our peer, but we need to hold on to our parser as we parse each payload
# received! Psych, this isn't a problem with Supply.scan:
react whenever $connection.scan: &parse-payload, Connection::Grammar {
    .made.say;
    # OUTPUT:
    # Nil
    # [foo bar]
    # [baz]
    # [foo]
    # [bar baz foo]
    # [bar baz]
}

proto sub parse-payload(Connection::Grammar:_, Str:D --> Connection::Grammar:D) {*}
multi sub parse-payload(Connection::Grammar:U $acc, Str:D $payload --> Connection::Grammar:D) {
    $acc.subparse: $payload, actions => Connection::Actions.new;
}
multi sub parse-payload(Connection::Grammar:D $acc, Str:D $payload --> Connection::Grammar:D) {
    $acc.subparse: $acc.orig.substr(max $acc.pos, 0) ~ $payload, actions => $acc.actions
}

method fold(::?CLASS:D: &folder is raw --> ::?CLASS:D)

method fold(::?CLASS:D: &folder is raw, $init is raw --> ::?CLASS:D)

method scan(::?CLASS:D: &scanner is raw --> ::?CLASS:D)

method scan(::?CLASS:D: &scanner is raw, $init is raw, Bool:D :$keep = False --> ::?CLASS:D)