**27 Jan 2001** (updated 31 Jul 2003 at 00:48 UTC)

»
**Scheme**
graydon:Yes, I think
the right question
is: "why isn't '() coerced to #f." More generally, why
isn't the coerce-to-boolean form specialized for
every type? The whole idea of everything being coerced to
#t except for #f, in a way already violates the disjointness of types. If the system were totally pure, it would raise an error when anything
other than a boolean constant is tested for truth. So,
there is already a coerce-to-boolean form for each
type; it just happens to be a constant function. I
don't think it is any worse to define a more
interesting coerceion function, different for each type.

**Cognitive Illusions**

I saw an interesting lecture yesterday on cognitive
illusions in human/computer interaction; these are places
where the mind incorrectly applys a hueristic to come up
with a solution, instead of using logical thought.

One place this pops up is conditional probability. Write
down the solution to the following problem before you check
the answer.

**Your friend has two rabbits. One of them is a girl.
What
is the probability the other one is a girl too?**

For now, I've put the answer at the bottom of notes on
my account page, because I can't figure out which of the
allowed tags will make background black to do the hidden
text trick.

**Update**: sarum is
right, your
friend wouldn't pick the rabbits randomly. So lets say,
your friend has a rabbit that just had two children.

If you don't believe the answer, think about these other two problems, and how they are different.

There are two rabbits in a cage. The bigger one is a
girl. What is the probability the other one is a girl too?

**Update**: and to clarify this one too, this question is not about rabbit physiology.

There are two rabbits in a hat. You pull one out, it is
a girl. What is the probability the other one is a girl too?

Really tough to wrap your mind around, huh? If you still
don't believe me, I'll make you a truth table.