I'm still trying to wrap my brain around Lisp and the fact that (eq 2 2) may be false.

And this is by design, not a bug?

Apparently yes, it is by design. == is used for pointer equality while = is used for structural equality. I'm guessing that since floats are 8 bytes, they can't be stored as a primitive value. Therefore == compares pointers instead of values. But I'm probably wrong about that.

This actually makes a lot of sense. If you're comparing values, 0.0 = 0.0 +. 0.0 is true, whereas == simply checks to see if each value references the same object.

Eric, I disagree. If you're adding ints, then == works fine. It's only if you're adding doubles that it breaks. In my world, any language that treats ints as non-reference-types also treats doubles as non-reference types. Therefore this is confusing and unexpected.

You should be using = instead of ==. Only use == when you know exactly what it does, e.g. when you're writing code to traverse cyclic data structures.