[yellowapple]

> The printer is probably cheating and rounding to the output you expect

    northrup@Topaz:~$ sbcl
    This is SBCL 1.5.8, an implementation of ANSI Common Lisp.
    More information about SBCL is available at <http://www.sbcl.org/>.
    
    SBCL is free software, provided as is, with absolutely no warranty.
    It is mostly in the public domain; some portions are provided under
    BSD-style licenses.  See the CREDITS and COPYING files in the
    distribution for more information.
    * (= (+ 0.1 0.2) 0.3)
    T
    * (= (- (+ 0.1 0.2) 0.3) 0)
    T

> Aside from financial applications

"Financial applications" happen to be pretty common reasons for number crunching :)

[reitzensteinm]

This just means that 0.3 has the same internal representation as the result of (0.1 + 0.2).

Internally, they're probably both 0.30000000000000004 (depending on precision), so an equality check returns true.

It could also be that they're both 3/10 rational numbers, but given other tests in this thread that's likely not the case out the box.

[yellowapple]

    northrup@Topaz:~$ sbcl
    This is SBCL 1.5.8, an implementation of ANSI Common Lisp.
    More information about SBCL is available at <http://www.sbcl.org/>.
    
    SBCL is free software, provided as is, with absolutely no warranty.
    It is mostly in the public domain; some portions are provided under
    BSD-style licenses.  See the CREDITS and COPYING files in the
    distribution for more information.
    * (- 0.3 3/10)
    0.0
    * (= (- 0.3 3/10) 0)
    T
    * (= (- 3/10 0.3) 0)
    T
    * (= 0.3 3/10)
    NIL

So yeah, 0.3 and 3/10 are definitely distinct, but still apparently net out to exactly 0 nonetheless.

[fiddlerwoaroof]

    CL-USER(3): (rational (+ 0.1 0.2))
    
    5033165/16777216
    CL-USER(3): (rational 0.3)
    
    5033165/16777216