?- phrase(joy_parse(Expression), `truedat`).
    Expression = [bool(true), symbol(dat)] .

Oops!

I knew about:

    ?- phrase(joy_parse(Expression), `23dat`).
    Expression = [int(23), symbol(dat)] .

should probably fix it too

still want:

    ?- phrase(joy_parse(Expression), `[2[3]]`).
    Expression = [list([int(2), list([int(3)])])] .

    ?- phrase(joy_parse(Expression), `[true[false]]`).
    Expression = [list([bool(true), list([bool(false)])])] .

...to work.  That is, you shouldn't need spaces around '[' and ']'.