Why does common lisp's case construct always match True (T)?

Question

This is with SBCL 1.0.55 on Debian squeeze. I'm probably missing something obvious, but I'm a beginner, so please bear with me.

CL-USER> (defparameter x 0)

CL-USER> (case x (t 111) )
111

So it looks like case here is matching the variable x with the truth symbol t. This happens with everthing I've tried; this x is just an example. I don't see why this would happen. Since case uses eql for matching, I tried

CL-USER> (eql x t)
NIL

So, eql does not match x and t. What am I missing? Thanks in advance.

Answer 1

In the case construct in Common Lisp, t, used by itself, is equivalent to default in C; that is, it's evaluated if the expression doesn't match any of the other cases. If you want to match the actual symbol t, use (t) instead.

Answer 2

Described in the CASE documentation.

otherwise-clause::= ({otherwise | t} form*)

The syntax says that an otherwise clause is either (otherwise form-1 ... form-n) or (t form-1 ... form-n). Note that the syntax says {otherwise | t}. The vertical bar is an OR in a syntax specification. So the marker for an otherwise clause is either otherwise or t.

That means, if your case clause begins with otherwise or t, then we have an otherwise-clause.