There's an easier way to specify long floats in Common Lisp: use the exponent marker "L" e.g. 9999999999999999.0L0. No need to bind or set reader variables.

That said, even in Common Lisp I think its only CLISP (among the free implementations) that gives the correct answer for long floats.

CLISP:

    [1]> (- 9999999999999999.0L0 9999999999999998.0L0)
    1.0L0

SBCL, CMUCL and Clozure CL:

    * (- 9999999999999999.0L0 9999999999999998.0L0)
    2.0d0

The standard only mandates a minimum precision of 50 bits for both double and long floats, so there's no guarantee that using long floats will give the correct answer, as we can see.

http://www.lispworks.com/documentation/HyperSpec/Body/t_shor...